Flood Watches & Banding Of Rainfall

Today's weather, while unsettled, will seem rather nice in comparison to what we have coming at us for Tuesday and Wednesday.   The potential for heavy rainfall in the region on Tuesday and Wednesday from an upper level low tapping into tropical moisture (see the satellite picture below) has lead to flood watches getting issued for the I-95 corridor for the next couple of days.

The satellite image shows an upper level low over the Southeastern US and a broad southerly flow setting up along the East Coast.   A bit of a backdoor front has drifted south across the region today and will act as a rainfall enhancer over the coming two days as the southerly flow of moisture lifts across this boundary as well as the higher terrain to the north and west of Philadelphia.   This will result in periods of showers and storms, some of which will be heavy at times, as the flow around the upper low pushes rainfall north, then northwest around its circulation.

Rainfall will be heavy at times and in spots over the coming two days, with some potentially significant rains in some spots by the time we get out of Wednesday.  Some spots could pick up over three inches of rain, with many of us getting over an inch over the next two days.   Due to the showery nature and banding nature of the rainfall, predicting the heaviest amounts is low confidence but there will be two periods when our region could see heavier rainfall.  One is from Tuesday morning, with a second heavier period possible on Wednesday afternoon.  Showers/storms in between could be locally heavy but guidance seems to suggest the heaviest and most widespread rainfall in Philadelphia will be in those two time frames.   The heaviest totals overall are focusing on New York City according to the NAM, with four inches possible if the midday guidance is right.

The Euro is a bit less aggressive on heavy rainfall, with two inches of rain across areas north and west of Philadelphia.   The GFS has an area of heavy rainfall west of Philadelphia (three inches plus) and up near New York City (three inches there as well).   Needless to say, figuring out the "exact" location of heavy rain is not a perfect science but if you are north and northwest of Philly, the odds are pretty good that a couple of inches or more of rain will fall by Thursday morning.

We'll have another update in the morning.