National Weather Service-Pennsylvania State University Weather Events

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1 National Weather Service-Pennsylvania State University Weather Events The February Thaw February 2011 by Richard H. Grumm National Weather Service State College PA Abstract: After a cold January with a lack of a typical January thaw, a rapid warm up and thaw affected much of the eastern United States on January. The thaw began earlier in the plains and came on the heels of record snow and cold event in the southern plains just days before the thaw began. The warm episode ended with a strong cold front which came in on strong winds. A widespread high wind event impacted the eastern United States on 19 February Winds of 50 to 60 mph were common from Virginia to New Hampshire. A few wind gusts over 70 mph were observed during this event. 1. INTRODUCTION A brief warm up in the eastern United States brought record high temperatures to portions of the eastern United States on February The warmest day in Pennsylvania was the 18 th when many locations set or tied new record highs for the data (Table 1). The warm temperatures and relatively moist air rapidly diminished the snow cover over much of the State on the 18 th. Slow snow melt on the 16 th and 17 th was rapidly accelerated on the 18 th. A cold front with swept through the region on the 18 th and 19 th bringing high winds to the region. Wind gusts cause problems with fires and power outages. Maryland was hardest hit by fires where smoke closed roads and taxed fire companies from Princes Georges northward to Delaware. The strong wind 30 to 40 mph and gusts over 55 mph caused most of the problems on the 19 th of February (Washington Post). The warm air came on the heels of a southern plains snow nearly a week earlier which was followed by a surge of cold air into the southern plains (Fig. 1). On the morning of 10 February, Bartlesville, Oklahoma hit a record low of -28F and Nowata hit -31 (KOCO News). The latter site is a mesonet site. Across the southern plains, many record low temperatures were set or tied on 10 February. The anomalously cold air at 850 hpa (Fig. 1b-c) was rapidly replaced by unseasonably warm air by 13 February (Fig. 1d). Thus, the February thaw of 2011 represented a rapid transition from cold to warm. This paper will document the February thaw focusing on the exceptionally warm days of February in the Mid-Atlantic region and the post frontal winds. Farther south the warm began on the 13 th and lingered through the 19 th in the East. The focus her is on the Mid-Atlantic region and value of standardized anomalies (Graham and Grumm 2010;Hart and Grumm 2001) in identifying meteorologically and climatologically significant weather events. 2. Methods and Data The overall pattern was reconstructed using the 00-hour forecasts from the operational GFS. The anomalies were derived using the GFS and comparing it to the 30-year mean and standard deviations computed from the NCEP/NCAR re-analysis data (Kalnay et. al 1996). All anomalies herein are shown as standardized anomalies (Hart and Grumm 2001). The GFS is run on a 27 km grid. However the data shown here is on a 1x1 degree grid. This should mitigate some of the resolution issues between the coarser climatology and the model forecast grids. These effects are normally of minimal impact for parameters above

2 the planetary boundary layer. Some variables such as PW are sensitive and will show higher values in higher resolution models than in the re-analysis dataset. Forecasts from the NCEP Ensemble Forecast systems (EFSs) will be presented. Standardized anomalies will be presented as described above, computing anomalies from the ensemble mean and the NCEP/NCAR re-analysis data. Probabilities are derived using the ensemble output. These will be raw and uncalibrated probabilities unless specified otherwise. For brevity, times will be denoted in the format 01/1200 UTC to signify 1200 UTC 18 February 2011 and time such as 18/1200 would signify 1200 UTC 18 February The Storm system and impacts i. The pattern and key anomalies The 500 hpa pattern in 24-hour intervals from 0000 UTC February 2011 (Fig. 2) show the emergent 500 hpa ridge in the southwestern United States (Fig. 2a) which rapidly built poleward and brought above normal 500 hpa heights, with 1 to 3σ 500 hpa height anomalies to much of the eastern United States from the Rocky Mountains to the East Coast during this 6 day period. Beneath the ridge, abnormally warm air was present at 850 hpa (Fig. 3) and a large portion of the central United States was experienced 2 to 3σ above normal 850 hpa temperatures. The large ridge also brought above normal precipitable water values into much of the eastern United States (Fig. 5). Warm air combined with high dew points if an effective means to erode snow cover. Thus, these high PW values imply an air mass which was capable of eroding the snow cover from the southern plains into the southern Great Lakes. j. Regional pattern The regional pattern is focused over the eastern United States. The 850 hpa temperatures with the standardized anomalies in 6-hour increments (Fig. 5) show the broad are of 5C or greater air over the reigon with 1 to 3s above normal temperature anomalies. The warmest air was over the Midwest (Fig. 1a-c) on the 17 th with 3σ anomalies and temperatures over 15C. The warm air peaked early on the 18 th (Friday) in the East with 10C air reaching into southern Pennsylvania at 18/0600 UTC. There were some areas of 3σ above normal 850 hpa temperatures. The surge of warm began to erode after 18/1200 UTC as colder air moved in from the northwest (Fig. 5e-f). The warm surge was associated with a surge of warm moist air ( Fig. 6) with 2 to 3σ precipitable water anomalies. Interestingly, there was snowmelt in Pennsylvania on the 17 th, but the rapid loss of snow on the 18 th was remarkable 1. The drier air moved into the eastern United States faster then the cooler air, not the return to near normal of the PW anomalies in the eastern United States while the 850 hpa temperatures were still around 1s above normal (Figs. 5g and 6g). As the warm air moved over the region, so did strong winds aloft. Most of the strong winds were decoupled until after frontal passage. The 850 hpa winds ( Fig. 7) were 1 Personal observation of the loss of 6 inches of snow from 1200 UTC through 2100 UTC 18 February % snow cover to less than 10% snow cover in less than 12 hours.

