National Weather Service-Pennsylvania State University Weather Events Heavy Rain 7-8 December 2011 by Richard H. Grumm National Weather Service State College PA 16803 Abstract:. A mid-level short-wave moved out of the southern United States and up the west side of a strong ridge over the western Atlantic. The resulting strong southwesterly flow brought a plume of high precipitable air up the East Coast resulting in heavy rainfall from the Carolinas in into New England. A large swath of the region saw 25 to 50 mm of rainfall with a few areas receiving over 75 mm of rainfall. Reagan National airport received 78.12 mm (3.1 inches) of rain on 7 December setting a new daily and monthly rainfall maximum. The rainfall resulted in mostly minor flooding in portions of Mid-Atlantic region where 16 points went over flood stage. All but 3 points experienced minor flooding. This was the first December flood event in the Mid-Atlantic region in 2011 and the eighteenth flood event of the calendar year. This fast moving rainfall event fell in a pattern often associated with heavy rainfall and was generally well predicted by the NCEP models and ensemble forecast system within about 48 hours prior to the onset of precipitation. Forecasts at 5-days out had the potential event but uncertainty issues arose and the events more persistent predictability horizon was limited to about 48 hours prior to the onset of precipitation. Like many winter events there was some snow on the northwestern edges of the precipitation shield.
1. INTRODUCTION A mid-level short-wave moved out of the southwestern United States and up the west side of a strong ridge over the western Atlantic producing heavy rain in the Mid- Atlantic Region and southern New England on 7-8 December 2011 (Fig. 1a). There was a swath of 1 to 8 inches of snow with some higher snowfall amounts in the higher elevations of Pennsylvania and New England (Fig. 1b) as cold air filtered in on the northwestern edges of the precipitation shield. The rain produced flooding in the Mid- Atlantic region with 16 points along rivers and streams reaching or exceeding flood stage. 1 The key features in this event were the short-wave moving out of the southern stream, a strong northern stream wave digging into eastern North America, and a persistent La Niña like ridge of the East Coast of North America (Fig. 2). These three systems brought a plume (Carlson 1980) or atmospheric river (AR: Ralph et al. 2004 and 2006) of high precipitable water air up the East Coast into the Mid-Atlantic Region and southern New England (Fig. 3). The association of heavy rainfall to plumes of abnormally high precipitable water based on standardized anomalies (Hart and Grumm 2001) has been documented by Junker (2009; 2009). This event was relatively well predicted 24 to 48 hours prior to the onset of heavy rainfall in the Mid-Atlantic region. It will be shown that the NCEP GEFS forecasts converged on the potential for heavy rainfall from forecasts initialized on and after 1200 UTC 5 December 2011 and the NCEP SREF converged from forecasts issue on and after 1500 UTC 5 December 2011. The use of ensembles can aid in predicting heavy rainfall events. However, much beyond 48 hours prior to many events deterministic models and ensemble prediction systems appear to have some limitations. This paper will document the pattern and the rainfall event of 7- December 2011. Standardize anomalies are used to show the pattern and explain the basic synoptic overview of the event. The NCEP GEFS and SREF are used to address the forecast issues and uncertainty aspects of this event. 2. Methods and Data The NCEP GFS is used to re-produce the conditions associated with the storm to include the large scale pattern. The standardized anomalies are displayed in standard deviations from normal as in Hart and Grumm (2001) and are computed using the climatology from the NCEP/NCAR global reanalysis data (Kalnay et al. 1996). The focus is on the pattern and anomalies associated with the storm. The value of EFS and anomalies with EFS data are presented. Ensemble data shown here are from the NCEP forecast systems. The NCEP short range ensemble forecast system (SREF) is used herein to show the probability of heavy rainfall during this event. GFS and NAM data were also examined to see how the event was forecast from a short-range perspective. The Stage-IV rainfall data was downloaded from NCEP to produce images of the rainfall and to conduct some verification of the rainfall forecasts. 1 Based on the Mid-Atlantic River Forecast Center flood climatology.
