Hi all,

We discussed the UK cold and as well as the then ongoing strong nor’easter in parts of the northeastern U.S. Eric Bunker and Tomer Burg assisted with map discussion. Links used in Friday map discussion can be found here: http://www.atmos.albany.edu/mapdisco/20180302/.

1. Background:

The attached 2-m temperature anomaly map for 25 Feb 2018, courtesy of Reanalyzer (http://cci-reanalyzer.org/), provides a very nice segue between the last three Friday map discussions and the 2 March Friday map discussion. The signal of a strong negative NAO is readily apparent in the form of highly anomalous warmth over the Arctic (e.g., https://mashable.com/2018/02/26/arctic-heat-wave-north- pole-february-sea- ice/#NZQuE3hmRkqK and https://www.theatlantic.com/science/archive/2018/02/its- 54-degrees-warmer-than-normal-in-the-arctic/554303/ , anomalous cold over much of the northern half of Eurasia, and the relentless westward advance of the anomalously cold (aka Siberian Express or “Beast from the East" in the British Tabloids) toward the UK. "Conventional wisdom” (usually defined as thinking about anything with a mean of one and a standard deviation of zero) is that negative NAO large-scale circulation patterns are usually associated with with well below normal temperatures in the central and eastern CONUS, something that has yet to happen even though the epic western Atlantic upper-level ridge in place in the latter part of February has mostly collapsed (see the 2-m temperature anomaly for 4 March 2018 from Climate Reanalyzer at http://cci- reanalyzer.org/wx/DailySummary/#t2anom). A WeatherBell link to an ECMWF NAO time series (analysis and forecast) can be found here: http://models.weatherbell.com/oscillation/ecmwf_nao_bias.png. Noteworthy, is the negative value of the NAO going off the scale below -5 around 1 March.

2. UK cold and snow:

Alicia Bentley’s suite of maps for the North Atlantic (http://www.atmos.albany.edu/student/abentley/realtime/atlantic.php) and Europe (http://www.atmos.albany.edu/student/abentley/realtime/europe.php) nicely tell the story of the very recent severe cold spell in the UK. The UK media, no shrinking violets when it comes to using over-the-top hyperbole like their US media counterparts, termed the severe cold spell the “Beast from the East” with some accuracy. The lowest Central England maximum temperature ever observed for the month of March occurred on 2 March with a reading of -0.8 C in records that go back to 1874 as posted by Tim Hewson (http://www.atmos.albany.edu/mapdisco/20180302/images/CET_series_M arches.png).

We briefly reviewed the structure and evolution of the large-scale flow pattern since the occurrence of a strong sudden stratospheric warm event induced a rearrangement of the NH circulation that culminated in the establishment of a strong negative phase of the North Atlantic Oscillation (NAO) as discussed in the 9 and 23 March map discussion summaries (http://www.atmos.albany.edu/mapdisco/). These summaries provide context for the ocean effect that adversely impacted parts of eastern England and Scotland a few days ago. In an earlier post to Tim Hewson and map, I opined that a combination of synoptic-scale and mesoscale processes likely contributed to the UK snowfall. Alicia Bentley’s maps show that the UK snowfall occurred subsequent to the establishment of a synoptic pattern consistent with a pronounced negative phase of the NAO that enabled Siberian air to reach the UK (see above). At 1200 UTC 28 Feb, an area of 700-hPa temperatures as low as -28 C approached the northeast coast of the UK from the North Sea where SSTs were in the 4–6 C range. As the surface-to-700-hPa temperature difference approached and then exceeded the dry adiabatic lapse rate in parts of the eastern UK and over the North Sea, multiple open cellular bands, indicative of surface-based instability, became evident in satellite imagery over the North Sea. These cloud bands spread westward across the central and northern UK with resulting ocean effect snow squalls occurring well inland.

The potential temperature on the DT during this period was between 276–282 K in the core of the PV anomaly centered over the UK. Alicia Bentley’s standardized anomaly loops illustrate the rarity of the circulation pattern over Europe as the calendar turned to March based on -5 to -6 SLP anomalies to the west of the Iberian Peninsula, +2 to +3 sigma SLP anomalies from Greenland to Scandinavia, and -3 to -5 sigma 850-hPa temperature anomalies in the cold air mass impacting the UK (http://www.atmos.albany.edu/student/abentley/realtime/europe_anom.php). Once a very cold air mass is in place over the UK, subsequent warm-air advection can contribute to a broader coverage of snowfall analogous to how cold-air outbreaks in the Pacific Northwest can end, sometimes with a big snowstorm, as low-level warm- air advection begins with the return of milder oceanic air on westerly flow. For example, subsequent warm-air advection that overspread the southeastern part of the UK (warm-air advection going from a 522 dam thickness to a 504 dam thickness from southeast to northwest is not something you see every day in the UK) (http://www.atmos.albany.edu/student/abentley/realtime/europe.php).

