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.
Here are the latest models with their rainfall totals –
The latest National Blend of Models (NBM version 4.0 ), often correct for QPF values, has even greater amounts—
The latest NAM NEST model has the heaviest rain more westward—
The Canadian Global (GDPS) is also a bit westward with higher amounts—
The German ICON model has a similar precip forecast—
So a generalized very heavy rain event is expected
Sat 10:31 PM Update — Tonight’s models just coming in seem to keep most of the thunderstorms early Sunday morning to our west and north.
Update Saturday 7:56PM— Some scattered thunderstorms also possible Sunday afternoon. The main events will be early Sunday morning and evening as outlined below.
A warm front will move through our area between 4 AM and 8 AM Sunday morning. Several models show showers and thunderstorms developing and moving through during this timeframe.
Showers end during the mid morning and we’ll have sunshine by the afternoon. It will be very hot and humid! Dew points move into the 70s and temperatures will reach a high of 91.7º sd 2.2º (high spread!)
Another round of thunderstorms are forecast during the early evening hours, between 6 PM and 9 PM Sunday.
As for Hurricane Isaias, the storm is expected by all models to move up the coast and affect our area Tuesday into early Wednesday. The chief concern will be very heavy rain in the Philadelphia area.Models are showing 3-5 inches of rain total.
Last season, the Navy experimental COAMPS tropical model did very well with forecast tracks—
As always, various hurricane models have different tracks—
The weak cold front that moved through on Friday will move north as a warm front later Saturday into Sunday. Showers expected with the warm front Saturday night. My regular readers know that a tropical system or hurricane near the continental US often plays havoc with the model forecast specifics.
Significant and increasing high level cloudiness (cirrus). Lower level clouds increase during the late afternoon and early evening.
High temp 87.0º sd 2.0º
Dew points mid 60s.
Winds calm to very light from the SE at 0-6 mph
The warm front moves north, but unstable and humid air may result in thunderstorms Sunday afternoon, especially west of Philadelphia.
Cloudy in the morning, sun breaks out during the late morning or early afternoon.
Showers linger early in the morning.
Thunderstorms and showers possible later Sunday afternoon.
High temp 91.3º sd 2.6º (larger than average spread)
Very humid! Dew points near 75º
Winds increasing 10-15 mph from the S and SW. Becoming gusty in the afternoon.
GFS (pre-release version 16) forecast Hurricane Isaias on Tuesday morning at 8 AM, currently the closest forecast approach to our area —
The model blend (NBM) shows much more rain to the west on Tuesday—