Daily Archives: Monday, May 18, 2026 7:34 am

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx #PhillyWx

Thursday Weather Update

Posted Thursday 05/21/26 @ 9:51 AM — First, here’s last night’s rainfall—

MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch)      (Click on image for a larger view.)

Thursday’s weather is unfolding as predicted. An area of rain has developed north of the frontal boundary and it will slide southeastward over us through much of the day.

Isentropic Analysis at 320 ºK with radar at 9:30 AM shows vertical motion triggers causing lift and rain. The entire area moves southeastwar through the day. (Click on image for a larger view.)

Here’s the latest REFS forecast rainfall for today—

06z REFS accumulated rainfall estimate. (Probability Matched Mean) (Click on image for a larger view.)

Friday looks to be the best day of the coming Memorial Day Weekend. The models continue to show rain for Saturday into Sunday morning. Then continued low clouds for Sunday and possibly Monday.

Storms Update

Posted Wednesday 05/20/26 @ 4:23 PM — There’s more indication that the storms developing to our west will be severe, especially northern sections near Allentown and upper Bucks Counties.

4 PM radar with superimposed Satellite image and vertical velocity at thermal level 320ºK. The storms will follow the orange arrows pointing along lines of equal thermal energy. (Click on image for a larger view.)

The latest hourly RRFS shows strong storms, especially far north of us at Strong storms with forecast radar echo heights approaching 45,000 – 50,000 feet are impressive.

RRFS forecast echo heights at 6 PM indicating strong to severe storms. (Click on image for a larger view.)

I’ve seen much more threatening forecast parameters with past storms, but this will hold its own. The timing as indicated below still holds.

Wednesday Storms Update

Posted Wednesday 05/20/26 @ 9:34 AM — This morning’s early models are just becoming available. Here are the trends:

  • Scattered Showers and thunderstorms may break out as early as 2-3 PM in western Montco, Bucks and Chester counties.
  • A more organized line of storms are forecast to develop and move into the city about 4-8 PM.
  • The models are favoring areas to the southwest and south of the city for receiving the heaviest rain and strongest thunderstorms.
  • Severe weather parameters (vertical shear, helicity, CAPE) are not that impressive, but strong storms are always possible.
  • Tornado risk is close to zero
  • As forecast, a wave will form along the stalling cold front. The heaviest rainfall may occur Thursday as this low lifts towards the northeast along the front.
This morning’s HRRR (12z run) showing ‘simulated radar’ and 1 hour rain accumulation at 7 PM (Click on image for a larger view.)

Unfortunately, the models continue to show this front moving back as a warm front over the Memorial Day Weekend. Friday may be the nicest day coming up.

Wednesday Evening – Active Weather

Posted Tuesday 05/19/26 @ 5:55 PM — Temperatures reached 99º at 4:40 PM at Philadelphia Airport this afternoon, a record high temperature. Several areas of thunderstorms developed late this afternoon; these were not predicted by this morning’s NOAA models.

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine violet contours) and superimposed MRMS radar.

A cold front will approach our area and move through Wednesday night before slowing and stalling on Thursday. Several ingredients are coming together to produce the potential for strong, possibly severe thunderstorms Wednesday evening.

Currently, the RRFS is suggesting the strongest storms will be just north and northwest of the immediate city. Currently, the ingredient for tornadoes (helicity) is not present, but some high wind gusts and localized heavy rain is likely.

These storms will develop spontaneously as well a move in a line. Storms may develop as early as 4:30 -5:00 PM especially in northern and western areas. Areas nearer the city will experience storms between 6 PM and 8 PM.

Generally, the degree of severe weather is not captured by the models until the morning model runs. Check back at that time.

With the exception of a break on Friday, the AI and medium range models continue to forecast considerable rainfall in our area for much of the Memorial Day Weekend as the stalled front moves back as a warm front and then stalls again. This could change.

Tuesday Forecast Update

Posted Tuesday 05/19/26 @ 9:01 AM — Monday‘s forecast record breaking high temp for the PHL airport (96º) by the new Model Blend (NBM version 5) was spot-on.
Here’s the Model Blend forecast for today, Tuesday—

06z NBM forecast high temperatures for Tuesday. Fine Black Contours are near ground (2m) temperatures in 1º increments. (Click on image for a larger view.)

The heat index (Model Blend Apparent Temperatures) will be somewhat higher—

06z Heat indices at 2 PM Tuesday. Fine Black Contours are 1º increments. (Click on image for a larger view.)

Temperatures on Wednesday are still expected to approach 96º in the city. It appears that thunderstorms will move through between 9 and 10 PM Wednesday evening, with heaviest storms forecast just south of the city.


Originally Posted Mon 7:34 AM —A very warm (hot) flow of air from the southwest will bring near record temperatures to our area through Wednesday. A cold front will pass through sometime Wednesday evening or night but will not move far to our south. The front will stall and then move very slowly back north as a warm front. This has the unfortunate timing of bringing a strong probability of rain for at least the first half of the Memorial Day Weekend.

The expected sequence is shown on today’s water vapor satellite image—

Water Vapor image at 8 AM with superimposed RAP model MSL pressure (black contours), GFS/NAM potential vorticity contours (yellow fine contours) with MRMS Radar and surface wind barbs, The arrows (1) depict the wind and moisture flow through Wednesday. The cold front (near blue arrow) will move through and the flow of moisture will move to the arrows position (2) (Click on image for a larger view.)

Most models are predicting the scenario described above. That said, the models often are incorrect about the future position of a stalled front and I wouldn’t give up yet on the possibility of a nicer weekend.

As for today, Monday, temperatures will reach into the mid 90s.

06z Model Blend forecast High Temperatures Fine Black Contours are near ground (2m) temperatures in 1º increments. (Click on image for a larger view.)

06z Model Blend Heat Index at 4 PM Monday. Fine Black Contours are near ground (2m) temperatures in 1º increments. (Click on image for a larger view.)

I’ll keep an eye on the weekend forecast and the chance of storms on Wednesday. Stay tuned for updates.