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Santa Anna Winds and Anomalies to Forecast Them by Richard H. Grumm National Weather Service Office State College, PA

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Santa Anna Winds and Anomalies to Forecast Them by Richard H. Grumm National Weather Service Office State College, PA Santa Anna Winds and Anomalies to forecast them By Richard H. Grumm National Weather Service Office State College, PA 1. INTRODUCTION pressure builds over the Great Basin low pressure develops offshore. Many During the latter part of October 2007, studies have been conducted related to strong Santa Anna winds and dry the meteorology of Santa Ana winds conditions produced devastating fires in events. Raphael (2003) provides and Southern California. News reports of excellent summary of these events. rapidly moving fires appeared by 20 Small (1995) provides a summary of a October 2007 and the destructive forces 1993 event which was also associated of these fires dominated news reports with brush fires. from 21-27 October 2007. Weaker winds and more humid conditions diminished Rapheal (2003) developed a 32-year the fire activity after the 26th. dataset to quantify Santa Ana events. He noted that there are approximately 20 Moderate Resolution Imaging events per season and most events lasted Spectroradiometer (MODIS) imagery about 1.5 days. The most active months captures the fires and NASA began a were November through January (see his case page with images from 21-29 Figure 3) though the season began in October capturing the event. Figure 1 September and lingered into April. Key show MODIS images from the NASA features appeared to be low dew points on the 23 and 24th of October 2007. The and a strong pressure gradient from the large plumes of smoke directed from Great Basin to coastal California. east to west characterize the strong Santa Ana winds which are common in many The key conditions with Santa Ana fire cases1 (Raphael 2003). The winds, events have not been associated with when coincidental with the fires drive anomalies in mean-sea level pressure them westward. fields. One goal of this study is to show the pressure anomalies over the Great The Santa Ana wind is a hot dry easterly Basin and southern California during or northeasterly wind that blows from known Santa Ana events and those the deserts of Nevada and southern events which were associated with fires California into the eastern Pacific when such data was available. The (Glickman 2000). The wind is also anomalies were computed as outlined in characterized as foehn-type event named Hart and Grumm (2001). after the Santa Ana river valley and the pass of the same name. The stronger The goal of this paper is to present the events normally occur in the mid- conditions associated with the Autumn to early spring when strong high devastating fires and the persistent Santa Ana wind event of 21-29 October 2007. 1 Many fires are of human origin and one of the Forecasts and analysis of anomaly data larger fires of October 2007 was likely set by a will be presented. These data will be young man. contrast to other known and documented Cases were obtained by searching the significant Santa Ana events. web and articles published on Fires and Santa Ana winds. This was neither a 2. METHODS AND DATA comprehensive or exhaustive study. The 6 January 2002 event was found on the All model data were retrieved in near- NASA MODIS site. Imagery of dust and real-time from NCEP. The data shown a few fires were available along with a here are primarily from the NCEP brief description of the event. The Global Ensemble Forecast System Shekell, Fire in California on 3 (GEFS). The data are shown with December 2006 was a massive fire in respect to the 30-year climatology. The southern California which was captured forecasts are subtracted from the 30-year on MODIS imagery. The fire of late mean and normalized by the value of 1 October 2003 was another large fire standard deviation on the particular day captured by MODIS imagery. Imagery and hour. Thus all data are shown as of the fire was archived by NASA on the standardized anomalies. 24-30 October 2003 for this massive event which had similar east to west Re-analysis data is used to show smoke plumes. Figure 5 shows this conditions of similar events. All re- image as a case study presented in this analysis data is displayed showing the document. field and its associated standardized anomaly (Grumm and Hart 2001). Table 1 shows Santa Ana winds recorded at the National Weather All re-analysis, model, and anomaly data Service office in San Diego, California. were displayed using GrADS. Not all events are recorded. Several of these cases were reviewed for this study. Figure 2 Conditions over the southwestern United States from the NCEP/NCAR global re- analysis data valid at 0000 UTC 25 October 2003. Data shown include a) mean sea level pressure (hPa) and anomalies, b) precipitable (mm) water and anomalies, c) 850 hPa winds and u-wind anomalies, and d) 850 hPa winds and v-wind anomalies. Anomalies are computed using the value minus the mean for that date divided by 1σ for that date. Figure 2 As in Figure 2 except valid at 0000 UTC 26 October 2003. In Figure 3 As in Figure 2 except valid at 0000 UTC 27 October 2003. 