The Historic Mid-Atlantic Snow storm of 5-6 February 2010-Draft By

Richard H. Grumm and John LaCorte National Weather Service Office, State College, PA 16803

1. INTRODUCTION A strong storm moved across the Gulf of Mexico and up the East Coast of the United States on 5-6 February 2010 (Fig. 1). This storm produced heavy snowfall from Virginia to New Jersey. Many sites in Pennsylvania, Virginia, Maryland, Delaware, and New Jersey received record snowfall. Snowfall totals at some sites exceeded 30 inches in Maryland and Pennsylvania. Reports of snowfall in excess of 20 inches were common in the Pittsburgh, Harrisburg, Philadelphia, Washington, and Baltimore metropolitan areas. This high impact East Winter Storm (ECWS: Hirsh et al 2000) caused massive power outages in the Washington and Baltimore areas where temperatures were close to freezing at the height of the storm. Heavy snow and blowing snow closed roads, rails, and major airports throughout the Mid-Atlantic region Friday into Saturday. The storm total snowfall is shown in Figure 2. In terms of record snowfall, the snowfall in Pittsburgh, 21.1 inches, was a record for the date and the 4th all-time snowfall for the City behind the record storm of 24-26 November 1950 (27.4), 16-18 December 1890 (25.9), 12-14 March 1993 (25.3). At Washington, Dulles airport the 32.4 inches crushed the previous record set in January 1996. The 24.8 inches at Baltimore nudged out the old

record of 24.4 inches set 16-17 February 2003 during the “Presidents Day snow storm of 2003”. Philadelphia’s 28.5 inches was the second largest snowfall, second only to the 30.7 inches recorded on 7-8 January 1996. Finally, the 25.8 inches of snowfall at Wilmington, Delaware beat the old record set on 7-8 January 1996. Many locations set records for the date and the month. Harrisburg, PA set record for the date with 18 inches which as the 8th largest snowfall for the City. At this time the largest known snowfall was 38.3 inches in Maryland with 3.34 inches of liquid equivalent. This was the second major to historic snowstorm to impact the Mid-Atlantic region during the winter of 2009-2010. Double digit snowfalls are in a winter are relatively uncommon events in the Mid-Atlantic region. The historic snow of 18-20 December 2009 (Grumm and LaCorte 2009) brought 1 to 2 feet of snow from North Carolina to southern New England. Both events occurred during a period when both the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) where negative. In addition to the negative NAO, the winter of 2009-2010 was a El Niño year which is often characterized by an active southern stream while the negative phase of the AO and NAO often produce relatively cold conditions over eastern North America. This storm was associated with strong low-level easterly winds at 850 and 925 hPa. Stuart and Grumm (2006) document the value of using u-wind

anomalies in identifying winter storms associated with potential heavy and record snowfall. It will be shown that this was forecast to and did contain anomalous low-level winds. Additionally, this storm had a deep surface cyclone with anomalies mean sea-level pressure anomalies, another characteristic associated with strong ECWS’s. From a forecast perspective, it will be shown that this storm was relatively well

predicted. The National Centers for Environmental Predictions (NCEP) deterministic NAM and GFS and the ensemble prediction system, including the SREF and GEFS predicted a high probability of in excess of 50 mm of quantitative precipitation (QPF) to fall as snow in the general area where the heavy snowfall was observed. Some higher resolution models indicated the potential for in excess of 75mm (3 inches) of QPF.

Figure 1. NCEP GFS 00-hour forecast of sea level pressure (hPa) and pressure anomalies from GFS inialized at a) 0000 UTC 5 February, b) 0600 UTC 5 February, c) 1200 UTC 5 February, d) 1800 UTC 5 February, e) 0000 UTC 6 February, f) 0600 UTC 6 February, g) 1200 UTC 6 February, h) 1800 UTC 6 February, and i) 0000 UTC 7 February 2010. Isobars every 4 hPa.

