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Extended-Range Forecasts of Atlantic Events during 2012 Using the ECMWF 32-Day Ensemble Predictions*

RUSSELL L. ELSBERRY,HSIAO-CHUNG TSAI, AND MARY S. JORDAN Department of Meteorology, Naval Postgraduate School, Monterey, California

(Manuscript received 5 September 2013, in final form 25 October 2013)

ABSTRACT

Previous studies have demonstrated the capability of the European Centre for Medium-Range Weather Forecasts (ECMWF) 51-member, 32-day ensemble to forecast tropical cyclone (TC) events (formation and tracks) in the western North Pacific on the extended range (5–30 days). In this study, the performance of the ECMWF ensemble in extended-range forecasting of Atlantic TCs during May–December 2012 is evaluated using similar approaches. The conclusion from this evaluation is that Atlantic TC events have lower fore- castability using the ECMWF ensemble than in the western North Pacific. Hurricanes Kirk and Leslie and Tropical Storms (TSs) Joyce and Oscar were successfully forecast in weeks 1–4 and, thus, are labeled as highly forecastable. Somewhat forecastable storms that are only forecast in three of the four weeks include Hurricanes Ernesto, Isaac, Nadine, and Sandy plus TS Florence. The limited forecastable storms that were successful in only the first two weeks include Hurricanes Gordon and Rafael plus TS Debby. The surprising result was that two hurricanes (Chris and Michael) and three TSs (Helene, Patty, and Tony) were not even forecast in week 1 before the starting time in the National Hurricane Center working best track (WBT) for these storms. As was the case in the western North Pacific, a substantial number of false alarm storms (no matches with any WBT) are predicted, with about 35% occurring in the first week. Except for the African wave–type false alarms, three other false alarm types may be easily recognized. A larger sample will be required to statistically verify the reliability of the probabilistic forecasts for the African wave–type en- semble storms.

1. Introduction Although verifications of the ensemble mean tracks might be expected to be less skillful than the tracks of the Ensemble predictions of tropical cyclone (TC) tracks ‘‘mother’’ deterministic high-resolution model because have become available from a number of numerical the ensemble model is integrated at a coarser horizontal centers for TCs that are already present in the initial resolution, some studies indicate the ensemble mean may conditions. For example, the European Centre for have marginally more skill at longer forecast intervals. Medium-Range Weather Forecasts (ECMWF) integrates Elsberry (2010) notes that the ensemble products are not the Variable Ensemble Prediction System (VarEPS) in available to the forecasters until 6 h later than the con- addition to its medium-range deterministic model (van der Grijin et al. 2005). The new generation of ECMWF sensus of deterministic models that many forecasters use products includes an extension of the tracking and strike as the primary track guidance, and this time delay must be probability maps from 5 to 10 days and probabilistic in- taken into account when comparing ensemble track er- formation on the storm intensity (Vitart et al. 2012). rors with the official track forecast errors. Rather than just generating a mean track that might be marginally better, a primary objective of the track * Supplemental information related to this paper is available at ensemble is to provide information on the uncertainty in the Journals Online website: http://dx.doi.org/10.1175/WAF-D-13- 00104.s1. the track forecast. The ECMWF constructs strike prob- abilities that the center of the storm will pass within 120 km of a point within the next 10 days. Superposing Corresponding author address: Russell L. Elsberry, Dept. of the ensemble mean track on these strike probabilities Meteorology, Naval Postgraduate School, 589 Dyer Rd., Monte- rey, CA 93943. then provides a visualization of the ensemble spread E-mail: [email protected] around the mean, and this swath is an indication of the

DOI: 10.1175/WAF-D-13-00104.1

Unauthenticated | Downloaded 10/03/21 09:33 AM UTC 272 WEATHER AND FORECASTING VOLUME 29 track uncertainty. Dupont et al. (2011) have evaluated Elsberry et al. (2010, 2011) used a two-step objective calibrated probabilistic track forecasts in the South In- plus subjective approach for verifying the ensemble dian Ocean based on the ECMWF VarEPS and found storm tracks relative to the Joint Typhoon Warning skill relative to climatology in predicting track un- Center (JTWC) best tracks. Their approach was to first certainty for lead times up to 3 days. Whereas Majumdar treat the JTWC track as another storm and compare all and Finocchio (2010) had found that the ECMWF was of the applicable forecast ensemble storms at each 12-h the most skillful ensemble they tested, the VarEPS forecast time to determine if at least one point on the forecasts in the Atlantic were underdispersive (i.e., en- ensemble storm track matched within a separation dis- semble spread underestimated the track uncertainty). tance «(t) the JTWC position at exactly that 12-h time. Yamaguchi et al. (2012) also found the ECMWF en- Second, a subjective assessment was made to assign semble was the best individual ensemble in their multi- a quality metric (excellent, above average, good, below center grand ensemble. average, and poor) of the match of the ensemble storm A new ‘‘genesis’’ prediction from ECMWF that was to the JTWC track. provided in 2012 is the strike probability for the occur- Tsai et al. (2013) have developed an objective track rence of a TC-like vortex with a maximum wind speed of analog verification technique to select all ensemble 2 at least 8 m s 1 passing within 300 km of a given location storm tracks predicted by the ECMWF 32-day ensemble during some period in the medium-range forecast that match the overall JTWC tracks. For ensemble (Vitart et al. 2012). A similar display of the TC activity storms within specified time and space differences of the within 300 km is available from the ECMWF 32-day JTWC track that are potential analogs, four metrics of ensemble integrated twice a week from initial conditions shortest distance, average distance of the matched at 0000 UTC on Mondays and Thursdays. points, distance at formation time, and distance at end- Vitart (2009) had demonstrated that an ECMWF ing time are calculated. An objective quality measure 15-member ensemble hindcast could reproduce the that assesses the overall track similarity between the main characteristics of the observed distribution of TCs, potential analogs and the JTWC storm is calculated in although the TC activity in the model was higher than terms of membership functions for the four track met- observed. Vitart (2009) had attributed the skill of these rics. Weighting factors multiplying these membership activity forecasts to the ability of the ECMWF ensemble functions were adjusted to approximately match the model to predict the Madden–Julian oscillation (MJO). subjectively determined quality measures for the Vitart et al. (2010) also demonstrated that after cali- ECMWF ensemble storm forecasts during the 2009 bration the ECMWF ensemble could predict the TC ac- season (Elsberry et al. 2011). tivity in the Southern Hemisphere up to 3 weeks with skill Objective verifications for the 2009 and 2010 seasons comparable to the statistical models. Belanger et al. (2010) were then summarized by Tsai et al. (2013) in terms of examined the predictability of North Atlantic tropical cy- hits, misses, false alarms, and correct negatives that no clones using the 32-day ECMWF VarEPS during the 2008 TC would be present in the western North Pacific. The and 2009 seasons. Predictability to 15–21 days was in- most important result was that the ECMWF ensemble dicated in the main development region for Atlantic TCs. was able to predict out to 4 weeks nearly all of the TCs in Belanger et al. (2012) have also studied the predictability both seasons and with only a small number of misses that of TC genesis and regional outlooks of TC activity for the generally were short-lived tropical depressions. Good Arabian Sea and the Bay of Bengal. They found similar performance in terms of correct negatives was achieved, levels of predictability as in the North Atlantic. especially during the 2010 season, which had the least Elsberry et al. (2010) evaluated the predictability of TC activity in recent times. western North Pacific TC formations and tracks out to False alarms were defined by Tsai et al. (2013) to be all 4 weeks during the 2008 season using the ECMWF ensemble storms that could not be matched with JTWC 32-day ensemble forecasts. Rather than a strike proba- storms within the specified thresholds. Evaluations of bility approach, they combined similar ensemble member the characteristics of the false alarms indicate seasonal vortex tracks and used a weighted mean vector motion and geographic biases and that about 50% of the false technique (Elsberry et al. 2008) to form ensemble storm alarms during the first week originate from the initial tracks. Elsberry et al. (2011) examined the more typical conditions in the model. After a minimum of false and active 2009 season using the same approach and alarms originating in the week-2 forecasts, which was found improvements in predicting the formations and attributed to the decrease in horizontal resolution in the tracks of typhoons and the improvement also applied model that occurs at day 10, a steady and nearly uniform to most of the tropical depressions on time scales of increase in false alarms originating in the week-3 and -4 5–30 days. forecasts was documented.

