Saturday, April 21, 2012

Timing Out Sunday's Rainfall

Sunday and Sunday night's coastal storm will bring the most significant rainfall that much of us have seen since early December.  It's a rather anomalous coastal storm for this late in the Spring but it does show that coastals can happen even this far into April...heck, they've even happened in May as it did in 2008.

It will not rain all of Sunday -- but the odds are rain will spread in as the day progresses, with rain moving in from the south and southeast.  Using the NAM as an example (not a forecast), any showers on Sunday morning (around 8 AM) will be concentrated along the coastline and across the Delmarva.

We should see rains approach the Philly metro by midday, continuing to slowly spread northwestward...the below graphic is from 2 PM and shows rainfall covering most of Southeastern Pennsylvania and beginning to approach the Lehigh Valley by this point.

The time of steadiest and heaviest rainfall will be from late Sunday afternoon into Sunday night.  Computer modeling varies in the placement of where the heaviest rainfall sets up.  The NAM brings the heaviest shield of rainfall over the I-95 corridor along both sides of the river.  By late Sunday evening (below), you can see the city about to get in on the brunt of rainfall from the storm...with the rest of the region getting steady rainfall.   Computer modeling is split on where the heaviest rainfalls end up occurring -- the NAM and SREF both suggest New Jersey gets in on rainfall that could reach two to four (or more) inches.

 The NAM totals from the 0z run last night (below) are in stack contrast with the GFS and EURO...not only placing the heaviest rainfall over parts of the region but in terms of how much may fall.  The Euro and GFS both are suggesting that an inch or so of rain would fall over the region, with two inches possible along the Shore.  The NAM from last night was trying to place 4-5" rainfall over South Jersey...the SREF was a bit less aggressive but still places as much as 4" of rain along the Shore in a few spots.   Ultimately, the heaviest rains will propagate north along the coast and then bend northwest at some point -- whether it's closer to Philadelphia or farther northeast towards New York City.  Exactly where this takes place remains to be seen.   Most of us, though, should see one to three inches of rain...maybe more in a few spots...maybe less farther west into Central Pennsylvania.

In addition to the potential one to three inch rainfall, winds will be gusty at times along the coast, reaching 40 miles per hour if not higher.  The strongest wind potential will be over the Atlantic but even at the Shore some strong gusts look possible.  Winds probably get into the 20's or 30's in the city in gusts at the brunt of the storm early on Monday.

We'll provide more information on this storm over the next couple of days but it does look like a decent to significant rainfall is on tap for the region.