Eastern United States Winter Storm of 1-2 February 2015-DRAFT Northeast Ground Hog Storm

Transcription

1 Eastern United States Winter Storm of 1-2 February 2015-DRAFT Northeast Ground Hog Storm By Richard H. Grumm National Weather Service State College, PA 1. Overview A major Winter Storm brought precipitation from the Great Lakes into the eastern United States primarily from 1-2 February 2015 (Fig. 1). North of the primary cyclone track heavy snow and blizzard conditions were observed. Heavy snow was observed northern Illinois and southern Wisconsin across northern Indian and southern Michigan on 1 February. Chicago s O Hare airport received 20.2 inches of snow, the 4 th largest snowfall observed at the site. The much of the region of heavy snow was buffeted by strong winds with gusts between 50 and 60 mph. This resulted in snow drifts in the range from 2 to 5 feet (WFO-Chicago). The strong winds were associated with a strong gradient between a strong anticyclone to the north and west and surface cyclone tracing into the Midwest (Fig. 2a-d). As the storm moved into the Mid-Atlantic region, (Fig. 2e-f) a secondary cyclone developed along the coast. The Miller-B type cyclone evolution with the primary cyclone moving into western Pennsylvania and southwestern New York (Fig. 3) allowed warm air to change the snow to rain over most of Pennsylvania, dramatically limiting snow amounts. Based on hourly HRRR 0-hour forecasts, the secondary cyclone developed along the coastal plain in southern New Jersey (Fig. 3d-e). This secondary cyclone became an East Coast Winter storm (ECWS: DeGaetano et al. 2002) producing rain, a wintry, mix and heavy snow from Long Island into New England. The total precipitation for this phase of the event (Fig. 4) indicated that the higher QPE amounts occurred just north of the cyclone center from eastern Pennsylvania across Long Island. Some locally higher QPE was observed over southeastern New England and eastern New York. Most of the precipitation fell as rain in the 32 to 48 mm QPE band along the coast. Most inland areas received snow with heavier snow amounts in eastern Massachusetts. The Miller-B storms in the Mid-Atlantic region typically allow for an intrusion of warm air ahead of the primary storm. Thus, despite some optimistic snow forecasts across central Pennsylvania, the surge of warm air create periods of sleet, freezing rain and rain. This severely limited snowfall. It will be shown that the NCEP SREF and GEFS both indicated that the track of the primary cyclone might be farther north and that the surge of warm could severely limit snow amounts. Ensemble forecasts suggested uncertainty with the cyclone track, surge of warm air, and forecast precipitation types. Despite this uncertainty human produced forecasts (not shown) projected relatively high snow across the region. This paper will document the pattern and anomalies associated with the winter storm of 1-2 February Section 3 will focus on the pattern and standardized anomalies to put the event

2 into context. Section 4 will present some of the uncertainty information provided by the NCEP EFS suite. The human forecasts, which were based on a mix of models and the European Center forecast system were not readily available and are not presented here. 2. METHOD AND DATA The large scale pattern was reconstructed using the Climate Forecasts System (CFS) as the first guess at the verifying pattern. The standardized anomalies were computed in Hart and Grumm (2001). All data were displayed using GrADS (Doty and Kinter 1995). For storm-scale details the 00-hour analysis from the hourly NCEP HRRR was used though most times 3 hourly increments are display, hourly data was examined (see Fig. 3). The precipitation was estimated using the Stage-IV precipitation data in 6-hour increments to produce estimates of precipitation during the even in 6, 12, 24 and 36 hour periods. Snowfall was retrieved from National Snow Analysis website. Snowfall data was obtained from both NWS public information statements and the National Snow site. The NCEP SREF and GEFS were retrieved and examined in real-time and archived locally. These data helped identify the different predictability horizons of the forecast systems. The NCEP EFS data may not reflect public forecasts or perceptions of the forecasts. Many forecasters use a diverse set of forecast tools and often lean on the European Center model and post processed forecast data. 3. Pattern overview The evolution of the 500 hpa pattern (Fig. 5) implied fast northwestern flow over western North America at 0000 UTC 31 January This strong flow was in the strong gradient between the strong ridge to the southwest and the trough to the east. Confluence developed over the eastern United States and a short-wave came over the ridge. This wave was evident at 0000 UTC 1 February (Fig. 5c) and it deepened as it moved eastward. The height anomalies in the wave were on the order of -1s below normal. As the short wave moved into the plains a surface cyclone developed and intensified as it moved eastward (Fig. 2). Typical of most Mid-Western and eastern snow storms, an area of high pressure was present north and extended east of the surface cyclone. South of the surface anticyclone a strong baroclinic zone developed (Fig. 6). This baroclinic zone was in close proximity to the confluent 500 hpa flow (Fig. 5) and implied exiting jet entrance region to the west. A strong easterly jet developed on the cold side of this boundary (Fig. 7). The strong 850 hpa winds and -3 to -4s u-wind anomalies lined up relatively well with frontal circulation and the region of heavy snowfall in the Mid-West. The concept of heavy snowfall with strong u 850

