21z Forecast Discussion for 9-30-10

Synopsis…

Tropical moisture continues to roll up the eastern seaboard, causing some impressive rainfall totals and hazardous travel conditions. Western PA is already cool and dry, and Eastern PA will continue to be inundated through the evening, but Central PA is the boundary zone. Farther east, the rain will continue through Friday morning. A cold front will work its way slowly through the area overnight and Friday, leading to some of the coolest temperatures the state has seen in quite a while. Other than isolated sprinkles this weekend, we will finally be able to dry out over the next week.

 

Short Term…

As previous shift aptly named it, this dual-wave system continues to move through the mid-Atlantic. Any rain in the west is shutting off now, and even KUNV's heaviest precip is over as of 21z. But, flood watch continues for estrn PA thru Friday aftn due to excessive rainfall topping 7" in spots. New 18z WRF run seems to have a good handle on the timing and placement of the precip shield across PA, and it suggests a secondary wave pushing through the SE and NE zones from about 12z-18z Friday afternoon, causing some concern with our "rain tapering off" forecast. We do expect areas west of about I-81 to stay dry tomorrow, and can definitely see clearing after FROPA, so we kept these areas mainly dry. Tight pressure gradient should lead to breezy conditions Friday.

 

Long Term…

Cold air returns! The 540-line makes its first appearance in a long time and dips well south of Erie for early next week. Rural areas of wstrn PA will see lows in the 30s, with cold spots (BFD?) approaching freezing. WRF has some odd-looking precip output Saturday associated with leading edge of the coldest air. We are not sure if there is enough instability for shwrs, so we kept most areas dry and mentioned a slt chc of a sprinkle for the weekend. Much-below-avg temps look to continue through the week, with highs stuck in the 50s in many areas as a strong trough dominates the pattern.

 

Forecaster: Matt Mahalik