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FIG. 1. Ensemble storm 1 (dark gray) generated from the weighted-mean vector motion of ECMWF 32-day ensemble vortices (light gray) in the week-1 forecast initiated at 0000 UTC 30 Jul 2012, which was then matched by the analog track verification technique with the NHC WBT of (AL05, dashed black) with an LHV 5 0.55. The first position in the WBT is at 1200 UTC 1 Aug, which is 2.5 days after the first position of ensemble storm 1.

The purpose of this study is to evaluate the ECMWF Following Elsberry et al. (2010, 2011), the evaluations 32-day ensemble forecasts, which were kindly supplied of the ECMWF ensemble predictions will begin for the by ECMWF in support of the National Aeronautics and Monday or Thursday forecasts before the National Hur- Space Administration (NASA) Hurricane and Severe ricane Center (NHC) begins to follow the system, which Storm Sentinel (HS3) field experiment, for TC events in will be defined as week 1. An example of the 0000 UTC the Atlantic during May–December 2012. Since essen- 30 July 2012 ECMWF ensemble forecast of pre–Hurricane tially the same techniques for generating ensemble Ernesto is used to illustrate several aspects of the en- storms as in Elsberry et al. (2010, 2011), and for objec- semble storm generation and validation as applied in the tively verifying the forecasts as in Tsai et al. (2013), Atlantic (Fig. 1). The first entry in the NHC working best were utilized, these will only be briefly summarized in track for Ernesto (AL05) was at 1200 UTC 1 August at section 2. An overview of the ECMWF ensemble per- 11.68N, 46.78W(Table 1). Notice that the ECMWF en- formance during the 2012 Atlantic season will be given semble forecast initiated 2.5 days earlier already had in section 3. Some case studies of individual storms will a well-defined track that began about 148 longitude to the be presented to illustrate some successes and some east. The ensemble vortex tracks that make up ensemble failures in extended-range forecasts. A description of storm 1 then had a small spread about the Ernesto track the false alarms in the Atlantic will be given in section 4. until 708W, when a number of vortex tracks began to turn Finally, a summary and future tasks will be provided in poleward. However, the majority of the vortices contin- section 5. ued to the west-northwest, so the weighted-mean vector motion ensemble storm track was almost perfect until it passed to the west of the Yucatan Peninsula. Whereas 2. Methodology Ernesto actually moved inland over and decayed, While the reader will find the details in Elsberry et al. ensemble storm 1 was predicted to remain and moved (2010, 2011) and Tsai et al. (2013), a number of features over the western until it sharply turned of the technique and methodology will be summarized to the northeast and moved over the southeast United in the appendix, including (i) characteristics of the States. As Elsberry et al. (2011) documented in the ECMWF ensemble compared to the earlier studies and western North Pacific, the ECMWF ensemble vortices (ii) a detailed summary of the modified track analog are often predicted to continue large distances over land, technique for verifying the ensemble storm tracks that is as in Fig. 1. essential for understanding the following verifications The ensemble storm 1 prediction in Fig. 1 is referred during the 2012 Atlantic season. to as a week-1 forecast because it was initiated on

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TABLE 1. Storm number and name, as well as time, intensity category, and space characteristics from the 2012 Atlantic WBTs grouped according to their forecastability by the ECMWF 32-day ensemble (see text and Fig. 2 for the corresponding LHVs during weeks 1–4).

Time and center position of Time and center position Time and center position Storm No. Name starting point of TS stage of max wind speed (a) Highly forecastable EF4 storms 10 TS Joyce 1200 UTC 19 Aug 12.08N, 20.08W 1200 UTC 23 Aug 14.88N, 41.28W 1200 UTC23 V Aug 14.88N, 41.28W FORECASTING AND WEATHER 11 Hurricane Kirk 0600 UTC 24 Aug 12.98N, 21.28W 0000 UTC 29 Aug 23.88N, 44.58W 0600 UTC 31 Aug 29.68N, 50.78W 12 0600 UTC 29 Aug 13.28N, 33.68W 1200 UTC 30 Aug 13.88N, 42.68W 1800 UTC 5 Sep 25.88N, 62.78W 15 TS Oscar 0000 UTC 1 Oct 9.58N, 30.48W 1800 UTC 3 Oct 18.28N, 41.88W 1800 UTC 4 Oct 21.08N, 41.48W (b) Somewhat forecastable EF3 storms 5 Hurricane Ernesto 1200 UTC 1 Aug 11.68N, 46.78W 1200 UTC 2 Aug 13.08N, 53.68W 0300 UTC 8 Aug 18.88N, 87.78W 6 TS Florence 1200 UTC 2 Aug 11.58N, 17.88W 0600 UTC 4 Aug 14.08N, 28.58W 0000 UTC 5 Aug 15.88N, 32.58W 9 0600 UTC 18 Aug 12.18N, 26.88W 1800 UTC 21 Aug 15.38N, 53.28W 0000 UTC 29 Aug 28.98N, 89.58W 14 0000 UTC 10 Sep 15.08N, 33.28W 0000 UTC 12 Sep 17.58N, 44.68W 1200 UTC 30 Sep 36.98N, 38.38W 18 1200 UTC 21 Oct 14.78N, 75.88W 1800 UTC 22 Oct 12.78N, 78.48W 0600 UTC 25 Oct 20.18N, 75.98W (c) Limited forecastable EF2 storms 4 TS Debby 1200 UTC 22 Jun 22.68N, 88.18W 1800 UTC 23 Jun 26.08N, 87.68W 0600 UTC 24 Jun 27.08N, 87.38W 8 0600 UTC 15 Aug 26.18N, 54.08W 0000 UTC 16 Aug 30.58N, 55.48W 0000 UTC 19 Aug 34.38N, 35.08W 17 Hurricane Rafael 1200 UTC 8 Oct 8.38N, 37.58W 1800 UTC 12 Oct 14.78N, 62.78W 0000 UTC 16 Oct 24.68N, 65.68W (d) Not forecastable EF0 storms

Unauthenticated |Downloaded 10/03/21 09:33 AMUTC 3 Hurricane Chris 0000 UTC 17 Jun 28.88N, 68.88W 1800 UTC 19 Jun 39.58N, 58.08W 1200 UTC 21 Jun 40.58N, 43.98W 7 TS Helene 0000 UTC 7 Aug 11.78N, 26.68W 1800 UTC 17 Aug 20.38N, 95.98W 1800 UTC 17 Aug 20.38N, 95.98W 13 Hurricane Michael 1800 UTC 2 Sep 26.48N, 40.18W 0600 UTC 4 Sep 26.28N, 43.08W 1200 UTC 6 Sep 29.98N, 41.48W 16 TS Patty 0000 UTC 11 Oct 25.88N, 72.78W 1800 UTC 11 Oct 25.88N, 72.38W 0000 UTC 12 Oct 25.58N, 72.48W 19 TS Tony 1200 UTC 21 Oct 19.88N, 50.08W 0000 UTC 24 Oct 26.48N, 49.88W 1200 UTC 24 Oct 28.18N, 46.48W OLUME 29 APRIL 2014 E L S B E R R Y E T A L . 275

FIG. 2. Quality metric LHV (ordinate) for weeks 1–4 (symbols in inset) for the AL03–AL19 TSs and H1–H3 hurricanes during the 2012 season. An LHV $ 0.2 is considered a hit and an LHV , 0.2 is considered a miss. Note that LHV 5 0 for all four weeks for AL03, AL07, AL13, AL16, and AL19.

