Memorial Day Weekend 2013: Snow and Cold

Transcription

1 Abstract: Memorial Day Weekend 2013: Snow and Cold By Richard H. Grumm National Weather Service State College, PA A slow moving 500 hpa low and associated unseasonably cold air in the lower troposphere brought unseasonably cold weather and a record late season fall to the higher elevations of northern New York and New England. Over 30 inches of snow fell on the slopes of Whiteface Mountain in northern New York. Many of the higher elevation locations, where ski resorts are often located had heavy snow. Trace amounts of snow were reported as far south as Binghamton, New York and Scranton, Pennsylvania. The deep trough and cold air delayed the start of summer for a few days on the normally busy Memorial Day weekend. Despite the time of year, this storm shared many of the characteristics often associated with East Coast Winter Storms including a strong low-level northeasterly jet. The heaviest rainfall and heavy snow fell in and near the axis of this feature. The 850 hpa temperatures over the northeastern United States were below 0C during the precipitation and many locations were -2C, despite the deep cold air, accumulating snows were generally limited to elevations above about 2000ft. The overall pattern and the potential for cold and rain was well predicted by the NCEP models and ensemble forecasts systems.

2 1. Overview Memorial Day weekend is considered to be the first official weekend of summer. But in the northeastern United States, summer 2013 got off to a chilly start as a slow moving mid-tropospheric cut-off low (Fig. 1) and a pocket of sub-freezing air at 850 hpa (Fig. 2) combined to produce a record breaking late season snow fall over northern New York and New England over the Memorial Day weekend of Whiteface Mountain, New York received 36 inches of snowfall (AP 2013) forcing the closure of the main highway on the mountain. Heavy snow was observed at other high elevation sites to include Mount Mansfield, VT where 13.2 inches of snow was reported. Snow was reported as far south as Binghamton, NY which set a new daily record of a trace of snow for the date. The higher snow amounts were generally associated with the higher elevations of northern New York and New England. The lumbering cut-off and cold air created unseasonably cold temperatures in the Mid-Atlantic regoin into New England. Despite the cold a significant number of record lows or record low high tempeature records were not observed (Table 1 & 2), though late season frosts affected many inland areas. The critical part of this event was foreasting the cold cold and potential snow. To that end, this paper will document the event of May 2013 over the eastern United States to show the pattern and how the anomalies may aid in identifying patterns with meteoroloigcally signficant events. Model and ensemble data are used to show how well this event was predicted. 2. Data and Methods The larger scale pattern was reconstructed using the Climate Forecast System (Saha et. 2010). The means and standard deviations were used based on the method described by Hart and Grumm (2001) suing the NCEP/NCAR reanalysis data (Kalnay et al. 1996). Daily values of raw CFS data were compared to the 21-day centered means and standard deviations. Record low high-temperature data and record low temperature data were obtained from NCDC. The data are listed in Tables 1 & 2 respectively. 3. Pattern and precipitation Figure 3. AP picture courtesy of AP and ORDA/Whiteface Mountain showing the snowfall on Veterans Memorial Highway, 26 May The evolution of the larger scale pattern over the United States (Fig. 4) from 0000 UTC 23 to 27 May 2013 showed a deep trough over the northwestern United States from May and the developing deep trough over the eastern United States. The eastern trough was a more transient feature which closed off over the northeast between 0000 UTC 25 to 0000 UTC 26 May 2013

3 (Fig. 4d-e) and then lifted to the northeast. The cold trough over the western United States was a more persistent feature and it too was associated with cold weather and late season snow in the higher terrain of the northwestern United States. The regional evolution of the 500 hpa low over northeastern United States on Memorial Day weekend from 1200 UTC 24 to 0000 UTC 27 May 2013 (Fig. 1) showed the deep 500 hpa low with a negative tilt. This system closed off over Long Island by 25/1200 UTC (Fig. 1c) and drifted to the north. The 850 hpa temperatures were -1 to -3σ below normal near and west of the trough axis (Fig. 2). At 25/0000 UTC there was a sharp north-south frontal boundary with the coldest air over New York and Pennsylvania with warmer air over New England. The point near Saranac Lake was already below 0C by 25/0000 UTC and remained so through 26/0600 UTC (Figs. 2a-f). At the surface, a surface cyclone was present along the East Coast (Fig. 5a) with a strong anticyclone to the west. The pressure anomalies were on the order of +2 to +3s with the anticyclone and only about -1s below normal near the surface cyclone. The critical aspect of this was the strong pressure gradient between the two systems and the implied northerly to northeasterly winds. The 850 hpa winds (Fig. 6) showed strengthening northeasterly flow over northern New England, peaking at -3 to -4σ below from 25/1200 UTC through 26/0600 UTC. In the winter strong northeasterly flow with -3 to -4σ u-wind anomalies and temperatures below 0C at 850 hpa have been repeatedly shown to be strong signals associated with high impact snow storms (Stuart and Grumm 2006). It should be noted that there was strong northerly flow in the cold air and the v-wind anomalies peaked near -3 to -4σ west of the 850 hpa cyclone (not shown). The stage-iv observed precipitation for the event (Fig. 7) indicated that the higher QPF amounts, over 64mm, were confined to the Saint Lawrence Valley into the eastern Adirondacks of New York and the Champlain Valley region. The 12-hour accumulations (Fig. 8) show the larger scale bands were aligned with the features shown in Figures 4-6. The low-level 850 hpa northeasterly jet, interacting with the terrain, played a critical role in the location of the heavier precipitation amounts. Snow is not plotted as the accumulating snows were primarily restricted to the higher elevations, mainly above 2000ft and the heavy snow fell mainly at locations above 2500 ft. Lower elevations, as far south as Binghamton, New York and Scranton, Pennsylvania had rain mixed with snow. 4. Forecasts The deep trough over the eastern United States was relatively well predicted with about 7 days of lead-time by the global models and ensemble prediction systems (EFS). However, it was not until about 4.5 days in advance did the NCEP GEFS converge on a solution of at least a -1s below normal trough over the northeastern United State by 25 May This reflected the fact that the trough, though predicted had considerable uncertainty with the timing, location and