3. The Storm system and impacts i. The large scale pattern The 500 hpa heights and anomalies (Fig. 2) showed a strong ridge over the western Atlantic with +2σ height anomalies over easternmost Canada (Fig. 2a). The strong ridge off the East Coast of North America is a common feature in many La Niña winters. A strong short-wave with -1 to -3σ height anomalies moved into the western side of this ridge (Fig. 2c-f) and up the East Coast. The strong implied southerly flow produced an above normal 250 hpa jet (not shown) and brought a plume of deep moisture up the East Coast (Fig. 3a-e). Satellite loops of total precipitable water (TPW) suggested the source of the deep moisture was from the Gulf of Mexico which can be inferred from Figure 3. ii. Regional pattern and key anomalies Most of the precipitation fell on 7 December 2011 and early on 8 December 2011. The focus on the regional pattern is the period covering the heaviest precipitation. The precipitation was associated with a relatively modest surface cyclone (Fig. 4) which Figure 1.StageIV rainfall data showing total accumulated rainfall (mm) for the period of 0000 UTC 07 December through 1200 UTC 8 December 2011.
zipped up along a southwest to northeast oriented frontal boundary (Fig. 5). Most of the precipitation in the Mid-Atlantic region was over by 08/0600 UTC. Strong southerly winds (Fig. 6) along and ahead of the frontal system (Fig. 5) produced the heavy rainfall. The moisture flux (MFLUX) peaked over Maryland, Virginia and Delaware between 07/1800 and 08/0000 UTC (Figs. 7b-c). The 6σ MFLUX over the Delmarva at 08/0000 UTC is a relatively well known heavy rainfall signal. The strong winds and high MFLUX values quickly moved over the western Atlantic after 08/00000 UTC. iii. observations The total rainfall from the Stage-IV data for the periods of 07/0000 through 08/0000 UTC and 07/0000 through 08/1200 UTC show the swath of precipitation over the Mid-Atlantic region (Fig. 8). These data show to axis of maximum rainfall, one over Pennsylvania and New York and another to the south and east extending from Virginia across Maryland into southern New Jersey. The southern axis of precipitation has both a closed 64 mm with some embedded areas covered by a 75 mm contour across northern Virginia into southern New Jersey. The axis of heavy precipitation to the northwest had a close 48 mm contour and most of this precipitation fell prior to 08/0000 UTC while there was clearly a period of heavy rainfall in the southern band after 08/0000 UTC (Fig. 8 bottom). The 6-hour accumulated precipitation (Fig. 9) shows how the precipitation event evolved mainly after 07/0600 UTC and peaked over Pennsylvania between 08/1200 and 08/0000 UTC (Fig. 9c-d). Over the Capitol District, the heavy precipitation was more enduring and included a period of heavy rainfall between 08/0000 and 08/0600 UTC. With the exception of the Capital District area, most locations, the entire event spanned 12-18 hours. The period of heavy rainfall for the 2 6-hour periods ending at 08/00000 and 08/0600 UTC line up relatively well with the peak time of high MFLUX (Fig. 7b-c). The general pattern of the rainfall suggests that the frontal boundary and the strong low-level jet ahead of this boundary (Figs. 5&6) dictated the orientation of the rainfall of the rain bands with produced the resulting rainfall. The rainfall produced mainly minor flooding in the Mid-Atlantic Region based on the Mid-Atlantic River Forecast Center (MARFC) flood database. Of the 16 points that reached or exceed flood stage, 13 sites observed minor flooding and 3 points reached moderate flooding. Moderate flooding was observed at Langhorne, Pennsylvania, Blackwells Mills, New Jersey, and Richmond, Virginia. These points were located within or in the edges of the band of heavier rainfall indicated in Figure 8. iv. Forecasts The general pattern associated with this event was relatively well predicted; however the predictability horizon of the rainfall in the Mid-Atlantic region was relatively short, on the order of 48 to 72 hours in advance. There was a general convergence toward a higher likelihood of rain and heavy rainfall over the Mid-Atlantic region from NCEP GEFS forecasts issued at and after 05/1200 UTC. To illustrate this, three GEFS forecasts, initialized at 05/0600, 05/1200, and 06/0600 UTC are shown in Figures 10-12. All three forecasts are valid at 08/1200 UTC.