Monday, 30 June, in the range of 1–7 days before pre- common in the western North Pacific (Elsberry et al. Ernesto existed according to the NHC WBT. The next 2010) and code was developed to merge two ensemble forecast initiated on Thursday, 2 August, is not evalu- storms that were subjectively determined to have similar ated because Ernesto (AL05) began on 1 August and the tracks. As this capability to combine two similar tracks interest here is on the formation and subsequent track. had only been applied during the later part of the 2012 The forecast on Thursday, 27 July, would also be a week-1 Atlantic season, in this evaluation the ensemble storm forecast since it is also in the range of 1–7 days before pre- track with the higher LHV will be selected if multiple Ernesto existed. The Monday, 23 July, and Thursday, ensemble storms from the same forecast were matched 20 July, forecasts would be designated as week-2 fore- to an NHC storm. casts, etc. Fewer ensemble vortex tracks exist to form an ensemble storm at longer forecast intervals than in week 3. Evaluation of EMCWF 32-day ensemble 1(Fig. 1), but a minimum of three vortices and a track of performance during the 2012 Atlantic season at least 3 days was imposed to include a maximum number of potential ensemble storms without consider- In addition to the period of the NASA HS3 field ing short-lived systems that are not likely to be repre- campaign of August–November, ECMWF also pro- sentative of tropical storms and hurricanes. vided the forecasts starting from 31 May and extending When the ensemble storm 1 track is validated with the to 17 December. Specifically, forecasts of Tropical Storms objective track analog technique of Tsai et al. (2013; see (TSs) Alberto (AL01) and Beryl (AL02) were not in- summary in the appendix), the quality measure called cluded in the study period. the likelihood value (LHV) is only 0.55, even though the a. Overview of the 2012 season track overlies the Ernesto track from 408 to 908W. This reduction from LHV 5 1.0 is due to the deduction for The performance of the ECMWF 32-day ensemble the forecast ending point being far to the northeast, and forecasts of Atlantic TC events during the 2012 season is partly for the forecast starting point being to the east. summarized in Fig. 2 in terms of the quality measure However, the NHC forecasters may suggest the LHV for the ensemble storm that matched an NHC weighting factors for the ending point and the formation storm (see the LHV definition in the appendix). For point may need to be reduced to better reflect the en- example, the summary for Ernesto (AL05) in Fig. 1 is semble storm 1 track forecast overlapping most of the LHV 5 0.58 (0, 0.94, 0.29) for week 1 (2, 3, 4), which Ernesto track and be more accepting of the early start of would be designated as good (miss, excellent, below the forecast track. Nevertheless, the same weighting average) in Tsai et al. (2013). factors of the four metrics in the quality measure LHV An immediate conclusion from comparing the 2012 will be specified here as they were in Tsai et al. (2013) for Atlantic summary of LHVs in Fig. 2 with the western the western North Pacific. North Pacific summary for the 2009 and 2010 seasons Occasionally, two or more ensemble storms with (Tsai et al. 2013) was that the ECMWF ensemble per- similar tracks existed in a forecast and were both formance on extended-range time scales was more lim- matched with a NHC storm by the track analog verifi- ited in the Atlantic. Since a small sample of 17 Atlantic cation technique. Such multiple similar tracks are more storms during 2012 cannot be used to assess predictability,

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FIG. 3. Ensemble storm, member vortices, verifying WBT, and LHV as in Fig. 1, but for Hurricane Leslie (AL12, black dashed) based on ECMWF 32-day ensemble forecasts initiated during (a) week 1 on 23 Aug, (b) week 2 on 20 Aug, (c) week 3 on 13 Aug, and (d) week 4 on 6 Aug. this evaluation of the ECMWF ensemble performance which the ECMWF 32-day ensemble forecasts failed to will be characterized in terms of a ‘‘forecastable’’ metric have a match in all four weeks. for certain types of TC events. That is, the objective is to 1) HIGHLY FORECASTABLE (EF4) STORMS inform the NHC forecaster that some Atlantic TC events may be forecastable in the extended range, but Only two hurricanes (AL11 and AL12) and two other types of Atlantic TCs are not. Indeed, the varia- tropical storms (AL10 and AL15) were successfully tion of forecastability is from 4 weeks to no forecast- forecast in all four weeks, which is in strong contrast with ability even 1 week before the storm has appeared in the Tsai et al. (2013), who found that nearly all of the ty- NHC WBT. phoons and even many of the tropical depressions in the western North Pacific during 2009 and 2010 were suc- b. Forecastability of individual storms cessfully forecast out to 4 weeks. Some characteristics of Four groupings of the storms (Table 1) will be de- these EF4 storms are given in Table 1a. scribed. These groupings are characterized as 1) highly The ensemble storm forecasts that could be matched forecastable (EF4), which is defined as cases where all with Hurricane Leslie (AL12) during all four weeks are forecasts for weeks 1–4 had a match; 2) somewhat given in Fig. 3. Ensemble storm 8 in the Thursday, forecastable (EF3), which will be for ensemble storm 23 August, forecast that is designated as week 1 (Fig. 3a) forecasts that had LHV $ 0.2 in only three of the four had a track that overlapped the Leslie track for much of weeks; 3) limited forecastable (EF2), which were en- the forecast period. As in Fig. 1, the week-1 forecast for semble storm forecasts that had LHV $ 0.2 in only the Leslie had an LHV of only 0.54 (Fig. 2) due to de- first two weeks; and 4) not forecastable (EF0), storms for ductions for a large forecast track ending difference and