3 3 to 5σ above normal over the Great Lakes at 18/1200 UTC (Fig. 7a). These strong winds moved over Pennsylvania and western New York by 18/0000 UTC (Fig. 7c). The 850 hpa winds, behind the front, remained 2 to 4s above normal through about 19/1800 UTC. There was a period of 4-5σ 850 hpa winds over Maryland around 19/1200 UTC (Fig. 7e) which slowly weakened by 19/1800 UTC. Behind the front these strong winds (25ms -1 ) were able to reach the surface. These data show the strong winds which likely caused the power failures in Pennsylvania, Delaware and Maryland. These winds likely fanned the flames of the numerous reported wildfires. k. Forecasts The overall pattern was relatively well predicted and a brief set of images area presented to show the warm up in the east. NCEP GEFS 850 hpa temperatures from 9 forecast cycles are shown in Figure 8. These data show the surge of above normal 850 hpa temperatures in the East peaking around 18/1800 UTC along with the rapidly approaching cold from the northwest. s the warm air moved over the region, so did strong winds aloft. Most of the strong winds were decoupled until after frontal passage. The 850 hpa winds (Fig. 7) were 3 to 5σ above. Shorter range forecasts and forecasts from the SREF did slightly better at the timing of the warm surge. But for brevity are not shown. The cold frontal passage and the potential for strong winds behind the front were relatively well predicted. SREF 850 hpa winds, valid at 19/1200 UTC are shown in Figure 9. These data show the pattern of strong winds and 3 to 4s total wind anomalies which were observed in the GFS 00-hour forecasts (Fig. 7). Longer range forecasts clearly showed more convergence toward a solution and thus large mean winds and larger anomalies. From a probabilistic perspective, the SREF showed a high probability of a high wind event. The probability of 3σ 850 hpa wind anomalies from the 21-SREF members is shown in Figure 10. These data show that at 19/1200 UTC nearly 100% of the SREF members predicted 3σ above normal 850 hpa winds and winds in excess of 20ms Conclusions After a cold surge and some record lows in the southern Plains on 10 February, a surge of above normal air in conjunction with a strong 500 hpa ridge (Fig. 2) produced a rapid thaw over most of the eastern United States during from February The thaw in the Mid-Atlantic resulted in a 2-day period of warmth, with record high temperatures in the 60s across Pennsylvania on the 19 th. The combination of warm air, moist air, and gusty winds substantially eroded the snow cover over large portions of the eastern United States. A cold front moved across the eastern United States on the 18 th and early on the 19 th. Strong winds along and behind the frontal boundary reached the surface. High wind warnings were posted in advance of this system. These warnings were likely based on the strong winds and above normal winds in the model and ensemble guidance. The NCEP SREF showed nearly a 100% chance for 3σ above normal 850 hpa winds (Fig. 10) above normal behind the front, where isentropes are typically steeply sloped and thus may allow winds from 850 to 700 hpa to reach the surface. The high winds caused power outages