Second-year graduate student Massey Bartolini then took over map discussion to explore interesting mesoscale aspects of the oceanic effect snow that impacted parts of the eastern and northern UK to include a comparison of ocean-effect snow in the eastern UK with lake-effect snow downhill of the Great Lakes over the U.S. A lively discussion of this very interesting UK event ensued. Massey will post a summary pdf of his presentation to map at a later time.

An important previously discussed science issue that commands attention is that given the occurrence of a sudden stratospheric warming (SSW) event sufficiently strong enough to split the polar vortex, what dynamical and thermodynamical processes govern where the split parts of the polar vortex will reach higher midlatitudes and why? In the present SSW event, the two pieces of the split polar vortex (and their associated PV anomalies) reached northern Eurasia and western Canada, respectively. The split polar vortex part that reached northern Eurasia was associated with rising sea level pressures that eventually drove a bitter Siberian air mass westward toward the UK. The split polar vortex part that reached western Canada reinforced cold conditions in western North America, especially over the Pacific Northwest. In between, an epic upper-level ridge over the eastern CONUS and western Atlantic resulted in a number of locations in the southeastern CONUS having their warmest February on record.

A science question arises as to whether there are preferred longitudes where the PV anomalies resulting from split polar vortices will track toward lower latitudes following SSW events and, if so, what dynamical and thermodynamical processes govern these longitudes and why? Can we hypothesize that strongly retracted North Pacific jet (NPJ) that has defined much of and early to date has played a role in favoring the western Canada track of one of the split polar vortex tracks? My somewhat muddled thinking here is based on the idea that a persistent retracted NPJ favors persistent jet-exit region cyclogenesis events west of the Dateline with subsequent downstream high-latitude ridge building toward northeastern Russia, the Bering Sea, and western Alaska. Under these conditions when the atmosphere loses its “berings” (so to speak) the aforementioned will favor downstream roughing over western North America, prompting my question as to whether there could be linkage between a persistent retracted NPJ and the preferred longitudinal location of a split polar vortex component given the occurrence of a major SSW event?

3. Nor’easter of 2 March 2018:

A succinct summary of the impressive oceanic of 2 March 2018 is that the predictability horizon was relatively long for the occurrence of a high- impact synoptic-scale event and relatively short for important mesoscale -related details such as distribution of significant snow [A regional snowfall map generated by NWS Albany (thank you, Tom Wasula) can be found here: https://www.weather.gov/aly/2Mar18Noreaster]. This forecast challenge often occurs with transition when the question becomes how to best distinguish between which areas will receive significant snow versus and how much will occur. In the synopsis of Friday map discussion for 23 Feb, I wrote "An important key to how the forecast is going to play out over the eastern CONUS toward next weekend, based on the weather maps for 1200 UTC 25 Feb, is likely to be the subsequent interactions between trough #1 (currently crossing northern Japan), trough #2 (currently situated near 55 N and just west of the Dateline), and trough #3 (currently approaching Vancouver Island)." As is evident from Alicia Bentley’s maps (http://www.atmos.albany.edu/student/abentley/realtime.html) this large-scale three-way trough interaction scenario mostly played out as envisioned and culminated in a high-impact nor’easter. Forecasters were talking about the high likelihood for a wind-driven high-impact potential coastal flooding event from coastal new Jersey northeastward to New England days in advance of the storm even as significant uncertainties remained about precipitation type and amount.

The devil was in the forecast details, particularly with regard to the areal distribution of rain versus snow especially in regions of complex terrain. Very warm weather in the second half of February and the disappearance of frozen ground likely made it difficult for people to envision fully that a significant snowstorm was possible in parts of the interior Northeast even at lower elevations. The global models provided generally good guidance as to the overall location and timing of a major coastal as evidenced by a d(ensemble)/dt analysis of the ECMWF EPS mean SLP and surface cyclone positions for the 144-h to 24-h forecasts all verifying at 1200 UTC 2 March (http://www.atmos.albany.edu/mapdisco/20180302/images/EPS_forecast.pdf). Between 144-h and 24-h, the ensemble mean SLP became better defined and shifted ESE away from just east of the Delmarva Peninsula to a location SE of Nantucket. The cyclonic circulation envelope of the offshore cyclone extended from northern Maine to southern Florida even in the 144-h EPS forecast, indicative of the likelihood of a widespread significant wind event.