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FIG.4.AsinFig. 3, but for Hurricane Kirk (AL11) for forecasts initiated during (a) week 1 on 23 Aug, (b) week 2 on 13 Aug, (c) week 3 on 6 Aug, and (d) week 4 on 30 Jul. a premature starting location more than 258 longitude to had a more westward path, so that a deduction was made the east. While the large-scale environmental conditions for a reduced average position metric and the large do not accurately predict the timing of formation ac- forecast ending position difference. Thus, the LHV 5 0.70 cording to NHC, they do determine the track type and is in the above average category. Finally, the week-4 fore- two factors in the LHV measure the track characteris- cast (Fig. 3d) that was initiated on 6 August, or 23.25 days tics, which are arguably more important than the precise before pre-Leslie started, had a forecast starting position formation time. and an early track that agreed well with the Leslie track, For the week-2 forecast (Fig. 3b), the ensemble storm but an excessive ending distance metric at the time cor- 19 track forecast starting position was quite close to the responding to day 32 in the forecast led to the quality actual position. In addition, the subsequent track fore- measure LHV 5 0.78. cast was quite similar to the Leslie track, so both the Hurricane Kirk (AL11) was the second hurricane that closest position and the average position difference met- was classified as a highly forecastable (EF4) storm with rics were highly scored. However, the forecast track a quality measure LHV . 0.4 in all four weeks (Fig. 2). ending position was quite poor, so the LHV was fully A large fraction of the ECMWF ensemble members reduced by 0.22 (weighting factor) to 0.56 (Fig. 2). For were predicting pre-Kirk in the 0000 UTC 23 August the week-3 forecast (Fig. 3c), the ensemble storm 39 forecast (Fig. 4a), which is the week-1 forecast, as Kirk starting position and the early portion of the forecast began 30 h later at 0600 UTC 24 August (Table 1a). The track agreed quite well with the Leslie track. However, good agreement in the initial positions and in the early Leslie turned sharply northward and almost stalled for track contributed to an LHV 5 0.73 (Fig. 2). However, several days. While one ensemble vortex track had Kirk had a relatively sharp recurvature that only a few a close similarity to this track change, other vortex tracks ensemble vortices predicted. Rather, the majority of the

Unauthenticated | Downloaded 10/03/21 09:33 AM UTC 278 WEATHER AND FORECASTING VOLUME 29 ensemble vortices recurved the storm about 108 longi- but the track lengths were short, similar to TS Oscar, and tude farther west than Kirk. A significant number of thus the LHVs of 0.77 and 0.68 were in the above av- vortex tracks had the recurvature much farther to the erage category. west, but the weighted-mean vector motion calculation In summary, these four EF4 storms were forecast by would give little weight to these tracks compared to the the ECMWF on extended-range time scales (5–30 days). larger number of vortices with tracks closer to the prior It should be noted that these four highly forecastable ensemble storm 3 track. storms generally occurred in the midseason and all de- Ensemble storm 33 in the week-2 forecast (Fig. 4b), veloped from African systems. Although the forecast which initiated at 0000 UTC 13 August, again had a start- tracks were frequently too long (and thus resulted in an ing point on the coast of and then had a more LHV deduction of 0.22), they are considered to be westward track that resulted in a recurvature about 178 highly forecastable and potentially useful outlooks of longitude west of Kirk. Although the shortest distance specific TC events in the Atlantic. metric is satisfied because the forecast track crossed 2) SOMEWHAT FORECASTABLE EF3 STORMS Kirk’s track, the deductions for the forecast starting point, average distance, and forecast ending point re- Four hurricanes and one tropical storm were classified sulted in an LHV 5 0.51 (Fig. 2). The week-3 forecast as EF3 storms (Table 1b), as the EMCWF 32-day en- (Fig. 4c) is rated at LHV 5 0.81 even though the starting semble was able to successfully forecast these storms in point is again on the coast of Africa and the initial track three of the four weeks (Fig. 2). In Hurricanes Isaac is more westward because two crossings of the Kirk (AL09), Nadine (AL14), and Sandy (AL18), the un- track lead to a zero shortest distance metric and a successful forecast was in week 4 (as might be expected), smaller average distance as the forecast recurvature point but for Hurricane Ernesto (AL05) and TS Florence is only 78 longitude west of Kirk. Finally, the week-4 (AL06), the unsuccessful forecast was in week 2. forecast (Fig. 4d)isalsohighlyrated(LHV5 0.92) due The successful week-1 forecast of Ernesto (AL05) was to an excellent forecast starting point, a forecast track illustrated in Fig. 1. The week-3 forecast (Fig. 5a) initi- crossing, a small average distance, and that the distance ated on 12 July was even more successful with an LHV 5 ending time is only considered to the end of the 32-day 0.94 (Fig. 2). Even though ensemble storm 16 started ensemble forecast (i.e., the later part of the actual track 48 h prior to the pre-Ernesto starting time of 1200 UTC is not verified). 1 August in the NHC WBT (Table 1b), the forecast lo- TSs Joyce (AL10) and Oscar (AL15), which are also cation was good. In addition, the forecast track overlaid classified as EF4 storms (Fig. 2), began off the west coast the Ernesto track and the forecast ending point was of Africa, as did Leslie and Kirk, but both had short relatively close to the location for Ernesto, which ac- tracks. In both the week-1 forecast—see Fig. S1a in the counts for the very high LHV for the week-3 forecast. By supplemental material—and week-3 forecast of Joyce contrast, ensemble storm 24 in the week-4 forecast (Fig. (Fig. S1c), the only LHV deduction of 0.22 was for 5b) had a starting position almost 158 longitude east of a much longer forecast track. For the week-2 (Fig. S1b) Ernesto, and although the first half of the forecast track and week-4 (Fig. S1d) forecasts, the track was only was parallel to Ernesto’s track, the latter half was slightly longer than for TS Joyce, and the primary de- northwestward rather than westward. Thus, the LHV 5 duction was for the average distance and again a forecast 0.29 (Fig. 2) is marginal and would be in the below av- starting on the coast of Africa rather than downstream. erage category of Tsai et al. (2013). Because the week-1 forecast for TS Oscar (Fig. S2a in Tropical Storm Florence (AL06) began just 24 h after the supplemental material) was initiated at 0000 UTC 10 Ernesto, but Florence formed 308 longitude to the east at October (Table 1a) that was also the starting time for 11.58N, 17.88W(Table 1b), which is just off the coast of pre-Oscar, the forecast starting position was good and Africa. The week-1 forecast for Florence (Fig. S3a in the the large number of ensemble vortices contributed to an supplemental material) initiated at 0000 UTC 30 July is excellent forecast of a reversed-S track. Again the LHV noteworthy, as ensemble storm 3 had a perfect quality deduction of 0.22 was for the longer forecast track. The measure LHV 5 1.0 (Fig. 2), which indicates that all four week-2 forecast (Fig. S2b) had a similar shape as the metrics that make up LHV were within tolerance. En- track of Oscar. However, it started on forecast day 17 semble storm 21 in the week-3 forecast (Fig. S3c), which and was 3 days late (rather than the usual premature initiated at 0000 UTC 16 July, had a parallel track rel- start), so the forecast track was shifted to the northwest ative to the Florence track, but the forecast starting time and the LHV was only 0.55. Both the week-3 (Fig. S2c) of 0000 UTC 5 August was 2.5 days late. Thus, the LHV and week-4 (Fig. S2d) ensemble storm-track forecasts for this storm forecast was 0.69. The first half of the started on the coast of Africa rather than downstream, ensemble storm 27 track in the week-4 forecast (Fig. S3d)

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FIG.5.AsinFig. 3, but for Hurricane Ernesto (AL05) for only forecasts initiated during (a) week 3 on 12 Jul and (b) week 4 on 9 Jul.