The 05/0600 UTC forecast shows the rainfall suppressed well to the south and is in general a non-event in the Mid-Atlantic region (Fig. 10). This forecast cycle did predict a modest rainfall event between 08/1200 and 09/1200 mainly in Virginia and North Carolina (not shown). The change in the forecast at the next forecast cycle, 05/1200 UTC was nothing short of remarkable. The GEFS forecasts initialized just 6 hours later showed a dramatic change with a higher probability of heavy rainfall in the Mid-Atlantic region and the potential for over 50 mm of QPF in the region impacted by the heavy rainfall (Fig. 11). The 06/0600 UTC GEFS (Fig. 12) showed the same and consistent forecast for the potential of heavy rainfall and in excess of 50 mm of QPF in close proximity to where the heavy rainfall was observed. The probabilistic forecasts of 25 mm or more QPF form forecast issued at 25/0600, 25/1200 and 26/0000 illustrate this dramatic change (Fig.13). The same convergence of forecasts, albeit slower than the GEFS, was observed in the NCEP operational 32km SREF (Fig. 14). Three of the four SREF forecasts from 06 December (Fig. 15) show that once the system converged on the forecasts they remained quite steady state. Similar result was observed in the GEFS (not shown). 4. Conclusions A mid-level short-wave moved out of the southwestern United States and up the west side of a strong ridge over the western Atlantic producing heavy rain in the Mid- Atlantic Region and southern New England on 7-8 December 2011 (Fig. 1). There was a swath of 1 to 8 inches of snow with some higher snowfall amounts in the higher elevations of Pennsylvania and New England (not shown) as cold air filtered in on the northwestern edges of the precipitation shield. Due to antecedent warm conditions and the terrain dependence on snowfall, the snow was relatively inconsequential. The rain did cause 16 river points to reach or exceed flood stage in the Mid-Atlantic region. More interesting about this event was the general low predictability much beyond 48 hours prior to the onset of precipitation. The distinct change in the forecast in the GEFS between 05/0600 and 05/1200 UTC is not an uncommon forecast dilemma. As forecasts length decreases model skill generally increases. At some point a system becomes more predictable. It is difficult to know a priori when a forecast event is within the predictability horizon of a particular model or ensemble system. There have been numerous events like this, some more notable than others. The post-christmas blizzard of 2010 (Uccellini et al. 2011) was a classic case where within 72 hours of the event forecasts rapidly converged on the correct solution. The NCEP forecast assessment noted as early as 1200 UTC 24 December, a significant number of model ensemble solutions began to point to a higher probability of a significant snow event although many forecasters remained skeptical since there still was considerable model divergence. During the post-christmas Blizzard most forecasts systems did not converge toward the correct forecast solution until 36 to 48 hours prior to the event. This was similar to the event of 7 December 201 shown herein where forecasts began to converge about 36 to 48 hours prior to the on to the onset of heavy precipitation. A similar forecast horizon was identified during the historic early season snow event of 29-30 October 2011 (Grumm 2011).
These rapid changes in predictability need to be identified and leveraged. The general concept of not changing forecasts dramatically with new data is a complete contradiction to the reality of predictability horizons. Though predictability horizons are unique for each and every event, the overall tendency suggests a rapid increase in predictability in many events in the 24 to 72 hour time range. Thus, significant changes in ensemble forecasts within this time range likely are useful data and should not be ignored. The cases shown here and other case studies continue to show that the NCEP GEFS tends to converge on the more likely outcome 1-2 forecasts cycles earlier than the NCEP 32km SREF. Though not shown here, an examination of GEFS forecasts from all cycles from 01/0000 through 07/1200 UTC revealed that on 3 December the GEFS had the correct concept of a rain event in the Mid-Atlantic region. The best forecast was the 03/0600 UTC forecast cycle. The forecasts degraded thereafter until 05/1200 UTC, with some runs on the 4 th suggesting 2 distinct precipitation events, one on the 7 th and another stronger event on the 9 th. They key to these forecasts was the timing of the southern stream wave moving out of the southwestern United States and its interaction with the northern stream wave. The spaghetti and spread plots at fields such as 500 hpa heights showed the rapid decrease in uncertainty (spread) on forecasts produced on and after 05/1200 UTC. Phasing waves appears to be a serious issue. Dealing with uncertainty remains a serious forecast issue. But empirical cases studies continue to demonstrate a remarkable change in predictability somewhere in the 24 to 72 hour range with many events. 5. Acknowledgements Rainfall data from NCEP and the MARFC provided data on flooding and flooding statistics. A special thanks to Charles Chillag of the MARFC for tables and data summaries of the flooding. 6. References Carlson, T.N 1980: Airflow through Mid-latitude Cyclones and the comma head pattern. Mon. Wea. Rev.,108,1498-1509. (See figure 4). Grumm, R.H. 2011: Historic Nor easter snows of 29-30 October 2011. Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic scale events objectively. Mon. Wea. Rev., 129, 2426 2442. 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. 445 453
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, 336 356. Kocin, PJ and co-authors 2011: The Blizzard of 25-27 December 2010: Forecast Assessment. NCEP Assessment document, 57pp. Ralph, F. M., G. A. Wick, S. I. Gutman, M. D. Dettinger, C. R. Cayan, and A. B. White, 2006: Flooding on California s Russian River: The role of atmospheric rivers. Geophys.Res. Lett., 33, L13801, doi:10.1029/2006gl026689. Ralph, F. M., P. J. Neiman, and G. A. Wick, 2004: Satellite and CALJET aircraft observations of atmospheric rivers over the eastern North Pacific Ocean during the winter of 1997/98. Mon. Wea. Rev., 132, 1721-1745.