The distribution of the locations of the 50 individual ensemble member cyclone centers (plus the control forecast) exhibited significant spread, even as close in time as the 24 h forecast. These cyclone positions stretch from NW to SE in the 24- h forecast, indicative of continuing forecast uncertainties with regard to the strength and duration of high coastal wind speeds, the location of a rain-snow line back from the coast, the location of the corridor of heaviest precipitation, and the timing and tracks of possible mesoscale circulation features embedded within the overall storm cyclonic circulation envelope. The mesoscale trough that extends WSW from the offshore cyclone to the Delmarva Peninsula in the 24 h forecast is also an important source of forecast uncertainty as to its possible role in the expected sensible weather distribution on the back side of the cyclone.

A comparison of the Albany sounding for 1200 UTC 2 March 2018 (http://www.atmos.albany.edu/mapdisco/20180302/images/alb_sounding.pdf) as produced by the SPC with a corresponding 24-h forecast sounding for Albany derived from the deterministic GFS run from 1200 UTC 1 February (attached) is informative. Although deep low-level warm-air advection is present in both soundings and the wind profiles match reasonable well, there is a significant discrepancy between the forecast and observed temperature and dew point profiles in the boundary layer below 900 hPa. The GFS forecast planetary boundary layer (PBL) is too warm, too dry, and too well-mixed in comparison to reality. This erroneous GFS forecast PBL structure no doubt contributed to the model’s difficulty in forecasting snow versus rain in lower elevation locations as well as the erroneously high GFS-MPOS forecast maximum temperature (47 F vs, 34 F observed) for Albany on 2 March.

On the basis of an admittedly cursory analysis it appears that the GFS was unable to simulate properly the tendency for the atmosphere to produce an isothermal lapse rate along the 0 C isotherm due to evaporative cooling from precipitation falling into initially dry air and sensible cooling from melting snow in the PBL in the presence of vigorous dynamical lift. I have seen this movie before, namely in the infamous 4 October 1987 early season snowstorm in the Northeast in which the then operational NMC MRF and NGM models were clueless. Fred Sanders and wrote a paper about this forecast bust (http://www.atmos.albany.edu/mapdisco/20180302/images/Bosart_and_Sanders(19 91).pdf). We demonstrated how both evaporative and sensible cooling acted to stabilizing the PBL by creating an isothermal temperature (0 C) profile that enabled accumulating snow to fall on interior valley floors where dynamically driven lift and precipitation intensity were highest.

An important science question is that given knowledge of the predictability horizon for a synoptic-scale cyclogenesis event, what dynamical and thermodynamical processes govern how much shorter the mesoscale predictability horizon will be for the same event? For example, does the time of the year, whether the cyclogenesis event is associated with cyclonic or anticyclonic wave breaking, the configuration of the large-scale flow, and the extent of any tropical-midlatitude and midlatitude-polar interactions influence how much shorter the mesoscale predictability horizon will be compared to the synoptic-scale predictability horizon.

4. High winds in the Virginia-Maryland area on 2 March 2018:

Parts of the Middle Atlantic region, and notably around the DC area, experienced maximum northwesterly wind gusts between 50–65 kt on 2 March. These observed high wind speeds occurred in the western periphery of the strong offshore cyclone. Appended metar observations from Dulles Airport (IAD) between 0752–2352 UTC 2 March are illustrative of these strong winds (see reported 62 kt peak wind at 1652 UTC). It’s difficult to explain the existence of these strong winds on the basis of the SLP gradient alone as evidenced from the NCEP-WPC high-resolution surface analysis archive with plotted observations (http://www.wpc.ncep.noaa.gov/html/sfc- zoom.php.; see maps for 1200/1500/1800/2100 UTC). A GOES-16 IR (Channel 14) loop for 1200–2100 UTC 2 March (source: NCAR-RAL) shows bands of high, cold cloud tops in the outer circulation of the strong oceanic cyclone working their way southwestward toward the DC area (http://weather.rap.ucar.edu/satellite/displaySat.php?region=US&itype=03&size=lar ge&endDate=20180302&endTime=21&duration=9). GOES-16 low-level (http://weather.rap.ucar.edu/satellite/displaySat.php?region=US&itype=06&size=lar ge&endDate=20180302&endTime=21&duration=9) and mid-level (http://weather.rap.ucar.edu/satellite/displaySat.php?region=US&itype=04&size=lar ge&endDate=20180302&endTime=21&duration=9) water vapor imagery indicate that the observed very strong surface winds were occurring beneath a moistening low- and mid-level atmosphere.

