Monthly Archives: September 2025

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx #PhillyWx

An Interesting Pattern- Not as static as first thought

Posted Friday 09/12/25 @ 8:21 AM — At first glance, the current pattern looked to lend itself to a simple forecast: dry, sunny and increasingly warm. (That’s the gist from last night’s TV forecast.) But, the current pattern has more variation and uncertainty than first thought.

First, the showers forecast for Sunday evening by yesterday’s RRFS seems to have been a red herring. While still showing a few pop-ups, most of Sunday now seems be mostly sunny and rain-free in most areas, consistent with yesterday’s AI model forecast. Still some uncertainty.

Getting back to the current pattern…

Here’s the current water vapor image with RAP model superimposed parameters:

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. (Water Vapor Imagery reveals emitted water vapor infrared TEMPERATURE, NOT REFLECTED LIGHT from clouds) (Click on image for a larger view.)

From the above, you can see that the wave pattern is an open wave pattern but it wouldn’t take much for it to become an Omega block:

Open Wave Pattern
Omega Block Pattern : Ω

Indeed, several models are showing the development of a closed low along the eastern seaboard next week which moves the current pattern closer to blocked pattern and increases the chance of clouds and rain, especially along the Jersey Shore.

ECMWF-AI forecast for next Wednesday. Not really an Omega block, but closer. One closed low and another almost closed, separated by a weak ridge.

There’s a lack of full agreement with the above ECMWF-AI forecast, so we’ll have to see. Stay tuned.

Posted Thursday 09/11/25 @ 5:25 PM — The latest experimental RRFS suggests a possible change in the forecast for Sunday. The AI forecast map I posted earlier today showed an mid/upper level trough moving through Sunday. The latest RRFS takes this further and shows widely scattered showers developing Sunday with the possibility of more significant showers by 8 PM Sunday evening as a result of a short wave moving through.

12z RRFS forecast for Sunday at 8 PM. Significant showers possible. (Click on image for a larger view.)

Too soon to know if the RRFS model is an outlier with this forecast.

Friday Update: it is an outlier.

Originally Posted Thu 9:37 AM —I’m back from a vacation. It looks like the weather continued here without me.

Quiet late summer/early autumnal weather continues. No major storms to affect our area. The general trend is for dry weather through much of the weekend and next week. A general warming trend is in store for us.

A mid level trough moves through on Sunday, with some cloudiness and perhaps a few, widely scattered light showers. Warmer, dry weather builds in for much of next week.

Today’s 06z ECMWF-AI forecast for 8 AM Sunday. Weak trough moves through. High pressure and mid level ridge, waiting to build in for next week. (Color code: green-rain, blue= low clouds, Maroon = mid level clouds, tan = high cirrus level clouds.)