Friday, April 20, 2012

Raw, Damp, Rotten...But Needed Rains

Considering the dryness factor around the region the past few weeks, the coalescing of computer modeling around the coastal storm taking a track up the coast is not a bad thing...unless you have outdoor plans on Sunday afternoon, evening, or Monday.   There are still differences in track, detail of intensity, and how much rain would fall over the region over the coming couple of days but the GFS, Euro, and NAM are all on board for a coastal storm in the Sunday PM/Monday timeframe.

This storm is a bit unique for late April -- it's low pressure that will be captured by a deepening, sharpening mid level trough -- in some respects similar to the Snowtober storm in it being an anomaly for the time of year and it being a strong, captured storm system although the final results will be different...if this were two or three weeks earlier it might not have been though.   It really is a two piece storm system -- the first is Saturday's cool front that will bring showers and thunderstorms to the chances city and west during the afternoon hours.  It's possible we get some steadier, heavier thunderstorms in a few spots but severe is not likely.

The main course is then on Sunday afternoon into Monday -- a slug of rain will push north as the low intensifies off of the Carolina coastline.  The differences come into play at this point -- the Euro takes this rain and pushes it along the coastline and into New England -- with the heaviest rainfall falling east of the city and into Connecticut where the potential for three to five inches of rain would exist if the Euro is right.

The GFS, in contrast, brings the heavier rains northwest into the region as the mid level trough fully captures the surface low and pulls it inland overhead as it pushes northwest through Pennsylvania...the Euro brings the surface low farther north before beginning to pull it to the west.  The GFS scenario would bring heavier rainfall into Pennsylvania....potentially more than five inches in Eastern Pennsylvania.   Rainfall would be significant across the entire region but the steadier and heavier rains would be west of the city and not east on the GFS.

The GFS has flopped quite a bit from yesterday where it was suggesting a push of the low farther to our east and a brush along the New England coastline.   That trend has abruptly halted with last night's run.

The rains are needed but the potential for this much rain might cause some stream and river problems -- it would certainly do a number on the increasing dry island along the East Coast but if the higher end rainfall totals end up verifying we could have some issues with stream rises and localized flooding in spots.

To add "fun" to all of this is the potential for higher elevation snows west of I-81 with this storm system...and perhaps even into the Poconos on Monday night as the low pulls through and colder air moves in underneath the deep surface low.  The best chances for a decent snowfall would be over the higher hills near Johnstown, Altoona, and the Appalachian Plateau where several inches of accumulation cannot be ruled out.  If this were early April and not late April, the resulting storm would probably have brought snow closer to Philly.

We'll keep you updated on this over the coming days -- just note that the best weather this weekend will be on Saturday...especially morning and early afternoon.  It won't be a total washout all weekend but by Sunday late afternoon, the weather may end up just ducky for many!