that initiated at 0000 UTC 9 July was displaced to the Elsberry et al. (2011) also found reduced forecast- north of the Florence track, but the second half overlaid ability by the ECMWF ensemble for late-season storms the Florence track and ended at almost the correct time. in the western North Pacific that had poleward tracks Thus, the LHV of 0.82 was quite satisfactory. from the beginning. Uncertainty in the week-1 forecast It is somewhat unclear why the week-2 forecasts of starting position was due in part to the spread among en- Ernesto and Florence were unsuccessful. Elsberry et al. semble member vortices over northern South America as (2011) documented a similar case of a failure in week-1 well as over the Sea. This uncertainty in initial forecasting a pretyphoon, but their proposed explana- positions also was a factor in the week-2 forecast (Fig. 6b) tion of a large-scale circulation interaction with the initiated at 0000 UTC 8 October, in which ensemble multiple coexisting typhoons would not apply for Ernesto, storm 19 actually begins over land.1 Again, the forecast which developed 1.5 months after TS Debby. As indicated of a slow translation to the north-northwest and a very above, Ernesto and Florence formed within 24 h, and TS poor forecast ending point led to an LHV 5 0.54 (Fig. 2). Helene formed in a similar region and just 4.5 days after The week-3 forecast (Fig. 6c) initiated on 4 October Florence (Table 1b). Thus, some degradation in the large- also had an initial position near the coast and a poor scalecirculationintheweek-2forecastforFlorencemay forecast ending point. However, the two forecast track have occurred. crossings and a small average position difference over Due to its significance, Superstorm Sandy (AL18) is the still resulted in an LHV 5 0.70 (Fig. 2). selected for extended discussion as an EF3 storm in The week-4 forecast (Fig. 6d) initiated at 0000 UTC which the ECMWF 32-day ensemble week-4 forecast 27 September did have a number of ensemble vortices in failed to predict its formation. As has been well publi- the region where Sandy formed on 21 October. However, cized, the ECMWF deterministic (and 32-day ensemble; these vortices occurred at the end of the ECMWF 32-day not shown) model provided excellent forecasts of the ensemble forecast and did not last the 3 days required to track after Sandy became well organized in the northern be designated as an ensemble storm. In summary, the Caribbean Sea. In the week-1 forecast (Fig. 6a) initiated ECMWF 32-day ensemble provided some advance no- at 0000 UTC 18 October (3.5 days before the start of tice of the formation and early poleward track across the Sandy; see Table 1b), a large fraction of the ECMWF Caribbean Sea, but the later (and more critical) track ensemble member vortices were indicating a storm was not predicted in those forecasts initiated prior to the would exist, but a large spread in tracks existed. Be- formation of Sandy, which justifies its classification as cause of this track diversity, the weighted-mean vector just somewhat forecastable. motion was first westward and then slowly northward Hurricane Isaac (AL09) was an EF3 storm that also with almost no indication from the member vortices of made in the . As indicated in Table 1b, the continued poleward track over the western Atlan- tic. In addition to the deduction in the LHV for the poor forecast ending point, the average position dif- 1 Dr. J. Beven of the NHC (2013, personal communication) in- ference was also poor, which resulted in an LHV 5 0.45 dicates that the global models often develop vorticity centers (Fig. 2). downstream of the Guajira Peninsula of Colombia.

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FIG.6.AsinFig. 3, but for Hurricane Sandy (AL18) for forecasts initiated during (a) week 1 on 18 Oct, (b) week 2 on 8 Oct, (c) week 3 on 4 Oct, and (d) week 4 on 27 Sep. pre-Isaac started just prior to TS Joyce at 0600 UTC Although almost all of the path of ensemble storm 11 18 August near 128N, 278W. In the week-1 forecast in the week-2 forecast (Fig. S4b) closely matches that of (Fig. S4a available in the supplemental material) initiated Isaac, the early (4.75 day) start and displaced starting at 0000 UTC 13 August, ensemble storm 3 starts at 1200 position plus the poor forecast ending position leads to UTC 17 August, but is about 108 longitude to the west of an LHV 5 0.48. In the week-3 forecast (Fig. S4c), the pre-Isaac. The forecast westward translation of ensemble starting time and position were excellent, the forecast storm 3 is parallel to the track of Isaac until about 708W, track was quite good, but the forecast ending position was but its translation speed is much faster. When those en- again not good. The resulting quality measure LHV 5 semble member vortices leading to this rapid westward 0.75 does indicate useful information. In conjunction with motion terminate over , the weighted- the similar week-2 forecast track (Fig. S4b), the NHC mean vector motion then continues with the remaining forecaster might have an increased awareness of a po- vortex tracks that were now moving eastward after re- tential landfall in the United States that will start from curving, which leads to the strange eastward track near the African coast in 2 weeks’ time. prediction across southern and Hurricane Nadine (AL14) was one of the longest- (Fig. S4a). With the deductions for a poor forecast ending lasting Atlantic storms in recent times, and was a primary point, large average distance, and somewhat deficient target of the NASA HS3 field experiment. Because the forecast starting point, the quality measure LHV is only initial time of 0000 UTC 10 September for the week-1 0.25 (Fig. 2). Nevertheless, the early track to about 708W forecast (Fig. S5a available in the supplemental mate- might still be useful to a forecaster who would recognize rial) coincided with the starting time of pre-Nadine, many the doubtful reality for the later portion of the track. of the ECMWF ensemble member vortices had an

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FIG.7.AsinFig. 5, but for Hurricane Rafael (AL17) for forecasts initiated during (a) week 1 on 8 Oct and (b) week 2 on 1 Oct. excellent initial position and early track to the recurva- (AL08) had their origins near 208N and had short pole- ture point near 308N, 508W. However, the track of Na- ward tracks before recurving to the northeast. dine then became extremely complex and the ensemble The week-1 forecast (Fig. 7a) initiated at 0000 UTC forecast track spread became very large. Because of the 8 October had a number of ensemble member vortices later portion of the track, and that the forecast ending that started near the first position (8.38N, 37.58W; Table 1c) point was very different from that of Nadine, the week-1 in the NHC WBT for Raphael. Whereas the WBT has LHV was only 0.64 (Fig. 2). In both the week-2 forecast gaps [note that the North Atlantic hurricane database (Fig. S5b) initiated at 0000 UTC 30 August and the week- (HURDAT) only starts Raphael at 35 kt (1 kt 5 2 3 forecast (Fig. S5c) initiated at 0000 UTC 23 August, the 0.51 m s 1) at 14.78N, 62.78W at 1800 UTC 12 October], forecast starting position and early track were somewhat the ensemble storm 2 track is continuous and parallel satisfactory, but the more westward track led to a forecast to the WBT until 608W. Whereas ensemble storm 2 is recurvature point that was about 158 longitude too far to forecast to sharply turn northward, this turn occurs the west. Consequently, the LHV was only 0.48 (Fig. 2) about 168 longitude to the west of Raphael’s turn. Rather for the week-2 and -3 forecasts. than continuing to move poleward, ensemble storm 2 is The inability of the ECMWF ensemble to forecast the predictedtostallat;258N,andsothefulldeduction formations and tracks in week 4 for Hurricanes Sandy, (0.22) for a poor forecast ending point is applied. In Isaac, and Nadine indicates that these are only some- conjunction with a deduction for the average distance, what forecastable EF3 storms. The late-season Sandy an LHV 5 0.63 (Fig. 2) is in the lower ranges of an above was strongly impacted by interactions with deep mid- average quality. While the track of ensemble storm 7 latitude troughs. Similarly, the extremely sharp re- in the week-2 forecast (Fig. 7b) initiated at 0000 UTC curvature and subsequent complex track indicate that 10 October has a similar shape to Raphael’s track, the Nadine was strongly impacted by midlatitude circulations, forecast starting point is far to the east at the coast of and these interactions are not forecastable on 4-week time Africa. While one member vortex track extends to scales. In the case of Isaac, it is only the LHV deductions 408N, the ensemble storm track 7 ends at 238N, which for the forecast starting and ending points that make the contributes to the low LHV of 0.25 (Fig. 2). week-2 and -3 forecasts appear less desirable. No expla- The week-1 forecast track of ensemble storm 4 (Fig. 8a) nation for the absence of a week-4 forecast of Isaac is was present in the initial conditions at 0000 UTC available at this time. 13 August; this forecast did not have the correct location for the formation of pre-Gordon at 26.18N, 548W(Table 1c) 3) LIMITED FORECASTABLE EF2 STORMS 2.25 days later. Similarly, the sharp recurvature was Two hurricanes and one tropical storm were classified forecast well. Because the forecast starting and ending as EF2 storms (Table 1c), as the ECMWF 32-day en- points were somewhat off, what many forecasters might semble was able to successfully forecast these storms in consider to be an excellent forecast is only rated at only the first two weeks (Fig. 2). Hurricane Rafael (AL17) LHV 5 0.55. In the week-2 forecast (Fig. 8b) initiated was a late season storm that seemed to have its origins at at 0000 UTC 6 August, ensemble storm 11 is predicted low latitudes, but moved poleward after reaching tropical to start at 0000 UTC 11 August, which is 4 days before storm stage at 14.78N, 62.78W at 1800 UTC 12 October pre-Gordon actually started. Although ensemble storm (Table 1c). Both TS Debby (AL4) and Hurricane Gordon 11 is made up of vortices that begin over a range of

