Thursday, July 19, 2012

Muggy, Storms Move In Later

Yesterday's failure to get any thunderstorms of note around Philadelphia and the western suburbs was a major frustration to those who were looking forward to a nice cooldown in the wake of 100 degree heat.  There are a couple of likely culprits -- one, despite plenty of instability the canopy of clouds that blew out ahead of the storms during the afternoon may have placed just enough of a lid on the atmosphere to cap out the potential instability for being utilized as the line of thunderstorms approached.   Two, there were pockets of dry air in the lower atmosphere about 3000 to 5000 feet up that developed due to westerly winds in the lower atmosphere. This could have been caused by the dry regime in place over our region or by compressional heating (downsloping) mixing in and scouring out some of the moisture as winds had nudged a bit more westerly at times in the afternoon in the city and in Pottstown.  Between the two, there was likely enough "there" to zap the thunderstorms as they moved in.

We'll have another shot at thunderstorms this afternoon and tonight...this time with a wave of energy riding east along the frontal boundary.  As of this morning, this complex of storms is over Chicago and the Great Lakes and is firing off a MCS (thunderstorm complex).  There should be a bit more instability, a bit better moisture support, and a bit more favorable wind trajectory at the surface and in the lower atmosphere today.  This should (fingers crossed) result in thunderstorms this afternoon and evening.  As the front is pretty much overhead this morning, the best chances for the heaviest thunderstorm activity will be across Philly and points south although any location could receive a drenching thunderstorm later on today and tonight.

We're starting out plenty warm locally -- 70's early this morning with some clouds around in the sky -- and we'll head towards the upper 80's to low 90's this afternoon.  We're pegging 90 for the city for a high -- which would mark the 5th day of 90+ in a row and 20th in 22 days.  It will not be *as* hot today but with dew point values still pushing 70 or above, it will feel quite muggy.