4 intensity of the trough. No attempt is made here to evaluate the longer range forecasts; instead shorter-term NCEP SREFs are examined to look at the cold rain and snow forecasts. For brevity the pattern is presented using 6 forecasts examining the 500 hpa heights, 850 hpa temperature forecast, and the u-winds anomalies valid at 0000 UTC 26 May 2013 (Figs 9-11). The QPF and probability of 25 mm of QPF (Figs ) are examined from the same 6 SREF forecasts along with the probability of snow as a precipitation type in the SREF (Fig. 14). Similar data were examined but are not shown from the NCEP GEFS. The pattern as forecast by the 6 SREF cycles showed the closed 500 hpa cyclone moving off the East Coast at 26/0000 UTC. The weakest trough with the smaller anomalies, and thus implied higher uncertainties were in the forecasts from 23/2100 UTC (Fig. 9a). The depth of the trough and the cold air at 850 hpa (Fig. 10) showed a clear signal for a deeper and colder trough at ever shorter forecasts lengths. Note the 0C contour was not present in the SREF mean at 23/2100 UTC (Fig. 10a) appearing at forecasts from 24/0300 UTC (Fig. 10b) and eventually a -2C contour appeared in the SREF mean by forecasts from 24/1500 UTC. This impacted the precipitation type forecasts (Fig. 14) with increasing chances of snow as forecast length decreased. Similar to the temperatures fields and the observed winds, the 850 hpa winds in the SREF showed a strong low-level northeasterly jet which showed a stronger signal at shorter forecast length. As the SREF members converged toward a solution, a strong northeasterly jet was predicted. Overall, the pattern the SREF predicted was similar to the pattern diagnosed by the CFS. The pattern and 850 hpa temperatures provided some confidence in an abnormally cold period and it contained a signal, with -2C temperatures at 850 hpa for some snow potential. The mean QPF over the period where most of the snow fell (Fig. 12) and the probability of 25mm or more QPF during this period (Fig. 13) showed that the highest probability of higher QPF amounts were confined to the regions impacted by the strong low-level northeasterly flow. Close to the axis of higher QPE (Fig. 7&8) as observed in the stage-iv data. Similar to the depth of the trough and the intensity of the cold air, the probability of QPF greater than 25mm was relatively slow to increase and the higher confidence forecasts of 25mm or more QPF at the 60% level were limited to forecasts issued on and after 24/1500 UTC. Though all the SREF QPFS showed that 25mm of QPF was possible over the same general area (Fig. 13). The cold air and higher confidence of higher QPF amounts likely led to incremental confidence by human forecasters for the potential for snow. As shown in Figure 14 the probability of snow in the SREF was slow to come rise much above 30% until forecasts issued on or after 1500 UTC 24 May These data show that the SREF had the general regions were snow was observed forecast quite well though most of the accumulating snow was confined to the higher elevations.

