Tue 08:41 AM Update — graphics added below

The 2 AM (06UTC) runs of the models show the following changes in the forecast for tropical storm Isaias—

  • The storm is moving faster than previously forecast-  peak winds and rain rate will reach us between 10 AM and 1 PM.
  • The track of the storm is a bit further westward.
  • Rain totals will be the highest to the west of Philadelphia, in Lancaster county.  Still an additional 2-4 inches to fall. The precipitation maps posted yesterday still hold.
  • Higher localized rain amounts in thunderstorms.
  • Wind gusts will peak about 11 AM and again at 1 PM.  Wind gusts around 60 mph possible in the Philadelphia area, higher in NJ.
  • Rain will be ending by 2-3 PM

Some differences exist with areas of max rainfall.

Below is the latest GFS model accumulated rain forecast (from 2 AM, which does not include the rain from last night prior to 2AM. )

GFS forecast accumulated Rain from 2 AM  onward (Click on image for a larger view.)


Here’s the very latest HRRR (High Resolution Rapid Refresh) hourly model from 7 AM —

HRRR accumulated Rainfall from 7 AM





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


As we can see, the forecast for today had shifted somewhat and the earlier showers and thunderstorms moved to our west before daybreak and early this morning.  This wasn’t picked up until last night’s models.

For today, Sunday, most of the day will be dry and hot.  High 92.6º sd 1.6º.  Dew points in the 70s!

There is a chance of showers/thunderstorms this evening, between 8 PM and midnight.

With a tropical system in the picture, I expect further shifts in the forecast.  The model forecasts just seem to be more ‘fluid’ when there’s a tropical storm around.

That said, the track of Hurricane Isaias appears to be taking a more inland path.  Here’s the Navy COAMPS model track—


With the more inland track, the GFS (pre-release version 16) is showing 3-5+ inches of rain for our immediate Philadelphia area on Tuesday—

GFS (v 16) 12 hour Accumulated Rain forecast for Tuesday 4 PM  (Click on image for a larger view.)

It will be a tropical storm or tropical storm remnant when it reaches us.  The main issue will be flooding rains.   Stay tuned,


Weather… and other things "up in the air"