In addition to the high QPF values, the models and EFS all showed a sharp cut-off to the QPF. The northern edge of the storm was extremely sharp. In this gradient forecasters were faced with the potential for snow, now snow and potentially heavy snow. This sharp gradient extended across Pennsylvania, northern New Jersey, Long Island, NY, and southernmost New England. Not

only was this sharp gradient in the forecasts, but the observation indicated that this sharp gradient was remarkably well predicted by the NCEP forecast systems. There are many aspects of this storm worthy of study. No doubt this will be a well studied event. Thus, our focus here is to document this event and show the

Figure 3. NCEP GFS 00-hour forecast of 500 hPa height (m) and height anomalies from GFS initialized at a) 0000 UTC 5 February, b) 0600 UTC 5 February, c) 1200 UTC 5 February, d) 1800 UTC 5 February, e) 0000 UTC 6 February, f) 0600 UTC 6 February, g) 1200 UTC 6 February, h) 1800 UTC 6 February, and i) 0000 UTC 7 February 2010. Heights are every 60m.

value of anomalies in anticipating significant and potentially historic snow storms. Some NCEP forecasts are shown to demonstrate how well the models predicted the potential for heavy snow and how the models indicated a sharp area of uncertainty for the region where the heavy snow would rapidly diminish to light snow. 2. METHODS AND DATA

Rainfall data used here were obtained from the unified precipitation dataset (UPD) and the Stage-VI rainfall data (Seo 1998). Both are gridded datasets and were displayed using GrADS (Doty and Kinter 1995). Snowfall data was retrieved from the National Snow site

and from National Weather Service Public information statements. The 500 hPa heights, 850 hPa temperatures, 850 hPa winds, and other standard level fields were derived from the NCEP NAM and GFS data using the NCEP/NCAR (Kalnay et al. 1996) data as the climatological comparison. The means and standard deviations used to compute the standardized anomalies were from the NCEP/NCAR data as described by Hart and Grumm (2001). Anomalies were displayed in standard deviations from normal, as standardized anomalies. All data were displayed using GrADS (Doty and Kinter 1995). The standardized anomalies computed

Figure 4. The Climate Prediction Center (CPC) high latitude height anomalies and Arctic Oscillation for the period of October 2009-February 2010. Update this image on 9 Feb as daily shows -4AO 4,5,6 Feb.

as: 3. SD = (F – M)/σ ()

Where F is the value from the reanalysis data at each grid point, M is the mean for the specified date and time at each grid point and σ is the value of 1 standard deviation at each grid point. NCEP determinist model and ensemble data shown here were primarily limited to the GFS and NAM and the ensembles systems shown are the NCEP GEFS and

SREF. Displays will focus on the forecasts of the pattern conducive for precipitation and snowfall and products showing the amount and gradients of the QPF used to produce snowfall forecasts. For brevity, times will be displayed in day and hour format such at 06/0000 UTC signifies 06 February 2010 at 0000 UTC.

4. RESULTS

Figure 5. As in Figure 3 except showing 250 hPa winds (kts) and 250 hPa total wind anomalies.

i. Large scale overview Figure 3 shows 500 hPa heights and anomalies, representing the large scale pattern over North America during the storm. At 05/0000 UTC the 500 hPa low with -3SD height anomalies was over Mexico (Fig. 3a). It should be noted that this system produced heavy rainfall and flooding in Mexico and Mexico City1. 1 Reuters report http://www.reuters.com/news/pictures/slideshow?articleId=USRTR29VLE#a=1

The 500 hPa wave then moved across the Gulf of Mexico and then along the East Coast of the United States (Figs. 3b-3i). Height anomalies were not overly impressive with this upper-level wave. These data show that the upper-level wave originated over the Pacific Ocean. Other key features with the pattern include the high latitude height anomalies over Greenland and northern Canada (Figs. 3d-i) and a deep trough and negative height anomalies over the north Atlantic basin (Figs 3a-i). These features were present with 18-20 December 2009 storm (Fig. 4) and are