The 1200 UTC 2 March SPC sounding for IAD (Sterling, VA), adjacent to Dulles Airport, offers a possible clue to the origin of these strong wind gusts (http://www.atmos.albany.edu/mapdisco/20180302/images/KIAD_sounding.pdf). A reasonably well mixed PBL between the surface and 850 hPa is capped by a moist layer that extends upwards to ~450 hPa. Northwesterly winds between 60–75 kt are found between 850–750 hPa. These strong northwesterly winds were able to mix down to the surface. Given that a DCAPE value of 98 J/kg was reported in the above IAD sounding, I hypothesize that evaporatively cooled downdrafts produced by falling precipitation that was not reaching the ground from the 850–450 hPa moist layer in a strongly sheared environment was the source of the high winds that reached the surface. The absence of low-level cold-air advection in the IAD sounding (if anything, there is weak warm-air advection based upon the observed veering wind profile between the surface and 500 hPa, consistent with warm air wrapping cyclonically around the strong offshore cyclone) makes it unlikely that cold-air advection-driven descent was the primary source of the downward mixing of higher wind speeds from aloft.

KIAD 022352Z 33024G39KT 10SM FEW080 SCT140 SCT250 05/M07 A2995 RMK AO2 PK WND 31044/2331 SLP142 T00501072 10078 20050 53044 KIAD 022252Z 33028G43KT 10SM FEW070 SCT140 SCT250 06/M07 A2989 RMK AO2 PK WND 33054/2231 SLP123 T00611072 KIAD 022152Z 33025G46KT 10SM FEW060 SCT120 BKN250 07/M06 A2985 RMK AO2 PK WND 31052/2131 SLP109 T00721056 KIAD 022052Z 32033G48KT 10SM SCT050 BKN140 BKN250 08/M05 A2982 RMK AO2 PK WND 31048/2046 SLP098 T00781050 53014 KIAD 021952Z 32032G46KT 10SM BKN047 07/M05 A2981 RMK AO2 PK WND 33053/1915 SLP094 T00671050 KIAD 021852Z 31026G48KT 10SM BKN048 06/M04 A2978 RMK AO2 PK WND 30052/1820 SLP087 T00611039 KIAD 021752Z 30035G47KT 10SM BKN047 06/M04 A2978 RMK AO2 PK WND 30052/1658 SLP084 T00611039 10061 20044 51019 KIAD 021652Z 31031G49KT 10SM BKN044 05/M04 A2978 RMK AO2 PK WND 29062/1638 SLP084 T00501044 KIAD 021552Z 31032G46KT 10SM BKN047 04/M04 A2975 RMK AO2 PK WND 30053/1517 SLP073 T00441039 KIAD 021452Z 31028G45KT 10SM BKN048 05/M04 A2972 RMK AO2 PK WND 32046/1441 SLP065 T00501039 51042 KIAD 021352Z 31027G37KT 10SM BKN045 04/M03 A2968 RMK AO2 PK WND 31047/1307 SLP053 T00441033 KIAD 021252Z 32028G48KT 10SM BKN045 OVC055 04/M04 A2964 RMK AO2 PK WND 32054/1241 SLP038 T00441039 KIAD 021152Z 31034G45KT 10SM BKN045 OVC055 04/M03 A2960 RMK AO2 PK WND 31049/1105 RAB10E19 SLP022 VCSH P0000 60000 70013 T00441033 10089 20044 53022 KIAD 021052Z 30030G42KT 10SM BKN050 04/M03 A2954 RMK AO2 PK WND 30048/1007 SLP004 T00441033 KIAD 020952Z 30031G41KT 10SM BKN050 05/M03 A2955 RMK AO2 PK WND 30052/0933 SLP006 T00501033 KIAD 020852Z 29022G49KT 10SM BKN047 BKN055 05/M02 A2953 RMK AO2 PK WND 27058/0840 PRESRR SLP999 T00501022 50008 KIAD 020752Z 29024G41KT 10SM BKN050 06/M02 A2955 RMK AO2 PK WND 30041/0745 SLP005 T00611022

5. New York State Mesonet Precipitation Map for 1–2 March 2018 (courtesy of Nick Bassill):

A preliminary map of storm-total precipitation (inches) for 1–2 March 2018 based on NYS Mesonet observations that was posted separately to map by Nick Bassill can be found here: http://operations.nysmesonet.org/~nbassill/archive/2018/03/03/recent/precip 2.png. A subsequent update will show some of the higher precipitation amount totals to the west and south of Albany upwards of where 40” (~1 m) of snow was reported. Nick’s analysis also suggests that Albany Airport received ~1.50” of melted precipitation (eyeball interpolation) versus an ASOS-reported 1.08” storm-total precipitation where 11.9” of snow was reported. These precipitation and snowfall amounts values equate to an unlikely snow-to-water ratio of ~11.0, given how wet the snow was and given that ALB temperatures varied between 0.0 and 1.1 C throughout the storm. The continuing under measurement of liquid water amounts during snowstorms by many operational ASOS sites such as ALB is a longstanding problem and one that is likely contaminating the winter and annual precipitation climatology in many northern locations.

Friday map discussion will not be held next week since many of us will be on route to the 43rd Northeastern Storm Conference in Saratoga Springs.

Lance