Preliminary synoptic climatology of cool season severe weather for the philadelphia national weather service county warning area and vicinity

Cool season severe weather occurrence in and near the Philadelphia National Weather Service in Mount Holly, NJ, County Warning Area while rare does pose a significant forecast challenge. While most wind events tend to be related to strong pressure gradients given the synoptic forcing common during the cool season, damaging wind events, as well as reports of hail and tornadoes associated with convective systems also take place. Although these are associated with progressive and/or intensifying weather systems they do not have a clear conceptual model for a forecaster to apply confidently in advance.

In an effort to better forecast the occurrence of these events a preliminary synoptic climatology was prepared to determine the associated synoptic features. The intent was to provide greater insight to the characteristic nature of these events, their associated attributes and patterns, and to provide some guidance as to what forecasters might look for in advance to recognize the potential for severe weather. In addition, it would assist in identification of what other work might be necessary to improve prediction and lead-time across such a major metropolitan area.

The study focused on local storm reports (obtained online from the Storm Prediction Center) for the months of October through March during the six year period 2000-2005. Preliminary results indicated that severe weather events occurred only one percent of all days included (for the dataset sampled) and these were dominated by damaging wind reports (80% of all local storm reports). Further examination of the events focused on their associated synoptic patterns as determined from the Daily Weather Map Series.

Results indicated three basic synoptic types driving the severe weather, two of which shared similar features and were dynamically driven. The third synoptic type was characterized by quasi-stationary frontal features and thus much more dependent upon localized thermodynamic forcing. This third synoptic type accounted for all of the hail reports and most of the tornado reports. The spatial distributions of the storm reports revealed little or no evidence of other local effects or population-biased reporting although frequencies were observed to be lower near the coast and in the Pine Barrens region of New Jersey.