Saturday, May 26, 2012

Forecast for Sunday, May 27, 2012

Pulse thunderstorms taking advantage of a strongly unstable air mass with CAPE Values up to 4,000 J/KG will continue to impact the northern portions of our area into the evening hours where a severe thunderstorm watch has been posted. The Lehigh Valley, Central Pennsylvania, and the Poconos are at risk to see some torrential rainfall, prolific cloud to ground lightning, large hail, and some wet downbursts of damaging winds in any thunderstorm.  Further south, across the Philadelphia Metropolitan area, further removed from the cold front sagging southward, isolated thunderstorms are possible in the strongly unstable air mass. The thunderstorms right across the north are moving very slowly if not stationary and make take some time to move south. So it is possible that they may weaken by the time they get to city area and if they ever do, but since there is plenty of moisture in place and there has been such a building up of instability, I cannot rule out one strong one surviving into the night.

The front stalls tonight across the north and lifts northward on Sunday. A piece of energy will swing by and will be the trigger mechanism for thunderstorms on Sunday. The highest chances for thunderstorms are in the northern areas, but with the heat and humidity in place, another strongly unstable day should transpire with daytime heating and that means a thunderstorm could really pop-up anywhere except near the beaches where the oceanic influences will cause locally stable air. Some localized severe weather in addition to flooding downpours is possible with any thunderstorm. Another weak trigger could create isolated thunderstorms on Monday. Again, heavy rain and possible severe weather may occur in any thunderstorm.

An Excessive Heat Watch has been issued for the urbanized areas of the Interstate 95 corridor. Some dangerous heat index values are possible as a few days of 90 degrees or better are expected.

A stronger cold front will approach on Tuesday and Wednesday. At the same time, Subtropical Storm Beryl will be moving west-southwest with a gradual turn to the northeast. It will be extremely critical for our weather to pinpoint when this system makes the northeast turn and how sharp the turn to the northeast will be. At this time, there are various model solutions to this and the impact on the Mid-Atlantic, if any, is unclear. We could be dealing with a near miss, remnant moisture, or a weak tropical cyclone.