Monday, September 17, 2012

Tuesday's Storm Won't Be Huge Flooding Threat

With a general one to two inches of rain expected for the upcoming storm tomorrow, there is a risk for some ponding of water in those usual suspect roads, backyards, and areas that seemingly flood rather frequently. However, the bigger issue and threat with tomorrow's storm system is one of wind. More on that in a moment.

Six hour flood guidance from the National Weather Service for the region suggests we can handle at least three inches of rain in a six hour period before flash flooding becomes an issue locally -- that threshold is a good bit lower up near New York City and Long Island (where more rain has fallen over the past few weeks) than down here. Some spots can handle a bit more than that but three inches in six hours is a lot of rain...and a lot more than what guidance is suggesting for Tuesday.  In general, computer guidance is suggesting one to two inches of rain, with the higher rainfall totals setting up to the west and northwest of Philadelphia and lighter amounts east of I-95.  It wouldn't surprise us if parts of New Jersey south/east of the city plus Delaware fell short of an inch of rain.

Most of the rain threat will be centered around two periods -- a warm-air advection burst of rain tomorrow morning (generally centered along/west of I-95) and a second thump of rain with the front itself.  In between, there could be showers or even a few rumbles of thunder in a "lull" period in the afternoon hours but widespread, nonstop rains don't look likely in and around the city at this point.

The bigger story will be the wind that accompanies the front and the breezes out ahead of the front.  Tomorrow afternoon and evening will be breezy to gusty with southerly winds gusting to 30 mph ahead of the front.  The front itself will be accompanied by a low level jet that could bring winds to over 75 mph at 5000' above the ground, perhaps up to 85 mph if the GFS is right (see below) to our west.  The wind energy aloft could transport to the surface assuming a squall line develops along the front...and parts of the Mid Atlantic could see some nasty wind gusts with the front itself.  The 85 mph winds aloft may not reach the surface completely but gusts over 70 mph can't be ruled out in parts of Central Pennsylvania tomorrow afternoon and evening.  The worst of worst in terms of severe may stay to our west but we still have a good shot at a nasty squall tomorrow evening.

We will highlight timing details/specifics this evening in another post with updates on the potential for severe weather.