Eastern United States Anafrontal Snow 4-5 March 2015-Draft By Richard H. Grumm National Weather Service State College, PA 1. Overview A 500 hpa ridge over the western Atlantic (Fig. 1) and an approaching shortwave over North America produced a significant wintry precipitation event from the Ohio Valley to the Mid- Atlantic region on 4-5 April 2015. The strong gradient between the two systems was associated with a strong 250 hpa jet and anomalous 250 hpa u-wind anomalies, on the order of 3 to 5σ above normal (Fig. 2). The verifying quantitative precipitation estimate (QPE: Fig. 3) showed that the axis of higher precipitation and heavy snow was well aligned with the strong jet and jet entrance region. At the surface (Fig. 4) a trough was present between the retreating anticyclone over the western Atlantic and approaching arctic anticyclone to the west (Fig. 4). Similar to the deformation snow of 20-21 February 2015, this snow event was not associated with a significant area of low pressure (Grumm 2015). An unseasonably cold air mass was associated with the arctic anticyclone. On the cold side of the frontal boundary, the 850 hpa temperatures (Fig. 5) were abnormally cold, and about 100 km ahead of the frontal boundary, the 850 hpa temperatures were about 1σ above normal. The point in Figures 3-4 is Louisville, KY which was on the warm side of the boundary at 1200 UTC 4 March and observed rain. Shortly after 0000 UTC 5 March, the 850 hpa temperatures fell below 0 C and heavy snow developed over Kentucky. Areas around both Louisville and Lexington would receive around 14 inches of snow. At least one location in eastern Kentucky had around 24 inches of snow. Dissimilar to many heavy snowfall events in the eastern United States (Stuart and Grumm 2006), this event was not associated with a strong surface cyclone or with a strong low-level easterly jet with significant u-wind anomalies (Fig. 6). The 850 hpa v-winds (not shown) also indicated modest southwesterly flow on the warm side of the frontal boundary. This event was a classic example of an anafront 1 with most of the precipitation falling on the cold side of the frontal boundary. The strong 250 hpa jet (Fig. 3); the frontal zone (Fig. 5) between the two anticyclones (Fig. 4); with the cold air from the arctic anticyclone under-cutting the warm air to the south and the jet entrance region (Figs. 3 & 5). The advancing cold air and warm air moving into the jet entrance region produced a nearly textbook anafront which produced a high impact winter storm from the Ohio Valley to the Mid-Atlantic region. To the north, a stronger cyclone was present in Canada in the jet exit region and a weaker surface 1 This system was identified by many sources as an anafront including Jon Nese at the PSU Map discussion on 5 March and within the Albany MAP. Many descriptions were shared within both communities.
cyclone was present in the jet entrance region (Fig. 4). The strong southerly flow and jet brought a surge of deep moisture (Fig. 7). The precipitable water (PW) anomalies were on the order of 2 to 3σ above normal on the warm side of the frontal boundary. The strong jet, frontal zone, and high PW likely contributed to this meteorologically significant late season snow. It should be noted that after the snow ended and the large anticyclone settled over the eastern United States, many record low temperature records were set or tied on 6 and 7 March 2015. Harrisburg, PA 2 set the all-time low for the month of March on 6 March 2015. The combination of over 6 inches of fresh snow, arctic air and a massive anticyclone produced nearly ideal radiational cooling conditions in southeastern Pennsylvania. This arctic air mass would also be the last of a series of such air masses to affect the eastern United States during the winter of 2014-15. This paper will document the pattern and anomalies associated with the anafrontal snow event of 4-5 March 2015. Section 3 section will examine the forecasts produced by the NCEP ensemble forecast systems. 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. Snowfall data was obtained from both NWS public information statements and the National Snow site. The NCEP GEFS and 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 2 RERs were sent 6 and 7 March as Harrisburg set a record low for the month on 6 March of -1 F. The previous record of +8 F was set in 1890. The low of -1 F was tied on 7 March 2015.
