1. Overview Mid-Atlantic Ice Storm 4 March 2015 By Richard H. Grumm National Weather Service State College, PA A large 500 hpa ridge over the southern United States (Fig. 1a-e)) built into the Mid-Atlantic region ahead of a strong 500 hpa wave. Near the surface (Fig. 2) a strong anticyclone was present at 0000 UTC 3 March (Fig. 2a). This anticyclone was associated with low-level cold arctic air (Fig. 3a) which was rapidly retreating to the east (Fig. 3b-e) as was the strong surface anticyclone (Fig. 2b-e). The 850 hpa temperatures, near -12C over much of central Pennsylvania (not shown) would rise to near 4C by 0000 UTC 4 March 2015 and remain above freezing until after 1800 UTC 4 March 2015. The rapid rise in the 850 hpa temperatures was associated with a strong southerly jet as shown by the 850 hpa winds and v-wind anomalies (Fig. 4). This period of rapid warm advection was associated with period of precipitation over central Pennsylvania (Fig. 5). The higher precipitation amounts fell in west-central Pennsylvania and as snow, sleet, freezing, and eventually rain in most central areas. The combined ice accumulations in State College were around 1 inch though the actual glaze due to freezing rain was generally lower than 0.25 inches. The METAR observations at State College indicated cold dry and clear skies 0953 UTC 3 March which gave way to clouds thereafter. Snow arrived around 1553 UTC. The snow turned to freezing rain and ice pellets around 0033 UTC 1. The temperature did not rise above freezing until around 0815 UTC on 4 March 2015: METAR KUNV 030955Z AUTO 00000KT 10SM CLR M10/M14 A3036 RMK AO2 T11031137 METAR KUNV 031553Z 20004KT 1/2SM -SN OVC010 M06/M10 A3025 METAR KUNV 040033Z 18006KT 3SM -FZRA OVC014 M02/M04 A2981 = METAR KUNV 032353Z 19008KT 2SM PL BR OVC016 M03/M05 A2984 = METAR KUNV 032253Z 18008KT 1 1/2SM PL BR OVC021 M04/M06 A2985 = METAR KUNV 040815Z AUTO 00000KT 2 1/2SM UP OVC018 01/00 A2973 RMK = METAR KUNV 050335Z AUTO 29011KT 10SM -SN SCT025 BKN050 OVC060 01/M02 A2998= Light rain or drizzle with temperatures in the 2 to 3 C range persisted through 2100 UTC. The cold air did not return until 2235 UTC when light snow was reported. West of the mountains, the low-level cold air did not remain in place. This lead to rain and temperatures reaching into the 40s with significant snow melt and ice jam flooding. 1 ice pellets and freezing rain likely began in the 22 to 23 UTC timeframe within 1-3 miles of the METAR. It is unclear why these observations do not reflect this. Personal observation while cross country skiing 2200 to 2330 UTC and photos available of the glaze.
This paper will document the warm advection precipitation event of 3-4 March 2015. The focus is on the forecasts of the precipitation type and how forecasts and predictions of this event were conveyed by the SREF and HRRR. A forecast here is considered less confident and more probabilistic while a prediction is considered to be more precise and to cover a very short time scale. 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). Data were displayed using GrADS (Doty and Kinter 1995) and Python. For storm-scale details the 00-hour analysis from the hourly NCEP HRRR was used (see Figure 2). The precipitation was estimated using the Stage-IV precipitation data in 6-hour increments to produce estimates of precipitation during the event in 6, 12, 24 and 36 hour periods. Snowfall was retrieved from National Snow Analysis website. The NCEP SREF 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. Forecasts i) SREF The probability of 12.5 mm or more QPF (Fig. 6) and the SREF mean QPF and each members 12.5 mm (Fig. 7) contour show that the SREF forecast near 12.5 of QPF or more over most of Pennsylvania from 1500 UTC 3 through 1200 UTC 5 March 2015. Though not shown, the SREF forecast a high probability of over 25 mm of QPF in southwestern Pennsylvania and thus the mean QPF (Fig. 7) showed a large area covered by the 25mm contour. Forecasts from 0300 UTC 2 March through 1500 UTC 2 March actually had a 35 mm contour in western Pennsylvania. Lower amounts were forecast at shorter forecast horizons. The SREF 850 hpa temperatures forecasts indicated that surge of warm air would over all of western and most of eastern Pennsylvania by 0000 UTC 4 March 2015 (Fig. 8). Thus the SREF precipitation type (Fig. 9) showed nearly all the QPF in western Pennsylvania would fall as rain. To the north and east lower rainfall amounts were indicated and lower probabilities. The snow (not shown) was mainly forecast over north-central Pennsylvania and southern New York with amount generally under 1 inch. No snow was forecast across the southern third of the State.
