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Posted: (July 22, 2008 11:46 pm)
Severe Thunderstorms likely...Eastern PA/NJ/DE/MD/DC/VA/Eastern NY/CT/RI/MA
We are going to be in a nice setup for most of the area to some severe weather. As the front pushes eastward several features will come together to support a line of intense storms capable of producing tremendous amounts of rain, damaging winds, small hail, and isolated tornado. The forcing is going to be strong along the front due to the strong low level and sfc convergence, right rear quadrant of the upper level jet, and positive vorticity advection. With storms coming in early and hanging around through the night instability will not be a problem with the lack of sunshine. Cloud cover will move in during the late morning hours-noonish. Do not let this fool you as any rain overnight tonight will increase moisture with the limited sunshine in the morning hours. The main feature is going to be the low level jet increasing to 35+kts. This will help increase the instability parameters and prevent the values decreasing during the nighttime hours.
Flash flooding will be one of the primary concerns with training. Training is when storms move over the same area over and over again producing several inches of rain in short amount of time. As the upper level trough moves into the Mid-Atlantic it will back the flow aloft with a southerly wind allowing for the training to take place.
Unlike the past few threats the winds will be sufficient enough for a line of storms with a few super cells. Latest soundings show Philadelphia reaching 3000j/kg of CAPE with some slight veering in wind direction with height. This slight veering combined with increasing low level shear will support the threat for an isolated tornado. Ample moisture in the lower levels will lower LCL heights, or cloud bases, below 500m AGL.
Damaging winds will be the main and widespread threat with the steep low level lapse rates and strenthening wind fields in the lower levels. Small hail is a threat but due to the high frz levels and unimpressive mid level lapse rates this will not be a major concern. The main concerns are damaging winds and flooding. An isolated tornado cannot be ruled out as stated above so keep an eye on the weather and be safe.
