Saturday, June 08, 2013

Dry Spell Busting Rainfall

Most of our region picked up a month's worth of rain in about 24-30 hours from Thursday evening through last night, with three to four inches of rain common across the immediate Philadelphia metro. Some spots picked up more than that, with as much as five inches having fallen in King of Prussia (thank you "Fall Line" enhancement).  The rains, while excessive, were enough to knock out a dry spell that had started nudging a bit more into prominence over the past couple of weeks as we were running with a rainfall deficit in the three to four inch range for the year.

Well, so much for that.  I-95 and 20 miles either side were the winners, with the rainiest locales generally in a swath from the western Main Line through the Lafayette Hill and lower parts of Montgomery County, which is not at all surprising given the setup with Andrea and a nearby frontal boundary.  Those two factors typically yield a ton of rain along and near the Fall Line (which runs through the city northeast through Trenton and southwest through Wilmington and Baltimore).

The frontal boundary that helped squeeze rains lined up pretty close to I-95 yesterday morning, which acted as a mechanism for moisture to be squeezed out as it was pulled inland off of the Atlantic from Andrea's approaching circulation.  The front, plus the slight increase in elevation that occurs near the Fall Line, acts as a lifting mechanism for moisture off of the Atlantic, squeezing it out near the boundary and enhancing rainfall totals. This is not unusual -- it happened in Floyd, Irene, and other tropical or transitioning to nontropical systems that come up the coast but track offshore along a frontal boundary.

Philadelphia's 3.82" total just about closed the gap on the yearly rainfall deficit -- there's still a little bit of one (about a couple of tenths) but we're definitely out of the "we could use a nice rainfall" category.  The biggest story out of yesterday was stream and basement flooding for quite a few locations but thankfully that was really the extent of the storm's impact locally...there wasn't much more to report on other than "usual suspect" flooding around here and a few flash flood reports across parts of Delaware and Southwest New Jersey.

More: List of rainfall totals (NWS)