The freezing rain forecasts for the period (Fig. 10) suggested around 0.25 inches of ice was possible though the probability was generally low. The 3-hourly values showed higher probabilities. These images show the probability for all 3-hour periods and the mean freezing rain accumulations in all the members over the period of time. The sleet forecasts (not shown) were quite similar and the combined sleet and freezing rain probabilities were on the order of 60% over portions of north-central Pennsylvania. The bottom line was that the SREF forecast a prolonged period of precipitation with the 2m and surface temperatures at or below 0C. ii) HRRR Hourly updated HRRR forecasts of 2m, 850, and 700 hpa temperatures valid at 2300 UTC3 March 2015 (Fig. 11-13) show that successive forecasts from as early as 0800 UTC 3 March indicated that the cold air, with 2m temperatures below 0C would remain in entrenched east of the mountains while warm air would stream in at both 850 and 700 hpa. West of the Appalachian Mountains, the 2m temperatures were well above 0C by 2300 UTC. The warm air aloft impacted the precipitation type forecasts in the HRRR. The 1200 UTC HRRR forecasts valid 1500 through 2000 UTC (Fig. 14) show the onset of snow (Blue) with freezing rain (yellows), ice pellets (Reds) and rain (Greens) moving in from the southwest. The hourly forecasts showed the rapid progression of the transition to frozen and rain moving in from the southeast. An examination of updated forecasts (Fig. 15) all valid at 2300 UTC showed the retreating snow and region of freezing and frozen precipitation over the Appalachian Mountains. The timing of the transitions was relatively well predicted in central Pennsylvania by the HRRR. Similar to the SREF, the HRRR focused the maximum QPF to near the boundary and the transition zone from freezing to liquid precipitation (Fig. 16). The 2300 UTC 3 March 2015 HRRR cycle (Fig. 17) indicated that the low-level cold air would remain in place of a significant period of time. It thus produced a prolonged period of sleet and freezing rain over eastern Pennsylvania through 0400 UTC. Sub-freezing areas existed though the model produced no QPF to type after 0500 UTC.. 4. Conclusions A significant cold air damming and mixed precipitation event occurred over northern and central Pennsylvania on 3-4 March 2015. This event was somewhat dwarfed by the significant snow event of 4-5 March 2015 which affected a broad region of the United States from the south into the Ohio Valley in Mid-Atlantic region. However, this event did produce significant ice, canceled schools, and disrupted transportation on a more regional and local scale. Additionally, this event was relatively well predicted and showed the value of using ensembles and high resolution models during complex issues related to mixed precipitation and cold air damming.
The NCEP SREF and HRRR both performed well during this event. The SREF clearly showed the potential for a modest precipitation event and the potential for warm air overrunning cold air. Thus the SREF produces rain in southwestern Pennsylvania and a high probability of both sleet and freezing rain in central and northern Pennsylvania. At shorter ranges, the HRRR showed the precipitation and cold air damming issues. The HRRR also provided the potential to time and provide relatively precise, within a few hours, information related to the freezing rain event. In this event, the HRRR showed and displayed the potential snow to rain transition rather well. Though not shown here, the HRRR showed a brief period of snow in State College before the snow went to sleet and freezing rain. The 2m temperatures remained below 0C few hours longer than implied by the HRRR forecast. During this event and experience with 6 other events in the November 2013 through March 2015 suggest that as good as the HRRR is at showing trapped cold air in valleys and east of the Appalachian mountains, the model is typically too fast in eroding the cold air. The error is typically on the order of a few hours but new model runs come in aiding in forecasting the potential prolonged persistence of the cold air 5. Acknowledgements: The Pennsylvania State University for data access. 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., 209-219. DeGaetano, A. T., M. E. Hirsch, and S. J. Colucci. 2002. Statistical prediction of seasonal East Coast winter storm frequency. Journal of Climate 15:1101 17. 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, 2753 2769. Stuart,N.A and R.H. Grumm 2006: Using Wind Anomalies to Forecast East Coast Winter Storms. Wea. and Forecasting, 21,952-968. 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, 676-684.
