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Northeast Severe Weather as A Function of Flow Regime
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When the database of severe weather events was stratified by 700 hPa flow direction, some interesting results appear (see Figures). There is a distinct preference for severe weather to occur north of 42.5ºN (in the Adirondack and southern Green Mountains) on a day with southwest flow at mid-levels. On days with northwest flow, the preferential areas for severe weather to occur are clearly south of 42.5ºN, into the Catskills and Berkshires. An interesting result is the pocket of preference for northwest flow days in south central Vermont, extending southeast into Massachusetts. This preference for severe weather to occur on northwest flow days may be a result of the Housatonic River Valley, which runs north-south through the Berkshires. Low-level terrain-channeled southerly flow in the valley beneath northwest flow aloft may help to create a favorable shear flow for severe weather in this region (Bracken et al. 1998). This maxima is approximately centered in and just downwind of the valley. While the population corrected plots did change the overall shape of the severe weather distribution by eliminating the maxima associated with population centers, the ratio of events per southwest flow severe weather day to events per northwest flow severe weather day changed very little (see Figures). Wind roses at 850 hPa, 700 hPa and 500 hPa are also presented for Hail events, tornadoes and straight line winds. Few or no severe weather events occur when the flow at any level has an easterly component. For each type of severe weather event (high wind, hail, and tornado), there is a high percentage of events which occur in southwest flow at 500 hPa, indicating that an approaching trough is preferential to other types of flow patterns. At 700 hPa, over 30 percent of tornado events occurred when the flow was out of 250º, suggesting that there is a strong preference for southwest flow. However, another small peak is apparent at 290º, perhaps suggesting that a preference exists for tornadoes to occur when there is an approaching shortwave trough in west-northwesterly flow. The distribution of hail events is also bimodal at 700 hPa, although the peaks are more equal in magnitude. The wind event plot has a much more rounded appearance than the hail or tornado events plots at 700 hPa. This may indicate that as long as the flow has a westerly component, there is not a preferential direction for wind events to occur. At 850 hPa, each type of event shows a peak at due west (270º), but hail events show a peak at northerly flow, while wind events show a clear peak in the south-southwesterly direction. Tornado events show a peak in the northwesterly direction, suggesting that perhaps the Mohawk Valley has some importance in modifying the flow direction at low- and mid-levels so as to be more conducive to tornado development in a favorable environment. It is interesting that a certain percentage of hail events seem to show a preference for north and north-northwesterly flow at 850 hPa. The intrusion of colder, drier air may help the formation of hail in an environment where storms are already occurring.
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