It will be shown that the pattern associated with this event was generally well-predicted by the NCEP ensemble forecasts systems (EFS). These relatively successful forecasts included useful QPFs. The choice between standardized anomalies were shown here in lieu of mean spread charts. This masks some of the uncertainty in the forecasts but it would be prohibitive to show both.. It should be noted there was considerably spread in the temperature and mean sea-level pressure fields which produced some issues with the amount and location of the heavy snowfall. i) NCEP-GEFS The NCEP GEFS forecasts from 6 GEFS cycles show the forecasts of the 250 hpa jet (Fig. 8), the 850 hpa isotherms, the probability of 25 mm or more QPF, and the mean QPF with each members 25 mm contour. These forecasts suggest that the from 1200 UTC 27 February through 0000 UTC 4 March, the GEFS was able to capture the strong 250 hpa jet and the +4 to +5σ u- wind anomalies within the jet core (Fig. 8). At 850 hpa (Fig. 9) the GEFS forecasted the strong frontal zone and had some timing issues with the rain/snow transition as the 0 C isotherm moved south and east of the Ohio Valley and a point near Louisville, KY. The overall pattern at 850 and 250 hpa was well aligned with the CFSRV2 analysis. The probability of 25 mm or more QPF (Fig. 10) and the mean QPF (Fig. 11) during the critical 24 hour period showed the relatively well predicted pattern of precipitation to include the axis of the higher QPF aligned well with the QPE (Fig. 3). ii) SREF The SREF forecasts were taken from a shorter forecast horizon and the key fields in the SREF showed similar signals to those indicated by the GEFS. The SREF MSLP forecasts (Fig. 12) show the frontal trough and the arctic anticyclone moving into the Ohio Valley, the strong 250 hpa jet and anomalous u-winds within the jet (Fig. 13), and the probability of 25 mm or more QPF in the 24 hour period of heaviest snowfall (Fig. 14). The mean 24 hour QPF (Fig. 15) and the 12-hour 12.5 mm QPFs are also shown for the two periods ending at 0000 UTC and 1200 UTC 5 March 2015 (Figs. 16 and 17). For brevity, the similar 850 hpa temperature forecasts are not shown. These QPFs are shown as the two EFS and the CFSRV2 showed that the transition from rain to snow in the Ohio Valley was generally forecast to occur within about 6 hours either side of 0000 UTC 5 March 2015. In Louisville, the SREF forecasts had the transition time in the 2100 UTC 4 March to 0000 UTC 5 March timeframe. The times were derived from SREF 850 hpa temperatures in 3-hour increments on plan view and at points (not shown). The overall QPF forecasts and probabilities look reasonable verse the QPE (Fig.3). However, a close examination of these data showed that the northern edge of the QPF shield gradually shifted southward. Over time this greatly reduced the snow threat across Ohio and central Pennsylvania. The axis of higher QPF potential also increased over time in portions of
southeastern Pennsylvania and Maryland. Shorter range forecasts in Figures 13-17 reveal some of these subtleties. iii) HRRR The high resolution rapid refresh (HRRR) was examined to show the value of these data in shortterm prediction of the event. The focus is on the HRRR times around the transition time from rain to snow in Louisville, KY and Harrisburg, PA. Both locations received heavy snowfall and the region of north-central Kentucky had record snow for the date. The last report of rain in the Louisville was at 1856 UTC as shown in the METAR: METAR KSDF 041856Z 36013KT 2 1/2SM -RA BR OVC011 02/01 A300 = The transition of ice pellets to snow was short-lived and began around 2147 UTC: METAR KSDF 042147Z 36011KT 1 1/2SM -SNPL BR FEW009 BKN015 OVC020 01/M01 A3005 = Several HRRR runs prior to 2100 UTC indicated that the 2m temperatures would fall to 0 C around 2200 UTC (Fig. 18). The 1600 UTC HRRR precipitation type (PTYPE) forecasts showed the forecast transition from rain to mixed to snow. The snow transition in the 1600 UTC HRRR was forecast to occur between 2000 and 2100 UTC (Fig. 19d-f). The HRRR, like the observations, showed short period of mixed precipitation. Forecasts from 6 HRRR runs focused at 2100 UTC showed the rain-snow transition was closely focused around the 2100 UTC time period in HRRR runs from as early as 1000 UTC (Fig. 20e). The HRRR QPF from 6 runs (Fig. 21) showed the band of high QPF across Kentucky. Note some of the longer forecast ranges do not encompass the period through 1200 UTC. These forecasts end at the maximum duration of the HRRR forecast which spans 15 hours. Thus, the 0000 UTC 5 March HRRR covers the entire period and shows a broad area of over 25 mm of QPF after the rain to snow transition. The synthetic radar (Fig. 22) from the 0000 UTC 5 March HRRR showed the