3 hpa winds as defined by Stuart and Grumm (2006) worked well during this event in both the Mid-West (Fig. 7b-d) and southern New England (Fig. 7e). In Pennsylvania, as the primary low tracked into western Pennsylvania, it allowed for a surge of warm (Fig. 8) into the State. This limited snowfall. This scenario, with the primary low tracking into the State with a secondary cyclone developing along the coast is one of the primary winter event types associated with mixed precipitation events. A primary low tracking into western and central Pennsylvania is a climatologically known scenario which limits snowfall with a significant percentage of these events. It will be shown in the following section that the SREF forecast the cyclone to track into Pennsylvania and how far was indicated in the spread north of the cyclone center. The SREF also showed the surge of 0 to +2 C airs at 850 hpa into northcentral Pennsylvania, conditions favoring a mixed precipitation event. 4. Ensemble Forecast There are nearly an inexhaustible number of model and ensemble forecasts which could be presented. Using a single model at longer ranges is a fool s errand. Thus the focus here is on ensemble forecasts. Additionally, forecast indicated 8-10 inches of snow across Pennsylvania on 30 January are a good starting point for this section which will emphasize the uncertainty information which may have tempered forecasts which may have been biased by the EC and EC- EFS which are not shown here. i) SREF forecasts Forecasts form 6 NCEP SREF initialized on 30 and 31 January 2015 show the approximate event total probability of 25 mm or more QPF. The time window covers the close to the onset time snow at Chicago O Hare and toward the end of significant snowfall at Boston Logan airports. These SREF probability forecasts indicated that over the Midwest, the axis of high QPF slowly shift northward. Initially (Fig 9a), the axis of higher QPFs was focused south of Chicago but slowly moved northward and matched well with the QPE (Fig. 1). The probability of 25 mm or more QPF over central Pennsylvania was quite variable. Farther east and like the Midwest, as the predictability horizon shortened the QPF shield shifted farther to the north. The SREF mean QPF and each members 25 mm contour during the peak time of the event over the Midwest (Fig. 10) and East (Fig. 11) show the uncertainty and the areas where the SREF anticipated the higher QPF. These data also show the northward shift in the QPE shield with the highest QPFs focused in the New York and New Jersey area. The SREF precipitation type issues (Fig 12) over central Pennsylvania were the result of uncertainty with the cyclone track and how far north the warm air would surge over the region. The precipitation type issues are summarized in the SREF plumes over State College, PA. The

