SEVERE WEATHER ASSOCIATION OF JET STREAKS AND VORTICITY ADVECTION PATTERN WITH SEVERE THUNDERSTORMS IN THE NORTHEASTERN UNITED STATES

The association of 8501300 mb jet positioning and 500-mb vorticity advection with severe thunderstorm events in the Northeastern United States was determined for the period 1980-84. An objective criterion was de velopedfor classifying severe thunderstorm and tornado cases into a four categOlY system using both the frequency (number of severe weather cases) and type of severe weather event. Thefour non-mutually exclusive categories chosen for this study were called SELS Events, SELS Episodes, Tornado Events, and Tornado Epi-sodes. Each of the jet and advection factors were then ana-lyzed in terms of these four classifications. The principal results from this study are as follows: The 850-300-mb jet streak analysis primarily revealed that the right-rear (RR), and secondarily the left-front (LF) , quad-rants of the 300-mb speed maximum are the most favored for severe weather (i.e., 73 % of all events), consistent with divergence considerations, unless cyclonic curvature is pres-ent upstream. At 850 mb the only determination which can clearly be made is that the severe weather tends to occur to the left of the 850-mb jet axis (72 % of all events). A verage wind speeds in the speed maxima at 850 mb and 300 mb were 40 kt and 95 kt respectively, which are thought to be considerably larger than those of the non-SELS envi-ronment of summer, when most SELS events studied here occurred. In only 9 % of the events was a 70-kt 300-mb wind lacking in the domain used. The 500-mb vorticity advection analysis revealed that prior positive vorticity advection was not likely before severe weather occurred in the Northeast, suggesting that some other large-to-medium-scale lifting source is often present before severe weather. 1