Figure 1.. CFSRV2 500 hpa heights and height anomalies in 24 hour increments from a) 0000 UTC 1 March through f) 0000 UTC 6 March 2015. {Point latitude and longitude of black do is 40.7914 N, 77.8586. Return to text.
Figure 2. As in Figure 1 except for 250 hpa wind vectors and 250 hpa u-wind anomalies. Return to text.
Figure8.gs 15Z03MAR2015 12Z04MAR2015 12.5 PA 24 SREF SREFMAR2015 POP 40.79-77.85 BLACK Figure 3. As in Figure 2 except for 850 hpa temperatures ( C) and 850 hpa temperature anomalies from a) 0600 UTC 3 March through f) 1200 UTC 4 March 2015. Return to text.
Figure 4. As in Figure 3 except for 850 hpa winds and 850 hpa v-wind anomalies. Return to text.
Figure 5. Accumulated rainfall (mm) in 6-hour increments from a) 1200 UTC 3 March through e) 1200 UTC 4 March 2015 and f) the total accumulated precipitation during the period ending at 1200 UTC 4 March 2015.. Return to text.
Figure 6. NCEP SREF forecasts of QPF to equal or exceed 12.5 mm and the ensemble mean position of the 12.5mm contour for the period from 1500 3 March through 1200 UTC 4 March 2015. SREF forecasts initialized at a) 0300, b) 0900, c) 1500, and 2100 UTC 2 March and e) 0300 and f) 0900 UTC 3 March 2015. Black dot is State College, PA. Return to text.
Figure 7. As in Figure 6 except for ensemble mean QPF and each members 12.5 mm contour. Return to text.
Figure 8. As in Figure 6 except for SREF 850 hpa temperature forecasts valid at 0000 UTC 4 March 2015. Value in degrees C shading shows the standardized anomalies. Return to text.
Figure 9. As in Figure 6 except for SREF probability of rain and the total rain QPF in inches. Return to text.
Figure 10. As in Figure 9 except for SREF Freezing mean freezing rain QPF and the probablity of freeziing rain averaged over the 21 hour period. Return to text.
Figure 11. HRRR forecasts of 2m temperatures valid at 2300 UTC 3 March 2015 from HRRR initialized at a) 1400, b) 1300, c)1200, d) 1000, e) 0900 and f) 0800 UTC 3 March 2015. Contours every 2C and shading is value less than 0C. Return to text.
Figure 12. As in Figure 11 except for 850 hpa temperatures. Return to text.
Figure 13. As in Figure 11 except for 700 hpa temperatures. Return to text.
Figure 14. HRRR forecasts of 2m temperatures ( C) and precipitation type (color key) from the 1200 UTC 3 March HRRR showing the conditions forecast at a) 1500 through f) 2000 UTC. Return to text.
Figure 15. As in Figure 14 except for all forecasts valid at 2100 UTC from HRRR initialized at a) 0800, b) 0900, c) 1000, d) 1200, e) 1300, and f) 1400 UTC 3 March 2015. Return to text.
Figure 16. As in Figure 15 except for total accumulated QPF during the time from 1400 through 2300 UTC 3 March 2015. Return to text.
Figure 17. As in Figure 14 except for 2300 UTC HRRR with forecasts valid hourly from a) 0000 UTC through f) 0600 UTC 4 March 2015. Return to text.