A brief summary of last winter’s storms and the potential severe weather event over the Central U.S. this weekend

April 25th: Zhenhai Zhang and Minghua Zheng led a discussion titled, “A brief summary of last winter’s storms and the potential severe weather event over the Central U.S. this weekend”
The discussion began with a brief review of the winter storms during last winter (from November 2013 to March 2014). Comparing with the winter storm climatology (the mean of 1979-2012 winters), last winter is a normal winter overall. For example, the total number of cyclones during last winter over East Coast is 64, which is very close to the historical (1979-2012 winters) mean, 62. The intensity distribution (minimum central MSLP) of the cyclones doesn’t show any significant differences to the climatology either: there are slightly more (about 10%) weak (>1000hPa) and intense (<985hPa) cyclones, while there are about 15% fewer moderate (985-1000hPa) cyclones. However, more than 50% of these cyclones occur during February and March. Over the East Coast, during 2013 winter the cyclone frequency (cyclone center counts) decreases by about 30%, 23% and 10% for November, December and January separately, but it increases by about 18% and 50% for February and March comparing to the climatology. We can get a similar conclusion from the cyclone track map for individual month: there are significant more cyclones during later last winter, especially in March. One of the most important reasons for that is the temperature anomaly in that month. During 2014 March, the mean near surface temperature is 3~8K lower than the climatology (1979-2012 March mean) over most area of the continent, meanwhile, the near surface temperature is 1~2K higher than the climatology at low and middle latitude over Western Atlantic. As a result, the temperature gradient along East Coast, which is vital important to cyclone genesis and developing, is significantly larger in 2014 March than the climatology. To conclude, although 2013 winter is a normal winter for the total cyclone activities, most of these cyclones concentrate in later of this winter, especially in March, which is mainly caused by the significantly strong temperature gradient along East Coast. For early of this winter, there are fewer cyclone events, however, there are many wave packets over North America. We can see these nice wave packets in the figure of mean (30-55N) meridional wind, and many intense cyclone and heavy precipitation events occur over the downstream of coherent wave packets.
Attention then turned to the major weather systems for previous week.  An animation of the surface SLP and fronts showed a frontal system enters Great Lakes on 21st April (Monday). It crossed central PA and NY in the evening of 22nd April (Tuesday). The surface low moved to New England on 23rd (Wednesday) with a minimum pressure of 994 mb in the morning. In the upper level, the 500 mb height developed a cut-off low over NY and PA in the morning of 23rd April. This H5 Low then moved northeastward and left New England on 24th (Thursday).
The discussion then focused on the system that may bring severe weather in the weekend.  A short wave trough had move to the southern Plain in the evening of 24th. It then propagated eastward and reached the Southeast on 25th. At the same time, a surface low pressure system has developed over IN, OH and KY. The frontal system was crossing the western Southeast during the morning of 25th. The latest SPC soundings over TN and AL display the conditional instable environment, as well as the condition favoring supercell thunderstorms and hails. Based on SREF 12Z run, the shortwave trough and the surface low system will advance eastward in the afternoon and move through the northern Mid-Atlantic and the NE during this evening and Saturday morning. The NAM model shows that the low system will be over the southern NE. Considering the model consistency, heavy rain associated with strong to severe thunderstorms will develop across parts of the Mid-Atlantic and Southeast in the afternoon and the evening of Friday. On Saturday, the upper level low will move through and the cold from may trigger thunderstorms over Mid-Atlantic.
Out west, another shortwave trough from eastern Pacific had reached Northwest in the afternoon of Friday. Based on the regional models, the upper level dryline will mix eastward into central KS and across western TX in the southern Plains during Saturday evening. A strong low-level jet streak will favor the moisture advection through Saturday and aid in the development and provide a violent nature of updraft motion favorable for damaging hail, winds, and tornadoes over central and southern Plains. Due to the large-scale upward motion, the steep lapse rates will support a large hail threat across the southern Plains. The dew point will rise to 60s.The strong low pressure system will spawn supercell thunderstorms capable of generating large hail, damaging winds, and strong tornadoes. So, a large area of high populated regions in KS, OK and TX will be dealing with severe weather events on Saturday, Sunday, and Monday. The most dangerous day appears to be Sunday, and the SPC has issued their “Moderate Risk” forecast of severe weather over portions of OK, AR, TS, and LA that day.
Thereafter, the discussion focused on the Rossby wave packet moving across eastern Pacific. Based on GFS ensembles forecasts initialized at 00Z 25th, there was a wave packet over eastern Pacific, which will propagate enter southern Plains on 00Z 27th. This could be another case that the Rossby wave packets are associated with severe weather events: severe weather events tend to develop downstream of wave packets. This wave packet will be weakening on Monday and merge into the leading part of another robust wave packet which will be over the central Pacific on Tuesday. This new combined wave packet will sustain the existing short wave trough over the Central US and bring significant severe weather over central US and Mid-Atlantic.