Figure 6. As in Figure 1 except GFS 850 hPa winds (kts) and u-wind anomalies.

common features associated with a negative NAO and AO (Fig. 4). Figure 5 shows the 250 hPa winds and wind anomalies over the United States for the period of 05/0000 UTC through 07/0000 UTC. These data show the trough and Pacific jet energy moving across Mexico and then up the East Coast with an implied coupled jet (Uccellini and Kocin 2004a and 1987) over the eastern United States by 06/0000 UTC through 06/1200 UTC. The strong (+3SD) wind anomalies pre-positioned over the northeastern United States (Fig. 5a-e) were the result of the

blocked flow over the northern Atlantic and contributed to maintaining the cold air over the eastern United States. These data imply that the strong southern wave and the phase and the high latitude blocking, and thus the negative AO and NAO were contributor factors associated with this storm. ii. Regional view The evolution of the surface cyclone and the pressure anomalies were shown in Figure 1. Figures 6 & 7 show the 850 hPa winds and

Figure 7. As in Figure 1 except for GFS precipitable water (mm) and precipitable water anomalies.

u-wind anomalies and the precipitable water and precipitable water (PW) anomalies. These data show that the initial cyclone move up the west side of the Appalachian Mountains and a secondary cyclone developed along the coastal plain (Figs. 1d-f). The strong anticyclone with +1SD pressure anomalies and the strong gradient between the two cyclone centers implied a long fetch easterly jet north of the cyclone. The cyclone deepened to 986 hPa by 07/0000 UTC and had -3 to -4 SD pressure anomalies. The significant area of heavy precipitation and snowfall fell in close proximity to the strong, and highly anomalous low-level 850 hPa jet (Fig. 7). By 05/1200 UTC there was a -3 to -4 SD negative wind anomaly over the Midwest just north of the 850 hPa low over eastern Tennessee (Fig. 7c). The jet strengthened and the wind anomalies reached -5SDs below normal over Indiana

by 06/0000 UTC. The 850 hPa jet strengthened as the 850 hPa low tracked up the Ohio Valley (Figs. 7c-e). At 06/0000 UTC a secondary wind maximum began to develop along the Mid-Atlantic Coast (Fig. 7e) and the anomalies fell to -5SDs below normal by 06/0000 UTC. Note that the 850 hPa low as still to the west, over Kentucky and West Virginia at this time (Figs. 7f-g). As the secondary low developed along the coast, the 850 hPa winds increased and some areas saw -6SD wind anomalies (GREEN) off the Delmarva Peninsula. This strong 850 hPa jet and large wind anomalies moved eastward and thus spared New York and southern New England. iii. Deterministic forecasts and QPFs Figures 8 & 9 show total accumulated QPF from the GFS and NAM respectively. The data are focused over the Mid-Atlantic

Figure 2. Snow precipitation (in) for the period ending 1200 UTC 7 February 2010. Map will be replaced soon.

region due to the tight gradient and the area of concern was primarily the Mid Atlantic region. All forecasts showed the same extreme sharp northern edge to the precipitation shield. The northern extent varied from run-to-run but was present in both forecasts system and the NCEP ensemble forecast systems (not shown). The unified precipitation data (Fig. 10) showed that the same sharp edge to the precipitation shield as forecast by the two NCEP deterministic models. Additionally, these relatively course data showed around 48 mm or more (~2 inches) in the Washington, DC and Baltimore areas. From spatial perspective the sharp northern gradient and the high QPF amounts predicted by the models were extremely valuable information. Granted there was uncertainty with some run-to-run variability but overall these were excellent forecasts. The QPF pattern suggests a relatively well predicted system. Thus brevity only the 850 hPa winds and u-wind anomalies from GFS are shown (Figure 11). These data clearly show the 850 hPa cyclone tracking just south of the region where heavy snow was predicted. Additionally, all 9 cycles show predicted anomalous 850 hPa jet with all nine cycles showing at least some areas where the u-wind anomalies were -5SD or lower. These forecasts suggesting a record low-level jet aimed at the Washington and Baltimore regions. With conditions cold enough for snow (not shown) and the dominate precipitation also being snow (not shown) the conditions suggest, based the work of Grumm and Hart (2001) and Stuart and Grumm (2006) this going to be a record and potentially historic event. iv. Ensemble forecasts This event was equally well predicted by the NCEP global and regional ensemble forecast systems. For brevity only a few forecasts are shown here, focusing on the QPF as the QPF would reflect large scale pattern.