impressive band of snow forecast over Kentucky. The rain to snow transition at Harrisburg occurred in the 0156 to 0641 time frame as shown in the 3 select METARS for KMDT: METAR KMDT 050156Z 32005KT 6SM -RA BR SCT035 OVC070 03/02 A2993 RMK = METAR KMDT 050602Z 32007KT 4SM -PLSN BKN012 OVC050 01/00 A2996 = METAR KMDT 050641Z 31010KT 2 1/2SM -SN BKN010 OVC035 01/00 A2999= The HRRR was a bit aggressive with the transition of rain to snow at Harrisburg; failing to show the rain to ice pellets transition (Figs. 23 & 24). The 0300 UTC 5 March HRRR showed the rapid rain to snow transition occurring between 0500 and 0600 UTC (Fig. 23b-d). This transition was earlier than implied by observations. The HRRR seemed to under forecast the prolonged
period of mixed precipitation and thus longer transition to snow as the arctic air undercut the warm air aloft from multiple forecasts prior to the transition (Fig. 24). The HRRR simulated radar (Fig. 25-26) and accumulated QPF (not shown) at short ranges began to indicate the development of mesoscale snow bands over southeastern Pennsylvania. Similar bands developed on radar and produced locally heavy snow in the 6 to 12 inch range. The volume of HRRR data to display was limited here. It is difficult to show the subtle run to run changes. The limited data shown here imply that the HRRR can be used to make more precise predictions relative to longer range forecasts which need to be more probabilistic in nature. 4. Conclusions A nearly textbook example of an anafront brought heavy snow; with areas of 10 to 25 inches of snow across portions of Kentucky and heavy snow in southeastern Pennsylvania and Maryland. The snowfall lacked a strong surface cyclone, though it occurred in a region of strong baroclinicity with deep warm air on the south side of the boundary and cold arctic air on the cold side of the boundary. At 250 hpa a strong jet developed. Most of the enhanced snowfall on the cold side of the frontal boundary occurred as the jet entrance region moved along the boundary. The frontal boundary, strong 250 hpa jet with +4 to +5σ u-wind anomalies and the potential for significant QPF was well predicted by both NCEP EFSs. The GEFS and SREF predicted the anomalous jet, the area of high QPF and the arctic air under cutting the warm, moist air to the south. Relative to other high impact snow events, this event lacked a strong surface cyclone and was not associated with a strong 850 hpa low-level easterly jet. There was no cyclone or storm associated with this winter storm. This anafront was more about dueling anticyclones than cyclones. The famous Pennsylvania weather forecasting quote about predicting heavy snow forecast the high, forecast the snow was relevant in this case. There were forecast issues related to snow amounts and the timing of the transition from rain to snow. These issues were present in both EFSs and in the high resolution models. The timing of the transition was critical to the timing of the snow and potential for heavy snowfall. The HRRR was used to show the timing in the Ohio Valley and southeastern Pennsylvania. These forecasts show great promise for rapidly updated high resolution models. They are not perfect, but can be of value in making relatively precise and highly confident forecasts in the short-term. The volume of HRRR data processed and examined was significant. However, only a small amount of data was displayed relative to the data examined for over 24 different HRRR forecast cycles. Looping images will someday be embedded in documents facilitating the inclusion of more data.
5. Acknowledgements: The Pennsylvania State University Map discussion for details on the storm and type during its evolution and the Albany map for similar discussions about the event. Thanks to my son, Glenn Grumm in Louisville who got more snow than I told him he would. Elyse Hagner provided climate data and editorial support. 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. Return to text.
Figure 2. As in Figure 1 except for 250 hpa wind vectors and 250 hpa u-wind anomalies. Return to text.
Figure 3. Stage-IV QPE (mm) from 1200 UTC 3 to 0000 UTC 6 March 2015. Return to text.
Figure 4. As in Figure 1 except for mean sea level pressure (hpa) and pressure anomalies in 6-hour peroids from a) 1200 UTC 4 March through f) 1800 UTC 5 March 2015. Isobars every 4 hpa. Return to text.
Figure 5. As in Figure 4 except for 850 hpa temperature ( C) and temperature anomalies. Isotherms every 2 C. Return to text.
Figure 6. As in Figure 4 except for 850 hpa winds and u-wind anomalies. Return to text.
Figure 7. As in Figure 8 except for precipitable water (mm) and precipitable water anomalies. Return to text.
Figure 8. As in Figure 7 except for CFSRV2 250 hpa winds and u-wind anomalies. Return to text.
