4-12-2011 Technical Discussion

Technical Discussion

Synopsis:

A slow moving cold front that brought rounds of scattered showers and thunderstorms will gradually weaken throughout Monday night. Another low pressure system will develop over Tennessee Valley and ride along this quasi-stationary front on Tuesday, strengthening as it approaches the East Coast. As result, scattered showers will once again become periods of steady rain, especially for eastern PA. The rain will continue into Wednesday, gradually tapering off later in the day. There will be a brief period of clear weather for Thursday before another storm system approaches on Friday.

Short-term (Tuesday – Wednesday evening):

The low pressure that brought a severe weather outbreak across the Midwest and Northeast will continue to make its way into Canada. Its associated cold front will gradually lose its bunch and becomes stationary or quasi-stationary as it approaches the eastern seaboard. There will be a brief period of rain passing through central and southern parts of Pennsylvania Monday night but should come to end temporarily later into the night. However, as a 500-mb shortwave separates itself from the main long-wave over Canada, a low pressure system will develop within the zone of favorable upper-level divergence. The 500-mb trough will become increasingly negative-tilted and eventually closes off into a cut-off low once it nears the east coast on Tuesday. As result, the newly developed low pressure system will slowly ride along the aforementioned stationary front. The whole system should become fully occluded later Tuesday in response to the cutting-off of the 500-mb low. During the day, the stationary front will provide extra lift for the moisture flow coming from the Atlantic. As result, rain should be on the steady side for eastern PA. For the western portions, the rain should start as showers but there could be a period of steadier rain as deformation bands set up later in the day. The rain will gradually taper off from south to north as the storm system pulls away on Wednesday. May finally see some breaks in clouds for Wednesday night.

Mid-term (Thursday – Thursday night):

A high pressure ridge will build in on Thursday providing as some much needed drier air for the lower and upper-levels of the atmosphere. The cloud developments should be limited for much of the state. However, parts of northern counties could see some passing clouds as a weak cold front sinks a tad south from Canada. As we move into Thursday night, more clouds will build in as another storm system develops over the Midwest in response to a sharpening 500-mb trough.

Long-term (Friday – Saturday):

Much of the Friday should be cloudy but remain dry. However, showers will begin to break out during afternoon hours on Friday, though they should remain on the scattered side. The rain will gradually become steadier throughout the night as the storm system really cranks up over the Midwest. The models have a general agreement on the development and timing of this storm system. The latest model data suggest that the storm system will not arrive much later Friday night. If so, the main bulk of precipitation shall fall on Saturday. There is also chance for some embedded thunderstorms. This will once again be another long-duration rain event, as a series of low pressure will develop around the triple-point located near the east coast. The main 500-mb shortwave will spin over upper-Midwest while pulling ample amount of Atlantic moisture inland over the weekend.

David Wang