Mon 10:45 PM Update — More new model data. Wind gusts may exceed 60mph in Philadelphia and immediate suburbs.
Mon 10:15 PM Update — Tonight’s short range models just becoming available continue with the forecast of a generalized 3-5 inches of rain.  Heaviest amounts in the western suburbs. The precipitation maps posted below are still valid. The storm will be approaching sooner and moving faster. The precipitation rate peaks around 11 AM -2 PM with over an inch of rain per hour falling. Wind gusts are higher, especially in NJ.  

Tropical Storm/Hurricane Isaias continues to move up the coastline.  Very heavy rain continues to be the main threat from this storm. The heaviest rain may fall over a period of just a few hours.

The trend has been for the heaviest rain to fall slightly further west than yesterday’s model runs.  Some models are predicting amounts as high as 5-7 inches of rain in some spots!  The heaviest rain will fall between 11AM and 3 PM Tuesday.

The peak winds will occur around 1-2 PM.  Gusts around 35-50 MPH are forecast by the latest GFS in the PHL area.

GFS wind gusts forecast 1 PM EDT Tuesday  (Click on image for a larger view.)


Here are the latest models with their rainfall totals –

GFS model accumulated rain forecast  (Click on image for a larger view.)

The latest National Blend of Models (NBM version 4.0 ), often correct for QPF values, has even greater amounts—

NBM Accumulated Rain forecast. (Click on image for a larger view.)

The latest NAM NEST model has the heaviest rain more westward—

NAM NEST Precip forecast (Click on image for a larger view.)

The Canadian Global (GDPS)  is also a bit westward with higher amounts—

Canadian (CMC- GDPS)  Rain Forecast (Different color coding)  (Click on image for a larger view.)

The German ICON model has a similar precip forecast—

German ICON model  (Click on image for a larger view.)

So a generalized very heavy rain event is expected