Figure 12 shows the GEFS mean QPF from a series of forecasts initialized from 03/1800 UTC through 06/0600 UTC. Similar to the GFS and NAM, these forecasts showed the overall precipitation shield quite well from a spatial perspective. Additionally, these data too showed the sharp cut-off of the QPF, and thus snow on the northern edge of this system. Probabilistic forecasts from the SREF are shown in Figure 13. Only two forecast cycles are shown, but they capture the essence shown in previous forecasts including the sharp northern edge of the QPF shield, and the high probability of over 1 inch of QPF in the Washington, DC and Baltimore areas. Note there is a close 2 and at time 2.5 inch contour in these and other SREF forecasts not shown. Though not shown, the SREF to predicted a -5SD u-wind anomaly in the region where the heavy snow fell. v. Precipitation and snow fall Figure 10 showed the overall precipitation shield associated with this event from the UPD. These data show that higher precipitation amounts fell in Maryland, Virginia, and Delaware. The snowfall is shown Figure 2 and Figure 14. These data show what is obvious in the high resolution satellite imagery (Fig. 15) that the snow was concentrated in a narrow corridor and the sharp northern edge of the snowfall ran across north-central Pennsylvania, northern New Jersey, Staten Island and extreme southwestern Long Island. The shape is uncannily familiar to the NWP guidance.

5. CONCLUSIONS Storms with large anomalies of key fields often produce meteorologically and climatologically significant events. In this event, many locations received 33 to 150 percent of their seasonal snowfall.

Thus, in addition to being a meteorologically significant event producing heavy snowfall, this storm produced snowfall in many areas that was climatologically significant. For example, in the Baltimore area, the mean annual snowfall is around 17 inches. This storm produced 24.8 inches at the airport, more than a season’s worth of snowfall. Clearly, these few large snow events keep the climatology at Baltimore near 17 inches despite many winters where the seasonal snowfall is below 10 inches and in some years only a trace of snowfall has been observed. This storm was relatively well predicted by the NCEP deterministic model and ensemble prediction systems. One serious forecast issue with this storm was the sharp gradient in the QPF which implied a sharp drop of in snow fall north of the storm system. In retrospect this feature was well predicted in close proximity to the observed location. Similar highly successful forecast of historic storms have been made in the past (Uccellini et. al 1995). During the March 1993 Superstorm, the operational NCEP global model successfully predicted a deep cyclone and the potential for heavy snowfall 2-3 days before the event. These forecasts were relatively successful. From an anomalies perspective, this event had some of the most negative low-level u-wind anomalies yet observed in a major winter storm. With areas of -6SD anomalies along the Mid Atlantic coast being some of the lowest values observed in a snow storm. In addition to these observed values (Fig. 6). Based on the forecasts of 850 hPa u-wind anomalies in the GFS (Fig.10) and with conditions cold enough for snow and the dominate precipitation also being snow,