4 snowfall in State College was forecast to range from 6 to 10 inches. Locally about 3-4 inches was observed with about 0.61 inches of liquid equivalent. Sleet, freezing rain and rain limited the snowfall totals. Despite this heavy snow was observed about 50 miles to the north. The SREF precipitation type plumes all showed the potential for rain, freezing rain and ice pellets. The coldest run in the set was the 1500 UTC 30 January run and from 2100 UTC 30 January onward, warm air at 850 hpa and at 2m indicated a strong probability of rain and freezing rain. These forecasts indicated the potential for limited snowfall. The timing of the warm air aloft varied from run-to-run. The plan view of SREF 850 hpa and surface forecasts valid at 0900 UTC 2 February (Fig. 13 & 14) show the 0C isotherm at 850 hpa penetrating into central Pennsylvania. The spread in the 850 hpa temperatures and surface isobars was focused north of the surface cyclone. The spread at longer range 850 hpa temperatures over New York and northern Pennsylvania was 4 to 6C and 3 to 4C over a large portion of Pennsylvania and New York. The spread slowly came down with time as the SREF trended, in the mean, to push the warm air and track the surface cyclone farther north. The probability of the 850 hpa temperatures remaining at or below 0C at 850 in central Pennsylvania varied from 30 to 50% (not shown) implying a good chance of mixed precipitation. These precipitation type issues were reflected in the SREF snow precipitation type forecasts (Fig. 15) which showed higher probabilities in northern Pennsylvania and southern New York. These 12 hour forecasts ending at 1200 UTC 2 February also indicated that 5 to 7 inches of snow; using a 10:1 ratio; were limited to northern Pennsylvania and southern New York. Note that as the forecast length decreased the warmer solutions pushed the snowfall farther north into New York. ii) GEFS forecasts The GEFS forecasts were similar too but slightly colder than the NCEP SREF. Thus the GEFS and presumably the EC-EFS likely were used to produce more robust forecasts of snow. The near storm total QPF with the 25mm and greater QPF probability is shown in Figure 16. These data cover the same timeframe as the SREF data (Fig. 9). The corresponding regional mean QPF and 25 mm by members is shown in Figures 17 & 18. The GEFS 25 mm POP 1 shows that forecast issued on an prior to 1200 UTC 29 January did not indicated a high probability of significant QPF far enough north to affect Chicago and southern New York. With the potential storm coming up the coast Boston was on the edge of a significant event before Chicago (Fig. 16a-c). The forecasts issued on 30 January showed a shift to the north and west of the entire QPF shield and by 31 January it was quite clear that a significant precipitation event was likely from Chicago to Boston (Fig. 16e-f). 1 Note that these probabilities are derived from uncalibrated GEFS ensemble probability distribution functions and are not a statistical computation using regression. They should NEVER be used like a MOS POP.

5 The GEFS cyclone and 850 hpa temperature forecasts showed considerable uncertainty. There was significant spread north of the surface cyclone from forecasts issued form 0000 UTC 29 January through 0000 UTC 30 January. The spread decreased rapidly thereafter. During the period of high spread, the GEFS converged on a cyclone developing along the East Coast, well south and east of the verifying position. Also, a common error when no blocking high is to the north. Subsequent forecasts trended the cyclone track to the west as the spread decreased. The 850 hpa temperatures had very high spread too. Similar to the SREF the GEFS had precipitation type issues due to a few warm members. Though using on the ensemble mean it was not until the forecasts from 0000 UTC 31 January did the GEFS bring the mean position of the 0C contour into Pennsylvania. Several GEFS members were warmer and though not shown snow was not forecast in 100% of the GEFS members. Due to timing issues, mixed precipitation and rain members, the GEFS too was slow to converge on the region for heavy snowfall (Fig. 21). The GEFS did forecast the axis of heavy snow farther south than the comparable SREF forecasts. These longer range GEFS forecast suggest there were some serious predictability horizon issues in the forecasts. Despite large spread and high uncertainty rather specific human based snow forecasts were issued. 5. Conclusions A winter storm produced heavy snow from the Illinois eastward across northern Indiana, Michigan, northern Pennsylvania and into New England. South of the region of heavy snow a region of snow and freezing rain was observed. Forecasts from NCEP models indicated that uncertainty with the storm track, where the secondary cyclone would develop, and how far north the 850 hpa 0C would surge could impact where the snow fell. Simplistic mean spread in the SREF and GEFS and precipitation forecast aided in defining areas where higher QPF amounts, and where cold enough, snow might fall. The SREF precipitation type forecasts indicated that over central Pennsylvania, the snow would likely mix with or change to rain. This limited the total snowfall forecast by the SREF. These forecasts were inconsistent with forecasts which were produced by a variety of outlets which indicated from 6 to 10 inches of snow in areas were snowfall totals averaged below 4 inches. In the cold air and in the region of strong easterly flow north of the cyclone track, heavy snow was observed. The heavy snow in the western Great Lakes was well aligned it the strong lowlevel easterly jet. In Pennsylvania, when surface cyclones move into western and central Pennsylvania, warm air aloft often transitions the snow to ice pellets, freezing rain, or rain. This event followed that relatively climatologically know pattern. Once the secondary cyclone developed along the coast it shut-off the warm air.

