Monday, September 17, 2012

Slight Risk Continues For Tomorrow

A slight risk for severe weather remains in place for tomorrow for our region. We have a three in ten chance (as of this afternoon) of any one location getting severe criteria wind (58 mph or greater) within 25 miles of it, with the black "hatched" area picking up a one in ten chance of 75 mph winds. Those odds, especially the 30% one, may very well be bumped tomorrow and we could end up seeing a moderate risk for severe weather around the region. More on the timing and setup on the squall line in a bit.

The first thump of a one-two punch is coming tonight with a thump of rain moving in from the southwest later this evening.  Rain is already spreading into Southwestern Virginia and is moving northeast.  It should reach our region after 2 AM tonight and be steady to perhaps heavy west of the city as it lifts northeast for the morning commute.  This thump of rain is in advance of the storm system -- an overrunning/warm frontal band rainfall event that can produce some quick half to three quarter inch rains in some spots as this area of rain lifts through.  This rain will let up for a time around Philadelphia -- and for a longer while in South Jersey and Delaware where some sunshine can't be ruled out through a cloud-strewn sky tomorrow afternoon.  Odds of any sun are much lower west of I-95 where it could be showery almost all day.

The front is going to have some dynamics and pop to it -- wind at 5000 feet above the ground is modeled to be anywhere between 60 and 80 mph in our region as the front passes through tomorrow evening.  The low, which will pass to our northwest, is progged to intensify and as it deepens we should see a squall line of wind-whipped rain develop and march east with the front.  Timing on the front varies amongst the various models but a general consensus suggests the squall line should move through the region starting from 5-8 PM (west) and reaching Philadelphia anytime from 7-10 PM.   It might be notch later but I think these times will hold as a general rule of thumb.  

Rain will be heavy with the front as it moves through (illustrated below with the Euro's depiction via but winds will be the bigger story.  Winds could gust to 70 mph, if not higher, in the strongest of cells within the squall line.  While thunder may be limited, the gusty wind potential will not be.  A few tornadoes can't be ruled out in parts of the region as well tomorrow afternoon and evening...I think the "best" chance of that will be in Central into Northeastern Pennsylvania. 

You can track the rains tonight as they approach and move in through our current weather page.