4 and fanned wild fires in the Mid-Atlantic region. The winds also ushered in cold air, ending the warm episode. Wind gusts of 40 to 60 mph were common from Virginia to New Hampshire (Fig. 11). In eastern Massachusetts, a wind gust of 74 mph (Fig. 11c) was recorded. The winds caused power outages and fanned the flames of small fires creating wild fires. The 500 hpa height and temperature anomalies captured the potential for this warm event in the forecasts and analysis. The 850 hpa wind anomalies appeared to capture the potential for the snow melt during the warm period and the strong post-frontal winds. 5. Acknowledgements Jason Krekeler for data on the temperature records in Pennsylvania and adjacent States. John Lacorte for data analysis and wind gust images. 6. References Graham, Randall A., Richard H. Grumm, 2010: Utilizing Normalized Anomalies to Assess Synoptic-Scale Weather Events in the Western United States. Wea. Forecasting, 25, Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season Weather Events: Preliminary Findings. Wea. and Fore., 16, Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic scale events objectively. Mon. Wea. Rev., 129, Junker, N.W, M.J.Brennan, F. Pereira,M.J.Bodner,and R.H. Grumm, 2009:Assessing the Potential for Rare Precipitation Events with Standardized Anomalies and Ensemble Guidance at the Hydrometeorological Prediction Center. Bulletin of the American Meteorological Society,4 Article: pp Junker, N. W., R. H. Grumm, R. Hart, L. F. Bosart, K. M. Bell, and F. J. Pereira, 2008: Use of standardized anomaly fields to anticipate extreme rainfall in the mountains of northern California. Wea. Forecasting,23, Onogi, K., J. Tsutsui, H. Koide, M. Sakamoto, S. Kobayashi, H. Hatsushika, T. Matsumoto, N. Yamazaki, H. Kamahori, K. Takahashi, S. Kadokura, K. Wada, K. Kato, R. Oyama, T. Ose, N. Mannoji and R. Taira (2007) : The JRA-25 Reanalysis. J. Meteor. Soc. Japan, 85,

5 Location Previous Record Observed High COOP Location High State College Williamsport (KIPT) Harrisburg (KMDT) Capital City (KCXY) Lancaster (KLNS)* Lancaster 2 NE 67 York (KTHV)* York 3 SSW 70 Bradford (KBFD) Clearfield (KFIG)* Clearfield ( ) 65 Selinsgrove (KSEG) Selinsgrove 2 s 67 Johnstown (KJST)* Johnstown 69 Altoona (KAOO) Reading (KRDG) Allentown (KABE) Philadelphia (KPHL) Pittsburgh (KPIT) Washington DC (Reagan) Table 1. The location of the site, the previous high for 18 February and the recorded high for 18 February Values in red indicated new records for the date. Nearby NWS Cooperative location data are included. Locations with an asterisks have limited data since about Return to text.

6 Figure 1. GFS 00-hour forecasts of 850 hpa temperatures (C ) and temperature anomalies in 24 hour increments valid at 1200 UTC 8-13 February Return to text.

7 Figure 2. As in Figure 1 except for GFS 500 hpa heights (mm) and height anomalies in 24 hour increments from 0000 UTC 14 to 19 February Return to text.

8 Figure 3. As in Figure 2 except for 850 hpa temperatures. Return to text.

9 Figure 4. As in Figure 3 except for precipitable water (mm) and precipitable water anomalies. Return to text.

10 Figure 5. As in Figure 1 except for 850 hpa temperatures in 6-hour increments over the eastern United States valid from 1200 UTC 17 February through 1800 UTC 18 February Return to text.

11 Figure 6. As in Figure 5 except for precipitable water (mm) and precipitable water anomalies. Return to text.

12 Figure 7. As in Figure 5 except for 850 hpa winds (ms-1) and 850 hpa total wind anomalies in 6-hour periods valid from 1200 UTC 18 through 1800 UTC 19 February Return to text.

13 Figure 8. NCEP GEFS ensemble mean forecasts of 850 hpa temperatures (C ) and temperature anomalies from the 21-member GEFS all valid at 1800 UTC 18 February from forecast initialized at a) 0000 UTC 16 February, b) 1800 UTC 15 February, c) 1200 UTC 15 February, d) 0600 UTC 15 February, e) 0000 UTC 15 February, f) 1800 UTC 14 February, g) 1200 UTC 14 February, h) 0000 UTC 14 February and i) 1800 UTC 13 February Return to text.

14 Figure 9. NCEP SREF 850 hpa winds (ms-1) and wind anomalies valid at 1200 UTC 19 February 2011 from SREFs initialized at a) 2100 UTC 17 February, b) 1500 UTC 17 February, c) 0900 UTC 17 February, d) 0300 UTC 17 February, e) 2100 UTC 16 February, f) 1500 UTC 15 February, g) 0900 UTC 16 February, h) 0300 UTC 16 February and i) 2100 UTC 15 February Return to text.

15 Figure 10. NCEP SREF forecasts initialized at 0300 UTC 18 February 2011 showing conditions valid at 1200 UTC 19 February 21, 2011 to include a) 500 hpa heights and height anomalies lower than 3σ below normal, b) 850 hpa heights (m) and probability of anomalies below -3σ, c) 850 hpa winds (ms-1) and wind anomalies greater than 3σ above normal, and d) mean sea level pressure and pressure anomalies less than -3σ below normal. Return to text.

16 Figure 11. Wind gusts (mph) from National Weather Service public information statements from the event of 19 February Return to text.

17 Figure 11 continued showing regional views of the wind gusts. Return to text.