5 The SREF was hard pressed to forecast areas of 1-2 inches of snow, likely due to the limited resolution of the SREF and its terrain features. The 24 hour snow forecasts (Fig. 15) showed higher probabilities and snowfall amounts over 4 inches in Vermont. 5. Summary A deep 500 hpa cyclone brought unseasonably cold air to the eastern United States on May In the northeast the lower elevations had a cold rain while some of the higher elevations of New York and northern New England had a rare late season heavy snow event. Despite the time of year, numerical guidance did provide good lead-time on the potential for the cold weather during the Memorial Day weekend and the guidance provided useful information related to the potential for a rare late-season snow event. The pattern that evolved shared many characteristics associated with late and early season winter storms and elevations snows to include a deep cut-off cyclone. It also shared many of the characteristics of winter storms in general. In the winter strong northeasterly flow with -3 to -4σ u-wind anomalies and temperatures below 0C at 850 hpa have been repeatedly shown to be strong signals associated with high impact snow storms (Stuart and Grumm 2006). Clearly, in the higher terrain of northern New York and New England this same signal works well through, well at least Memorial Day. The heavy precipitation in general is well aligned with the concept of strong u-winds as described by Stuart and Grumm (2009) and Grumm (2011). From a forecast perspective, the event was relatively well predicted in 3-7 days lead-time, in terms of the cold and potential rainfall during the Memorial Day weekend. However, the issues related to snow were slower to fall out of the forecasts from the NCEP SREF (Figs. 8-15). It is encouraging that the state of NWP is such that models can predict record events. Clearly, the SREF and GEFS (not shown) were able to predict the potential for a rare very late season snow event over northern New York and New England. What is not clear is how well forecasters used these probabilistic data of such an event. 6. Acknowledgements Albany map for information and data on the storm. 7. References Associated Press, 2013: 3 Feet of snow blankets Adirondacks Whiteface Mountain after Memorial Day weekend storm (and related stories). 27 May 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.,

6 Grumm, R.H. 2011: New England Record Maker rain event of March NWA,Electronic Journal of Operational Meteorology,EJ4. Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40- Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, Saha, Suranjana, et. al., 2010: The NCEP Climate Forecast System Reanalysis. Bull. Amer. Meteor. Soc., In Press (DOI: /2010BAMS3001.1). Stuart, N. and R. Grumm 2009, "The Use of Ensemble and Anomaly Data to Anticipate Extreme Flood Events in the Northeastern United States",NWA Digest,33, Stuart,N.A and R.H. Grumm 2006: Using Wind Anomalies to Forecast East Coast Winter Storms. Wea. and Forecasting, 21,

7 Figure 1. CFS 500 hpa heights and 500 hpa height anomaly data valid in 12-hour increments from a) 1200 UTC 25 May through f) 0000 UTC 27 May Return to text..

8 Figure 2. As in Figure 1 except for 850 hpa temperatures and temperature anomalies in 6-hour increments from a) 0000 UTC 25 May 2013 through f) 0600 UTC 26 May Return to text.

9 Date Broken Tied Total Regions 23-May Midwest Pacfic Northwest 24-May Great Lakes-Midwest 25-May Ohio Valley to New England 26-May Northeast 27-May New England Table 1. NCDC data showing the date and record low high-temperature values set or tied and the total number of records affected. The region shows the generalized region or regions of the United States were a significant portion or all of the values were set. Return to text. Date Broken Tied Total Regions 23-May West 24-May Midwest Rockies 25-May Ohio Valley to Southeast 26-May Eastern US 27-May Northeast Table 2. As in Table 1 except for record low temperatures set or tied for the specified dates. Return to text.

10 Figure 4. As in Figure 1 except for 500 hpa heights over the United States in 24-hour increments from a) 0000 UTC 22 May 2013 through f) 0000 UTC 27 May Return to text.

11 Figure 5. As in Figure 4 except for mean sea-level pressure (hpa) and pressure anomalies. Return to text.

12 Figure 6. As in Figure 5 except 850 hpa winds and 850 hpa upwind anomalies. Return to text.

13 Figure 7. Stage-IV total accumulated precipitation (mm) from 1200 UTC 24 May 2013 through 1200 UTC 26 May Values in mm as indicated by the color bar and contours. No data values lower than 4mm are displayed. Return to text.

14 Figure 8. As in Figure 7 except for the 12 hour accumulation periods ending at a) 0000 UTC 25 May 2013, b) 1200 UTC 25 May 2013, c) 1200 UTC 25 May 2013, and d) 1200 UTC 26 May Contours start at 1 mm and from 2 on is powers of 2. Return to text.

15 Figure 9. NCEP 21-member SREF forecasts of mean 500 hpa heights and height anomalies valid at 0000 UTC 26 May 2013 from NCEP SREF initialized at a) 2100 UTC 23 May 2013, b) 0300 UTC 24 May, c) 0900 UTC 24 May, d) 1500 UTC 24 May, e) 2100 UTC 24 May, f) 2100 UTC 24 May. Contours every 60 m. Return to text.

16 Figure 10. As in Figure 9 except for 850 hpa temperatures ( C) and temperature anomalies. Isotherms every 2C. Return to text.

17 Figure 11. As in Figure 9 except for 850 hpa winds and u-wind anomalies. Return to text.

18 Figure 12. As in Figure 9 except for accumulated QPF (mm) for the 24 hour period ending at 1200 UTC 26 May 2013 and each members 25mm contour. The thick black contour is the SREF mean position of the 25mm contour. Return to text

19 Figure 13. As in Figure 12 except for the probability of 25mm or more QPF. Return to text

20 Figure 14. As in Figure 13 except for the probability of snow and the 1,2,4,8 and 10 inch contours for the 6 hour period of 1800 UTC 25 to 0000 UTC 26 May Return to text

21 Figure 15. As in Figure 14 except snowfall for the 24 hour period of 1200 UTC May Return to text.