based the work of Grumm and Hart (2001) and Stuart and Grumm (2006) this going to be a record and potentially historic event. The anomalies worked well in aiding in identifying a meteorologically and climatologically significant event. The NCEP higher resolution deterministic models did extremely well with the QPF shield with this event (Figs. 8 & 9). Relative to verification (Fig. 10) and from spatial perspective the sharp northern gradient and the high QPF amounts predicted by the models were extremely valuable information. The high QPF and thus heavy snow in Washington and Baltimore was a relatively easy forecast. But across Pennsylvania to Long Island, the uncertainty with the gradient was problematic and a far more challenging forecast issue. Clearly, one could foresee using higher resolution models, and then adjusting spatially and temporally to derive useful uncertainty and probability information. Clearly, forecast no to heavy snow in a deterministic sense was extremely difficult across northern Pennsylvania, northern New Jersey, and Long Island. Probabilistic forecast products would have made these forecasts less tedious. Finally, this event was relatively well predicted several days in advance. Not all storms are quite as well predicted. There still is uncertainty to deal with, hopefully forecasters will not be lulled into this single great success story like many were after the great success of the “Superstorm” of 1993. Somewhat short predictability horizon storms such as the surprise storm of 24-25 January 2000 (Zhang et al 2002) are still out there in our future.

6. Acknowledgements Snowfall records were summarized by Ernie Ostuno for use in this document. Snowfall

amounts from multiple National Weather Service Offices. Discussions, ideas, and forecast tidbits from the map blog. A thanks Lance Bosart for getting me on the blog.

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Figure 8. GFS total accumulated precipitation (mm) valid for 0000 UTC 7 February 2010. Data from GFS forecasts initialized at a) 0000 UTC 4 February 2010, b) 0600 UTC 4 February 2010, c) 1200 UTC UTC 4 February 2010, d) 1800 UTC UTC 4 February 2010, e) 0000 UTC 5 February 2010, f) 0600 UTC 5 February 2010, g) 1200 UTC 5 February 2010, h) 1800 UTC 5 February 2010, and i) 0000 UTC 6 February 2010. The model start times were used to accumulate thus forecasts on the 6th are lower as the event commenced on the 6th. No significant precipitation fell after 0000 UTC on the 7th so image through 1200 UTC were not used. Return to text.

Figure 9. As in Figure 8 except for NAM forecasts. Return to text.

Figure 10. Unified precipitation data showing total accumulated precipitation (mm) from 1200 UTC 5 February through 1200 UTC 7 February 2010. Return to text.

Figure 11. As in Figure 8 except showing GFS forecasts of 850 hPa winds (kts) and u-wind anomalies (standard deviations from normal) valid at 0600 UTC 6 February 2010. Return to text.

Figure 12. As in Figure 8 except showing NCEP GEFS ensemble mean QPF (mm) valid at 0000 UTC 7 February 2010 from forecasts initialized at a) 1800 UTC 3 February 2010, b) 0600 UTC 4 February 2010, c) 1800 UTC UTC 4 February 2010, d) 0000 UTC UTC 5 February 2010, e) 0600 UTC 5 February 2010, f) 1200 UTC 5 February 2010, g) 1800 UTC 5 February 2010, h) 0000 UTC 6 February 2010, and i) 0600 UTC 6 February 2010. Return to text.

Figure 13. SREF forecasts of the probability of 1.00 inches of QPF in 24 hours for the period ending at 0000 UTC 7 February 2010. Forecast initialized (left) at 0300 UTC 5 February and (right) 2100 UTC 05 February 2010. Upper panels the probability of 1 inch or more QPF and he mean 1.00 inch line, lower panels show the ensemble mean QPF (shaded) and each member 1.00 inch contour. Return to text.

Figure 14. Snow fall (inches) from public snow reports. Values are color coded with cyan for 1-10 inches, blue for 10-20 inches, red for 20-30 inches and purple for value over 30 inches. Return to text.

Figure 15. High resolution MODIS satellite imagery showing snowfall on Sunday 7 February 2010. Some of the white in the western mountains is cloud cover. Image from MODIS site courtesy of Aaron