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FIG.8.AsinFig. 5, but for Hurricane Gordon (AL08) for forecasts initiated during (a) week 1 on 13 Aug and (b) week 2 on 6 Aug. initial positions and have a variety of track types, the has almost the exact path of Debby. Had the second set shape of the forecast track is quite similar to that of of tracks been considered separately, that ensemble Gordon. Indeed, a shift of the forecast track about 68 storm would have been rated highly. However, the eval- latitude and longitude to the south and east would have uation of ensemble storm 4 is only rated at LHV 5 0.25 resulted in a near-perfect track. Consequently, the LHV due to its unrepresentative forecast starting and ending of 0.55 indicates a useful forecast of Gordon 2 weeks in points and poor average distance. advance. However, a storm such as Gordon beginning These three EF2 storms (Table 1c) represent the at 258N involves both tropical and midlatitude influ- limited forecastability of the ECMWF ensemble for ences, which are not forecastable beyond 2 weeks (EF2) somewhat different reasons. The low-latitude origin and in this case. poleward track of the late season Rafael has some of the Pre–TS Debby (AL04) began at 1200 UTC 22 June at same difficulties as Sandy due to interactions with a deep 22.68N, 88.18W(Table 1c) just north of the Yucatan midlatitude trough. The importance of interaction with Peninsula (Fig. S6 available in the supplemental material) the midlatitudes for the formation track and intensifi- and moved slowly northward before turning eastward cation of Hurricane Gordon can account for its limited and moving eastward across northern Florida. Because forecastability. Finally, Debby’s formation adjacent to the week-1 forecast was initiated just 36 h prior to the the Yucatan Peninsula and its short poleward track be- start of pre-Debby, a large number of member vortices fore recurving may indicate limited predictability due were predicting the system, but with at least two track to land and ocean interaction as well as midlatitude types: one group with a track quite similar to Debby’s interaction. track and another group with a larger number of vortices 4) NOT FORECASTABLE EF0 STORMS predicting a slow northwestward or westward motion. Thus, the weighted-mean vector motion was a slow The most surprising result of this evaluation of the northwestward track toward the Gulf of Mexico coast 2012 Atlantic season was that the ECMWF 32-day en- near the border of Texas and before turning semble forecasts on Mondays or Thursdays did not pre- northeastward. Given the slow motion of Debby and the dict even in week 1 the formations and tracks of five TCs early forecast track, the average distance plus the excel- (Table 1d). That is, the LHVs of three tropical storms lent forecast starting position still led to an LHV 5 0.48. [Helene (AL07), Patty (AL16), and Tony (AL19)] and Ensemble storm 4 in the week-2 forecast (Fig. S6b) two hurricanes [Chris (AL03) and Michael (AL13)] initiated at 0000 UTC 14 June is made up of two types of were equal to zero for all four weeks (Fig. 2). The tracks vortex tracks. The first set of tracks again begins over of these five storms from the NHC WBT are shown land in Colombia and moves northwestward along the in Fig. 9. Central America to the Yucatan Peninsula and the east In the case of TS Helene (AL07), the first entry in the coast of Mexico. This first set of tracks is then joined by WBT was at 0000 UTC 7 August at 11.78N, 26.68W five tracks that are more representative of the formation (Table 1d). This was a marginal tropical depression or and track of Debby. Indeed, one vortex track forecast even a 20-kt tropical disturbance along its long path across

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FIG. 9. WBTs for five Atlantic TSs and hurricanes in the not-forecastable EF0 category (see names and characteristics in Table 1d). the tropical Atlantic, Caribbean Sea, Central America, poleward (Fig. 9). Since Chris only attained TS status at and the Yucatan Peninsula. It was only for a 12-h period 1800 UTC 19 June when it was at 39.58N, 58.08W and over the Bay of that Helene was classified as moving eastward, baroclinic processes must have been a tropical storm. It was during this period that the a contributing factor to its intensification. While Hurricane Monday or Thursday ECMWF 32-day ensemble fore- Michael (AL13) was a midseason storm starting 1800 UTC casts failed to predict Helene. Pre–TS Patty (AL16) 2September(Table 1d), it was not a typical African started at 0000 UTC 11 October near 25.88N, 72.78Wat wave–type development. Rather, its starting position the southern end of a long, north-oriented cloud band was at 26.48N, 40.18W and it had a meandering poleward from the midlatitudes. Patty remained almost stationary track (Fig. 9).Giventhisstartinglocationandtheimplied (Fig. 9) and reached maximum intensity 24 h after weak steering, the formation and intensification of this formation. Patty had a compact structure that ap- storm was likely strongly influenced by baroclinic pro- peared to evolve from mesoscale processes, which (in cesses, which may account for Michael not being forecast addition to its short life) may explain the failure of a in advance. Monday or a Thursday ECMWF ensemble to predict its The tentative conclusion from four of these five EF0 formation and track. Pre–TS Tony was a late season storms that began in the central Atlantic is that strong storm (21 October; Table 1d) that began relatively far midlatitude influences make their formation locations north (19.88N, 50.08W; Fig. 9) and initially moved pole- and timing not forecastable by the twice-weekly ECMWF ward and then recurved quickly. Tony (AL19) achieved 32-day ensemble. Diagnostic studies performed to dem- tropical storm intensity at 26.48N, 49.88W, as it was moving onstrate the types and magnitudes of the interactions of into the midlatitude westerlies. As the intensification of the midlatitude circulations with the pre-TCs in the Tony was likely influenced by mixed tropical and baro- ECMWF ensemble fields are beyond the scope of this clinic processes, and given its short period as a tropical study. storm, it is perhaps not that surprising that Tony was not predicted as a week-1 storm in either a Monday or 4. Characteristic false alarm tracks Thursday ECMWF 32-day ensemble forecast. The more serious failures were the two hurricanes that As in Tsai et al. (2013), the false alarm tracks are the ECMWF ensemble failed to predict in advance of defined as those ensemble storm tracks that could not be their formation time. Hurricane Chris (AL03) was an matched with any of the NHC WBTs within the speci- early season storm that started at 0000 UTC 17 June fied tolerances. A substantial fraction (35%) of all false at 28.88N, 68.88W(Table 1d) and immediately moved alarms occurs during the first week and often are in the

