Thursday, June 13, 2013

A Hodgepodge of Threats

Today's going to be one of those "fun" days from a weather standpoint, fun being sarcastic and in quotes for a reason.  Fans of nasty weather like days like today although I don't think many of us want today's heavy rain and thunder impacts given how much rain has already fallen in recent days around the region.

First, let's mow through the severe weather risks.  Slight risk in the yellow shaded areas -- generally Philly, Montco, Burlington, Bucks, Camden, Ocean, Berks, Lehigh, and Mercer Counties.  North of that is in a limited to no risk area (Poconos, NYC) as of this morning.  Red shaded areas are in a moderate risk (Delco, Chester County, rest of South Jersey, and all of Delaware).  What will ultimately determine whether severe weather may be a bigger issue will be the position of the warm front later on today -- how far north the warm front gets (or doesn't) as the day progresses.

Radar as of 6:15 AM showed last night's squall line across Central Pennsylvania, with some additional thunderstorm activity parked over Eastern Pennsylvania working through the Lehigh Valley and towards North Jersey.  All of this is pushing east for the most part, with the southern edge of the squall line in Central PA pushing southeast.

Today is a two round type day when it comes to severe as two rounds of storms will impact the region. The first is this morning's activity, which is the leftovers of what moved through Chicago, Toledo, Cleveland, and Pittsburgh through the course of the night dropping a good bit of rain and gusty wind.  A severe thunderstorm watch is out for the DC area and Central Pennsylvania until 11 AM as a complex of thunderstorms moves through from the west. This morning complex of storms will bring heavy rain, some gusty winds, a bit of thunder as it moves through the region over the next few hours.  However, it's not the only act today.  More storms will probably develop later on; however, the odds for that thunderstorm development will be higher in the warm sector.

So, where is this warm sector going to set up?  Modeling does suggest the warm front lurks near the city most of the day, with areas south of it warmer than those to the north.  For instance, it could approach 90 in Dover later this afternoon while temperatures struggle to reach 70 in Allentown. Yeah, one of those days where 75-100 miles will separate clammy and soggy from hot and humid.  Philly will straddle that fence and it makes for a somewhat tricky forecast for us.

North of the city is pretty easy -- rain, lots of it. Limited risk of severe weather.  South of the city  is pretty easy -- less rain, a higher risk of severe weather since they will have a bit of sunshine, warmth, and the instability to produce severe.  We straddle that fence so the result is a tougher forecast here.

Once this morning's complex of storms pushes through, areas north of the city probably see some rain and thunder through the afternoon while city (or just south) and points south get a bit of a break before more storms develop later this afternoon into the evening hours. The higher resolution models like the NAM's high res (below) show a line of thunderstorms trying to develop after 5 or 6 PM across southern portions of the region, perhaps spreading a round of rain and thunder into Philadelphia by 7 or 8 PM (although stronger storms would line up to the south).  Other high resolution models are much less robust on the thunder threat for southern areas.  Where the thunder threat takes place is all dependent on where the warm front gets.

Rainfall could reach up to three inches north of I-78 later today with flooding a distinct possibility across the Poconos and Central Pennsylvania hills.  South of there, lesser rainfall amounts (an inch, perhaps two) down to Philly, with lesser rains likely south of that.

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More: Current Weather Page