Monthly Archives: September 2023

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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This Weekend’s Storm

Updated Wed 09/20 @ 8:37 AM — The models are in good agreement (with the exception of the ECMWF) that a major Nor’easter with some tropical characteristics will affect our area beginning Friday night.

The latest GFS shows this storm at 2PM Saturday—

06z GFS shows major low pressure system moving up the coast at 2 PM Saturday. (Click on image for a larger view.)

Anywhere from 1.5-4 inches of rain are possible with this system along with high winds and coastal effects.

The latest German ICON model shows very high rainfall possible—

Latest ICON model shows large areas of 3-4 inches of rain by Sunday afternoon. (This model is at the upper end of the rainfall forecast range.)
Wednesday Through Sunday Outlook

Update Tue 9/19 9:28 PM — Continued fair weather through early Friday. Increasing cloudiness expected during the day on Friday.

The models have come together forecasting a slow moving coastal storm, held back by a westward closed upper low. This is expected to move in Friday night.

NAEFS forecast for 8 AM Saturday. ( Click on image for larger view.)


The ECMWF, which had been holding back on the development of this system, has joined the other models forecasting this storm. It is a bit slower than the GFS and NAEFS, with the rain moving in during the day Saturday. Currently, it looks like it will be raining most of Saturday with somewhat light showers on Sunday.

Sometimes these storms move out faster than the model forecasts, so there’s some hope for Sunday.

Tuesday – Friday Outlook Update

Updated Mon 09/18 @ 6:23 PM —The low pressure system off of the coast will move northeast and high pressure builds in for Tuesday through Thursday.

Fair skies and seasonably mild temperatures expected.

ECMWF Temperatures forecast through Friday (Blue Bell, PA) (Click on image for a larger view.)

Clouds move in sometime on Friday.

There’s uncertainty with the low pressure development off the Florida coast. The NAEFS (combined GEFS and Canadian Global Ensemble) continues to forecast an “inverted coastal trough” moving in Friday into Saturday with clouds and an increasing chance of showers. The ECMWF is NOT currently on-board with this system moving northward. I’m leaning towards the NAEFS—

NAEFS forecast for Friday. Inverted trough along the Atlantic coastline of Florida is forecast to move northward toward us. (Click on image for a larger view.)

Update Sun 9/17 11:09 PM — Tonight’s models are forecasting a slower exit of the showers Monday. We may still see some showers in Philadelphia and eastward as late as 1-2 PM.

Previously Posted Sun 8:03 PM —

A weak coastal low will move northeast Monday. Showers may linger until 10 -11 AM and clouds will linger into the early to mid afternoon.

High pressure builds in for Tuesday through much of Friday—

NAEFS forecast for Wednesday at 11 AM. High pressure over the Mid Atlantic and Northeast will keep sunny skies and and seasonable temperatures. (Click on image for a larger view.)

Average seasonal high temperatures this week are 75º-76º. Temperatures this week will be close to slightly above seasonal averages.

So an uneventful weather week is in store for us.

Of interest is a potential tropical or semi-tropical system expected to develop off the Atlantic coast of Florida sometime on Friday. This is forecast to move north and bring clouds and rain late Friday into Saturday. Things will likely change by the time Friday rolls around.

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

Updated Sun 09/17 @ 6:03 PM — It appears that some rain will linger here in the Delaware Valley until about 10-11 AM Monday. Cloudiness will linger longer…until about 2-3 PM.

More info in This Week’s Weather to be posted this evening.

Update Sat 9/16 10:51 PM — It looks like rain will only affect eastern NJ and the shore on Monday. but clouds linger here with some clearing in the afternoon.

Update Sat 9/16 9:39 PM — This evening’s models are showing showers moving in as early as 3-4 PM Sunday.

A large dip in the jet stream will spawn a coastal low pressure system late Sunday and it may affect our weather with rain Sunday evening through early Tuesday.

Water Vapor image (channel 9) captures mid and upper level flow (jet stream level) Saturday afternoon. White arrows show the general location of the jet stream. A low pressure system (blue L) will be form and move up the coast later Sunday through Monday with rain.

Sunday

Sunny in the morning. Increasingly cloudy beginning 12 to 2 PM. Showers move in from the west between 5 PM and 7 PM.

Model temperatures have a higher than usual uncertainty (spread) due differences in forecast cloud cover. sd =2.5º

High temperature 76.7º ( NBM model— location Blue Bell, PA)
High temperature 79.3º ( NBM model— location Philadelphia, PA)