3. RESULTS addition to the strong low-level anticyclone, the event of October 2003 i. October 2003 event was associated with a massive upper- level ridge as shown in Figure 4. At The long-lived Santa Ana event of 0000 UTC 27 October the 500 hPa and October 2003 produced massive fires 700 hPa heights were +2SDs above and shared many of the characteristics normal over western North America and associated with the October 2007 event. the eastern Pacific. The low-level 925 Figure 2 shows the large scale conditions hPa winds were anomalously strong over the southwestern United States at from the northeast. 0000 UTC 25 October 2003. Key features include the above normal mean- An examination of other documented sea level pressure over the Great Basin Santa Ana events suggests that the (Fig. 2a), the below normal moisture in anomalous ridge is often associated with the precipitable water field (Fig. 2b), and stronger Santa Ana events. A thorough the strong northeast winds south of the study has not been accomplished to anticyclone (Figs. 2c&2d). The prove the significance of these features. anomalous easterly winds pushed The large ridge likely contributes to southward on the 26th (Fig. 3). larger scale subsidence and drying. The subsidence may play a role in the It should be noted that the anticyclone development of an inversion near the over the Great Basin is similar to the ridge tops. Other cases examined (not pattern presented by Rapheal (2003). shown) showed similar patterns with Figure 5 MODIS image of California fires 26 October 2003. anomalous anticyclones from the surface diminish slowly to about +1SD above into the mid-troposphere. normal by the 24th. ii. October 2007 event In addition to the strong MSLP anomalies, the NCEP GEFS forecast Forecasts of the surface pressure over significantly above normal easterly the United States from Forecasts winds over the region. Note the -4SD initialized at 0000 UTC 20 October 2007 wind anomalies at 850 hPa at 1800 UTC valid at 1800 UTC 21 & 22 October 22 October 2007. Anomalous easterlies 2007 are shown in Figure 6. were forecast to persist through 0000 UTC 25 October 2007. The key features in these forecasts was the emergence of above normal high A sequence of GEFS forecasts of 500 pressure over the Great Basin on the 21st hPa heights from 0000 UTC 19 October then the appearance of +3 to +4SD through 0000 UTC 21 October 2007 is pressure anomalies over Nevada and shown in Figure 8. These forecasts are California on the 22nd. In addition to the all valid at 0000 UTC 23 October 2007. strong anomalies, there was good The key feature shown is the anomalous agreement between ensemble members 500 hPa ridge to the west of the affected as to the location and magnitude of the area. The anomalous surface fields (not large anticyclone. Though not shown, shown) were east of the upper-level the MSLP anomalies were forecast to system over the Great Basin. The data show the ability of the GEFS to forecast Figure 6. NCEP GEFS forecasts of mean sea-level pressure (hPa) from forecasts initialized at 0000 UTC 20 October 2007 showing left) forecasts valid at 1800 UTC 21 and right) valid at 1800 UTC 22 October 2007. Upper panels shows each members 1008 and 1020 contours with spread and lower panels show the ensemble mean forecasts and the departures of these fields from the 30-year climatology in standard deviations from normal. the key features associated with these Southern California. News reports of events. rapidly moving fires appeared by 20 October 2007 and the destructive forces Figure 9 shows successive forecasts of of these fires dominated news from 21- the 850 hPa winds. Upper panels show 27 October 2007. Weaker winds and the u-wind anomalies. These forecast all more humid conditions diminished the showed abnormally strong u-winds and fire activity after the 26th. large u-wind anomalies over the region affected by the Santa Ana winds. These An attempt was made here to show some data imply the value of wind anomalies of the key synoptic scale features as in aid in forecasting these events. associated with Santa Ana wind events in southern California. The fires, when 4. CONCLUSIONS present or ignited during such an event can spread rapidly due to the During the latter part of October 2007, combination of dry conditions and strong Santa Anna winds and dry strong warm winds. No attempt is made conditions produced devastating fires in here to predict fires.
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