Friday, September 21, 2012

A Slight Severe Risk For Saturday...But...

Our region is under a slight risk for severe weather for Saturday with the approaching cool front that will bring another Taste of Fall into the region.  I'm not overly bullish on the prospects for severe weather but I do think a round of wind and rain, perhaps some thunder, is in the cards on Saturday evening as the front comes through.

First, the timing for the front is Saturday evening -- most of Saturday should be ok to decent weatherwise as skies will be variably cloudy to even mostly sunny at times.  It will be warmer than past days as temperatures get into the lower or even middle 80's (south of the city for the warmer temperatures).  The front crosses the region between 6 and 9 PM in the west, hitting the coast between 11 PM and 2 AM.  You'll get a good chunk of Saturday in without dodging drops, especially in New Jersey and Delaware. Southeastern Pennsylvania might have a few showers to deal with ahead of the front but odds favor dry into the afternoon.

One of my big concerns on this event not being a "big" storm event is a relative lack of energy and dynamics.  The GFS is showing a rather weak low level jet -- the best wind energy ahead of the front is across New England, while better wind energy locally exists behind the front as west winds push in and begin the cooldown to our west.  The NAM is a bit more aggressive with the low level jet and instability parameters than the GFS but the parameters for Saturday on the NAM don't approach those we had on Tuesday with our squall line that didn't get as nasty as modeled.

Knowing this, as there will be marginal instability (again, NAM parameters are more aggressive) and there will probably be some sunshine compared to on Tuesday, we *could* see a severe warning or two occur.  The NAM, below, shows the line pushing through South Jersey Saturday evening with some localized downpours and I think the round of downpours and some gusty winds is probably a more reasonable expectation than any sort of nasty "Fall Squall".   While I don't see a widespread threat for severe at this point a few warnings can't be ruled out in the strongest parts of the line.

In terms of rainfall totals from tomorrow's front, most people get a half inch or less from it.  There could be some localized higher totals to the north of the city that could yield some localized ponding of water on roads given it mostly comes with the front itself.  It should not as big an event as Tuesday's storm, though.