Figure 1. Precipitation plots showing a) the event total precipitation (mm) over the eastern United States for the period ending at 1200 UTC 8 December 2011 and b) the total snowfall (whole inches) over the eastern United States. Now values below 5 inches are black values of 5 inches or higher are blue. Return to text.
Figure 2. NCEP GFS 00-hour forecast of 500 hpa heights (m) and height anomalies (s) in 12-hour increments from a) 0000 UTC 06 December through f) 12000 UTC 08 December 2011. Heights every 60 m. Return to text.
Figure 3. As in Figure 2 except for precipitable water (mm) and precipitable water anomalies. Precipitable water contours are every 5 mm. Return to text.
Figure 4. As in Figure 2 except for mean sea level pressure (hpa) and pressure anomalies over the Mid-Atlantic region for the 6-hour periods spanning a) 1200 UTC 7 December through g) 1800 UTC 8 December 2011. Isobars every 4 hpa. Return to text.
Figure 5. As in Figure 4 except for 850 hpa temperatures ( C) and temperature anomalies. Isotherms every 2C. Return to text.
Figure 6. As in Figure 4 except for 850 hpa winds (kts) and total wind anomalies. Return to text.
Figure 7. As in Figure 4 except for 850 hpa moisture flux and moisture flux anomalies. Return to text.
Figure 8. Stage-IV gage and radar adjusted precipitation (mm) for the periods of (top) 0000 UTC 7 to 8 December 2011 and (bottom) 0000 UTC 7 to 1200 UTC 8 December 2011. Shading is as indicated by the color bar in powers of 2 with an additional 75mm contour added to capture the heavy rainfall in the Capitol district. Return to text.
Figure 9, As in Figure 8 except for precipitation in 6-hour increments with no 75mm contour in the color scale for the 6-hour periods ending at a) 0600 UTC 7 December, b) 1200 UTC 7 December, c) 1800 UTC 7 December, d) 0000 UTC 8 December,, e) 0600 UTC 8 December, and, d) 1200 UTC 8 December. Return to text.
Figure 10. NCEP 21-member GEFS initialized at 0600 UTC 5 December 2011 showing QPF forecasts valid for the period ending at 1200 UTC 8 December 2011 including a) the probability of 6mm or more QPF in the past 6 hours, b) the probability of 12mm of QPF in the past 12 hours, c) the probability of 25mm of QPF in the past 24 hours, and d) the probability of 50 mm of QPF in the past 36 hours. Probabilities indicted by the color bar and contours are every 25 mm. Return to text.
Figure 11. As in Figure 10 except for GEFS initialized at 1200 UTC 05 December 2011. The 50 mm contour in the lower panel is for a 33 hour increment. Return to text.
Figure 12. As in Figure 10 except for GEFS initialized at 0600 UTC 06 December 2011. Return to text.
Figure 13. NCEP GEFS forecast of the (a-c) probability of 25 mm or more QPF for the 24 hours ending at 1200 UTC 8 December 2011 from GEFS forecasts issued at a) 0600 UTC 05 December 2011, b) 1200 UTC 05 December and c) 0000 UTC 06 December 2011 and (d-g) the ensemble mean QPF (gray shaded) each member s 25 mm contour (color coded contours). Return to text.
Figure 14. As in Figure 13 except for NCEP 32km SREF forecasts of 25 mm of QPF from SREF initialized at a-d) 0900 UTC 5 December 2011, b-e) 1500 UTC 05 December 2011, and c-f) 2100 UTC 05 December 2011. Return to text.
Figure 15. As in Figure 14 except for SREF forecasts initialized at a-d) 0300 UTC 6 December, b-e) 0900 UTC 6 December and c-f) 2100 UTC 6 December 2011. Return to text.