6 Though not shown here, the secondary cyclone developed farther south than indicated in several forecast systems. This created a snow to rain and back to snow scenario as far south as Long Island. Farther north, most of interior New England and eastern New York had heavy snow. And similar to the heavy snow in the Midwest, the higher QPE and snowfall totals were observed near the strong low-level easterly wind anomalies. 6. References Doty, B.E. and J.L. Kinter III, 1995: Geophysical Data Analysis and Visualization using GrADS. Visualization Techniques in Space and Atmospheric Sciences, eds. E.P. Szuszczewicz and J.H. Bredekamp, NASA, Washington, D.C., DeGaetano, A. T., M. E. Hirsch, and S. J. Colucci Statistical prediction of seasonal East Coast winter storm frequency. Journal of Climate 15: Kahneman, D, 2011: Thinking Fast Thinking Slow. Farrar,Straus, and Giroux, NY,NY. 511pp. Kalnay, Eugenia, Stephen J. Lord, Ronald D. McPherson, 1998: Maturity of Operational Numerical Weather Prediction: Medium Range. Bull. Amer. Meteor. Soc., 79, Roebber, P.J., M.R. Butt, S.J. Reinke and T.J. Grafenauer, 2007: Real-time forecasting of snowfall using a neural network. Wea. Forecasting, 22, Stuart,N.A and R.H. Grumm 2006: Using Wind Anomalies to Forecast East Coast Winter Storms. Wea. and Forecasting, 21,

7 Figure 1. Stage-IV estimated precipitation form 0000 UTC 1 to UTC 3 February Return to text.

8 Figure 2. CFSR-V2 surface pressure and pressure anomalies in 6 hour increments from 1200 UTC 01 February through 1800 UTC 2 February Return to text.

9 Figure 3. As in Figure 2 except for HRRR 0-hour forecasts in 3-hour increments from a) 0600 UTC to e) 2100 UTC 2 February Return to text.

10 Figure 4. As in Figure 1 except for the focused over the East Coast. Most of the QPE fell after 0000 UTC 2 February Return to text.

11 Figure 5. As in Figure 2 except for CFSR-V2 500 hpa heights and height anomalies in 12-hour increments from a) 0000 UTC 31 through f) 1200 UTC 12 February Return to text.

12 Figure 6. Return to text.

13 Figure 7. Return to text.

14 Figure 8. As in Figure 4 except for HRRR 850 hpa temperature analysis and anomalies in 3-hour increments from 0600 UTC through 2100 UTC 2 February Return to text.

15 Figure 9. NCEP SREF probabilities of 25 mm or more QPF for the period of 0000 UTC 1 to 0000 UTC 3 February 2015 from SREF initlaized at a) UTC 30 January, b) 2100 UTC 30 January, c) 0300 UTC 31 January, d) 0900 UTC 31 January, 1500 UTC 31 January and 2100 UTC 31 January The mean 25 mm contour is shown in thick black. Return to text.

16 Figure 10

17 Figure 11. Return to text.

18 Figure 12. NCEP SREF plume diagrams of precipitation and precipitation accumulation by type for 0900, 1500, 2100 UTC SREF initialized on 30 and 31 January 201 Return to text.

19 Figure 13. As in Figure 11 except for SREF ensemble mean 850 hpa temperatures and the spread about the mean valid at 0900 UTC 2 February Return to text.

20 Figure 14. As in Figure 13 except for SREF mean sea-level pressure. Return to text.

21 Figure 15. SREF snow precipitation and the probability of snow with the mean snowfall (inches). Return to text.

22 Figure 16. Return to text.

23 Figure 17. Return to text.

24 Figure 18. Return to text.

25 Figure 19. Return to text.

26 Figure 20. Return to text.

27 .Figure 21. Return to text.

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