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FIG. 10. Four types and numbers (see inset) of false alarm ensemble storm tracks starting during the first week of the twice-weekly ECMWF 32-day forecasts between 31 May and 17 Dec 2012 (total of 58 forecasts). The starting location is indicated by a circle. initial conditions, which was also the case in the western a presumption that such a track forecast is highly likely North Pacific (Tsai et al. 2013). Of course, the NHC fore- to be a false alarm. caster would be aware that no potential tropical cyclone The type 1 (22 of 107) false alarm tracks in Fig. 10 that seedlings existed in the area by T 1 15 h when the begin near the west coast of Africa will not be easily ECMWF ensemble storms tracks are received and recognizable—especially those that have long, recurving processed. Since the ECMWF ensemble tends to begin tracks that resemble climatological tracks in the Atlantic. the storms 2–3 days (or more) before NHC begins its The shorter forecast tracks might be suspect because WBT, this may indicate the need for a modified vortex limited time would be available to form a hurricane, but tracker definition. Once the ECMWF ensemble begins some real tropical storms do have short tracks. Similarly, to predict a false alarm storm, it tends to sustain that the tracks that begin inland over Africa might be sus- storm in the next 32-day forecast. So if an ensemble pect, but the ECMWF ensemble tends to begin storms storm in the Monday forecast that may be associated 2–3 days early and may be detecting vigorous African with the same ensemble storm in last Thursday’s fore- waves that do have a chance to develop. While the Af- cast has still not shown signs of developing after 4.5 days, rican wave–type tracks were shown in section 3 to be the it is likely a false storm. more forecastable, those ensemble storms that verified A more straightforward detection of Atlantic false well did not include all 51 ensemble members. Indeed, alarm storms is simply from the geographical distribu- the fraction of the 51 ensemble members may be thought tion of their formation locations and tracks (Fig. 10). of as a raw probability of occurrence. Especially for the Type 2 false alarms that predominantly begin inland type 1 false alarms, their numbers compared to the hits over the United States or near the Gulf of Mexico coast reflect some measure of the reliability of the ECMWF are easily recognized. The most frequent (43 of 107) ensemble for African wave–type systems. Of course, type 3 false alarm tracks begin near the northern coast of a multiseason sample of Atlantic storms would be re- South America and along the coast of Central America, quired to statistically establish the reliability. or move inland over these regions. The least frequent The four types of false alarm tracks in week 1 continue (10 of 107) type 4 false alarms begin at or north of 208N to be present in weeks 2–4 with totals of 70, 72, and 54, and mainly have northward tracks. While these may be respectively (Fig. S7 available in the supplemental ma- representative of baroclinically forced systems, the re- terial). The numbers of type 2 (over the United States) sult in section 3b(4) that the ECMWF ensemble did not and type 3 (South America and Central America) false accurately predict the location and subsequent track of alarms are considerably smaller in week 2 (Fig. S7b) any such storms even 1 week in advance would lead to than in week 1, but the tracks are similar. Note that the

Unauthenticated | Downloaded 10/03/21 09:33 AM UTC APRIL 2014 E L S B E R R Y E T A L . 285 number of type 1 (African wave) false alarms in week 2 western North Pacific, this was not the case during the is almost identical to that of week 1, with 21 such false 2012 Atlantic season. Four categories of Atlantic storms alarms over the 58 forecasts in this sample. The type 2 have been tentatively classified as to whether they were and 3 false alarm frequencies in week 3 (Fig. S7c) are the forecastable on the extended-range time scale: 1) EF4 same as in week 2, albeit with shorter tracks. No de- storms in all four weeks in advance of the starting date in crease in the number of type 1 false alarms during week the NHC WBT, 2) EF3 storms in three of the four 3 is noted. While the overall number of false alarms is weeks, 3) EF2 storms limited to the first two weeks, and smaller during week 4 (Fig. S7d), only a small fraction of 4) EF0 storms that were not forecast 1–7 days in advance the reduction is attributable to the type 1 (African wave) of any Monday or Thursday ECMWF 32-day ensemble false alarms. In summary, this consistent pattern of false forecast. Two hurricanes [Kirk (AL11) and Leslie alarm tracks that may be easily recognized and removed (AL12)] and two tropical storms [Joyce (AL10) and (except for type 1) will make the ECMWF ensemble Oscar (AL15)] were successfully forecast in all four storm forecasts more acceptable to forecasters. weeks. The common characteristics of these EF4 storms were that they generally occurred in midseason from African wave type formations and the hurricanes had 5. Summary and future work long recurving tracks that may have been more con- This evaluation of the performance of the ECMWF trolled by the subtropical high than by an interaction 32-day ensemble predictions of Atlantic TC events (for- with an active midlatitude trough. In many of the Afri- mation and tracks) was made possible by the ECMWF can wave–type formations, the ECMWF ensemble has kindly providing their twice-weekly forecasts in near– a starting point at the coast rather than downstream, real time in support of extended-range mission planning which resulted in an LHV deduction of 0.22. Especially for the NASA HS3 field experiment. Although the for the TSs with shorter tracks, the ECMWF ensemble ECMWF also provided daily 15-day ensemble forecasts tracks were too long, which also resulted in an LHV that could be used for medium-range forecasting of ex- deduction. isting TCs, the focus in this evaluation was on the The somewhat successful EF3 events of Hurricane number of weeks in advance of the beginning of NHC Ernesto (AL05) and TS Florence (AL06) were not storms AL 03–AL 19 that the ECMWF 32-day ensemble forecast in week 2, which was surprising since they had predictions were successful in forecasting their existence. African wave origins with westward tracks. The expla- A single season is not adequate to make statements about nation for the week-2 failure may be related to these two the predictability of Atlantic TCs. Rather, this evalua- storms and TS Helene (AL07) having started in the tion is simply a description of the forecastability of the same area within 7 days. That is, multiple interactions AL 03–AL 19 tropical storms and hurricanes with the among the three storms in the ECMWF ensemble pre- ECMWF 32-day ensemble using essentially the same diction during that week may have led to degradations in technique as in Elsberry et al. (2010, 2011) to form en- the forecasts of the large-scale circulation over the At- semble storm tracks that could be verified with the ob- lantic during the second week such that formations of jective track analog technique of Tsai et al. (2013). AL05 and AL06 were not forecast in that week. Strong An important distinction is made here between fore- interactions with midlatitude circulations affecting the casts of TC formation (or genesis) alone versus both the formations and tracks of the late-season Sandy (AL18) formation and the subsequent track, which are also and the long-lasting Hurricane Nadine (AL14) are represented in the LHV quality measure by the average considered to be the reason limiting their forecastability track difference, shortest track difference, and the fore- to weeks 1–3. The tendency for the midseason TCs to cast ending distance. Given a multiyear sample of the recurve over the Atlantic in nature and in the ECMWF formation locations from these twice-weekly 32-day ensemble may be the explanation for the lack of a match ensemble forecasts, a comparison could be made with in week 4 for the long westward track of Hurricane the climatological distribution of formations. Similarly, Isaac. a multiyear sample of tracks could be compared with the Strong interactions with the midlatitude circulation climatological track distribution. At this time, only the during and following the formation at low latitudes of 2012 Atlantic season is available and such comparisons the late-season Hurricane Rafael (AL17) are consid- with climatology are not possible. ered to be the factor limiting forecastability beyond Whereas Elsberry et al. (2010, 2011) and Tsai et al. 2 weeks (EF2). Similarly, the strong midlatitude influ- (2013) had found that the ECMWF 32-day ensemble ence on the formation and track in the subtropics of could predict out to 4 weeks most of the typhoons, tropical Hurricane Gordon (AL08) was a factor in its limited storms, and even some tropical depressions in the forecastability. TS Debby (AL04) is a special case as it