Figure 9. As in Figure 7 except for CFSRV2 850 hpa winds and temperature anomalies. Return to text.
Figure 10.. NCEP GEFS forecasts from 6 GEFS cycles showing 24 hour QPF probabilities of exceeding 25 mm or more QPF in the period ending at 1200 UTC 5 March 2015. Shading is in percent and thick contour is the ensemble mean 25 mm contour. GEFS forecasts initialized at a) 1200 UTC 27 February, b) 1200 UTC 28 February, c) 1200 UTC 1 March, d) 1200 UTC 2 March, e) 1200 UTC 3 March, and f) 0000 UTC 4 March 2015. Return to text.
Figure 11. As in Figure 10 except for ensemble mean QPF shaded and each member 25 mm contour. Return to text.
Figure 12. NCEP SREF mean sea level pressure (hpa) and pressure anomalies valid at 0000 UTC 5 March 2015 from SREF initialized at a) 0300 UTC 3 March, b) 0900 UTC 3 March, c) 1500 UTC 3 March, d) 2100 UTC 3 March, e) 0300 UTC 4 March, and f) 0900 UTC 4 March 2015. Return to text.
Figure 13. As in Figure 12 except for SREF 250 hpa winds and u-wind anomalies. Return to text.
Figure 14. As in Figure 11 except for NCEP SREF QPFs from SREF initialized at at at a) 0300 UTC 3 March, b) 0900 UTC 3 March, c) 1500 UTC 3 March, d) 2100 UTC 3 March, e) 0300 UTC 4 March, and f) 0900 UTC 4 March 2015. Return to text.
Figure 15. As in Figure 14 except for SREF mean QPF and each members 25mm contour. Black dot is Louisville. Return to text.
Figure 16. As in Figure 15 except for the 12 hour period ending at 0000 UTC 5 March 2015. Return to text. Comment [EC1]: No caption
Figure 17. As in Figure 15 except for the 12 hour period ending at 1200 UTC 5 March 2015. Return to text. Comment [EC2]: No caption
Figure 18. NCEP HRRR forecasts valid at 2200 UTC 4 March 2015 from HRRR initialized at a) 2200 UTC, b) 2000 UTC, c) 1800 UTC, d) 1600 UTC, e) 1400 UTC, and d) 1200 UTC. The black dot is the location of Louisville, KY. Return to text.
Figure 19. HRRR forecasts of precipitation type (see key) based on the type and intensity and 2m temperatures from the 1600 UTC 4 March HRRR valid in hourly increments from a) 1700 through f) 2200 UTC 4 March 2015. The 2m temperatures are shown include 10, 2, 0, -2 and -10 contours. Black dot shows Louisville, KY. Return to text.
Figure 20. As in Figure 19 except for HRRR valid at 2100 UTC for 6 HRRR runs initialized at a) 2000 UTC, b) 1800 UTC, c) 1600 UTC, d) 1400 UTC, e) 1200 UTC and f) 1000 UTC 4 March 2015. Return to text.
Figure 21. HRRR accumulated QPF (mm) for the period of the run or through 1200 UTC 5 March 2015 from HRRR initialized at a) 0000 UTC 5 March, b) 2300 UTC, c) 2100 UTC, d) 2000 UTC, e) 1800 UTC, and f) 1600 UTC 4 March 2015. Black do in low right panel shows Louisville, KY. Return to text.
Figure 22. As in Figure 21 except for HRRR simulated from the 0000 UTC 5 March 2015 HRRR showing radar every hour from a) 0000 UTC through f) 0600 UTC 5 March 2015. Return to text.
Figure 23. NCEP 3km HRRR forecasts of precipitation by type and intensity from the 0300 UTC 5 March HRRR valid hourly from a) 0400 UTC through f) 0900 UTC 5 March 2015. Black dot is Harrisburg, PA. Return to text.
Figure 24. HRRR forecasts valid 0600 UTC 5 March 2015 from HRRR initialized at a) 0400 UTC, b) 0300 UTC c) 0200 UTC< d) 0000 UTC 5 March 2015 and f) 2200 UTC 4 March and e) 2000 UTC 4 March 2015. Return to text.
Figure 25. NCEP 3km HRRR forecasts of synthetic radar in hourly increments from a) 1200 UTC through f) 1800 UTC 5 March 2015. Red arrow shows the band of snow. Return to text.
Figure 26. As in Figure 25 except for successive forecasts from the 1100 UTC 5 March NCEP 3km HRRR. Return to text.