Unauthenticated | Downloaded 10/03/21 09:33 AM UTC 286 WEATHER AND FORECASTING VOLUME 29 formed early in the season in the Gulf of Mexico near mid-Atlantic are not that important because they posed no the Yucatan Peninsula coast, and did not have a pre- threat to the United States or Caribbean islands. However, cursor African wave that could be forecast for weeks in they included two hurricanes that were a maritime advance. Thus, the timing and location of the formation threat. Furthermore, these systems interact with and of Debby may have involved mesoscale processes and modify the large-scale environment (consider the long- land–ocean processes that are not forecastable beyond lasting Hurricane Nadine), which may affect the fore- afewdays. castability of a subsequent TC. A larger sample of Atlantic Among the five not forecastable EF0 storms, TS TC events is required to explore the source(s) of the EF0 Helene (AL07) near the east coast of Mexico and TS failures. Tony (AL19) in the eastern Atlantic were weak tropical This study has demonstrated that the ensemble storm storms for a short period of their lifetime. Similarly, TS generation techniques of Elsberry et al. (2010, 2011) and Patty developed for a short time in a limited region of the track analog verification technique of Tsai et al. favorable conditions. Elsberry et al. (2010, 2011) rea- (2013) can be applied for the ECMWF 32-day ensemble soned that the success in extended-range prediction for forecasts in the Atlantic. One of the advantages of this typhoons could be attributed to favorable environmen- technique relative to the more common studies of TC tal conditions that had to exist not only at the time of activity is the capability to associate the successes (hits) formation, but also over a large area to allow time for and failures (misses) to specific TC events. A second intensification to a typhoon, and that such large-scale advantage is the capability to study the false alarms circulation conditions were forecastable in the western [defined as in Tsai et al. (2013) as all ensemble storms North Pacific. By contrast, favorable environmental that could not be matched with an official track]. Un- conditions for formation plus intensification to hurri- derstanding these false alarms will be an important canes did not exist for these three tropical storms. For second step in developing an operational version of the the early season Hurricane Chris (AL03) that formed so technique for the extended-range prediction of TC far north, the ECMWF vortex tracker definition that events in the Atlantic. distinguishes between tropical cyclones versus extra- As suggested by a reviewer, it would be helpful to the tropical cyclones may have been a contributing factor. forecasters to know how the number of ensemble mem- However, it seems more likely that the mixed-phase bers that are forecasting a formation is related to an physics of baroclinic and convective processes may not actual formation. That is, if 40 of the members are pre- be well predicted in the ECMWF ensemble model, at dicting a formation versus only 20 members, can the least beyond a few days. The failure in forecasting the forecaster be twice as confident that a formation will formation in the subtropics of the long-lasting Hurricane occur? While it is generally true that the larger the Michael (AL13) may well be due to the treatment of number of ensemble members the more reliable are the baroclinic and convective processes in the ECMWF ECMWF ensemble forecasts, there have been notable ensemble. While the week-1 forecast of Hurricane exceptions. Additional seasons need to be examined to Gordon, which has some similarities to Hurricane Michael, document the reliability. A systematic study is also was successful, this success may have been due to an needed of the contributions of the twice-daily ECMWF African wave precursor (Fig. 8a). 15-ensemble forecasts in adding confidence in week 2 to Although based on a small sample, this evaluation has those events that were predicted in weeks 3 and 4 of the demonstrated a more limited extended-range forecast- prior ECMWF 32-day forecasts. That is, does week-to- ability of TC events in the Atlantic than in the western week continuity in events lead to higher and higher North Pacific (Elsberry et al. 2010, 2011; Tsai et al. 2013). confidence? It seems reasonable that the most forecastable TC events are those long-lasting that occur in midseason Acknowledgments. Dr. Hsiao-Chung Tsai is a Na- from well-defined African waves. The limited forecast- tional Research Council postdoctoral research associ- ability beyond weeks 3 (EF3) or 2 (EF2) may be due to ate. He and coauthors Russell Elsberry and Mary Jordan less strong tropical forcing (e.g., Madden–Julian oscil- are supported with funding from the Marine Meteorol- lation), the absence of a large-scale monsoonal influence ogy section of the Office of Naval Research. Dr. Frederic as in the western North Pacific, or the larger midlatitude Vitart of the ECMWF has been instrumental in the impacts on TC formation, intensification, and tracks in provision of real-time access to the ensemble predictions. the Atlantic. A larger sample of Atlantic TC forecasts Dr. Jack Beven of NHC provided some insights as to on the extended-range time scale will be required to physical processes involved in Hurricane Rafael and attribute these causes of limited forecastability. It might Sandy. Mrs. Penny Jones provided excellent manuscript be said that the four EF0 TC events that occurred in the preparation support.

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APPENDIX measures for the ECMWF ensemble storm forecasts during the 2009 season (Elsberry et al. 2011). The largest A notable consideration for this evaluation of the weighting factor of 0.3 is assigned to the average dis- 2012 ECMWF ensemble forecasts relative to the earlier tance to emphasize the similarities with the overall ob- studies has been an improvement in the treatment of served track. The next largest weighting factor (0.25) is convection in the tropics. In addition, higher horizontal given to the shortest distance, which in combination with resolution of T639 (32 km) for the first 10 days of the the average distance will favor the ensemble storm that forecast and T319 (65 km) from day 10 onward was most nearly overlaps with the NHC storm track. Finally, implemented in 2010 and an ensemble-based data as- weighting factors of 0.23 and 0.22 are used to represent similation technique was introduced in 2010. Lang et al. the differences in the formation and ending positions. (2012) had demonstrated the impacts of various per- Five quality measures of excellent, above average, turbation methods on TC track predictions. Thus, it good, below average, and poor are then assigned as might be expected that the 2012 track forecasts in the a linear function with an LHV interval of 0.2, with an Atlantic have been improved compared to earlier years. LHV , 0.2 not considered to be a real match. The same A major advancement prior to the 2012 season was membership functions and weighting factors as in Tsai that the ECMWF 32-day ensemble was available twice et al. (2013) are used with the following exceptions: a week (Mondays and Thursdays from 0000 UTC initial (i) The metric of the distance at the ending time is only conditions) rather than only once a week (Thursday). As applied to the end of the 32-day ensemble forecast. If an in Elsberry et al. (2010, 2011), the focus is on both the ensemble storm track ends at 32 days but the NHC formation and the track of the Atlantic tropical storms storm track extends beyond that date, the ending dis- and hurricanes. It is well known that the ‘‘mother’’ tance is calculated at day 32. (ii) An additional screening ECMWF deterministic model is quite skillful in pre- step for the distance at formation time is applied, which dicting the TC track after the TC warning center has requires the difference be less than 2220 km. named the storm, and this is also true of the ECMWF ensemble when as many as 51 members may be repre- REFERENCES senting the storm. Similar to the experience in the western North Pacific, the ECMWF ensemble predicts Belanger, J. I., J. A. Curry, and P. J. Webster, 2010: Predictability of vortices in the Atlantic that begin before the NHC be- North Atlantic tropical cyclones on intraseasonal time scales. Mon. Wea. 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