Daily Archives: Monday, June 23, 2025 9:22 am

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx #PhillyWx

Thursday Thunderstorms

Posted Thursday 06/26/25 @ 5:13 PM — The NWS has issued a severe thunderstorm watch. Some strong thunderstorms have developed far northwest of Philadelphia. Latest radar at 5 PM—

Radar with RAP model superimposed vertical motion (Omega). Note the upward vertical motion is centered west of the city. (Click on image for a larger view.)

The models have only been so-so with today’s forecast. Many models have forecast areas of storms that have not materialized late this afternoon. That’s particularly true of the experimental RRFS which forecast many areas of rain at this very time.

The trend mentioned in this morning’s update suggests the heaviest activity will remain just west of Philadelphia, as shown in the NBM graphic earlier.

The latest HRDPS and NAM-NEST capture the most likely forecast, again keeping the heavy rain to our west.

12z HRDPS forecast for 8 PM. The main activity remains to our west and eastern parts of the city may not see all that much rainfall. (Click on image for a larger view.)

These things are tough to forecast with high precision. I guess we’ll see what happens in the next few hours.

Thursday Forecast

Posted Thursday 06/26/25 @ 9:44 AM — Nailing down the forecast locations and timing of thunderstorms yesterday met with mixed results; the additional storms forecast towards midnight didn’t materialize.

Today, Thursday, presents another thunderstorm forecast challenge. Model forecasts range from storms developing as early as 2 PM near the city to storms mostly staying to our west and south (and South Jersey) through most of the afternoon and evening.

The NBM total rain accumulation captures the overall trend of any showers and storms staying to our west, south and east—

The 12z NBM just available keeps much of the Philadelphia area rain-free through 8 PM with any showers/storms staying to our west, south and South Jersey. (Click on image for a larger view.)

In contrast to this NBM forecast is the latest ECMWF-AI model, which does have showers and storms in the immediate Philadelphia area by mid afternoon.

Also in contrast to the NBM is the Canadian GDPS- with “AI spectral nudging” which has thundershowers in and near Philadelphia between 3 PM and 7 PM.

00z GDPS with AI spectral nudging. 1 Hour Convective precipitation at 6PM (Click on image for a larger view.)

A tough call. I guess we’ll find out.

Wednesday Night into Thursday

Posted Wednesday 06/25/25 @ 8:16 PM — Isolated to scattered storms developed. Very low wind shear has resulted in the storms forming and then collapsing. (Wind shear is needed to keep thunderstorms alive.)

Current radar at 8:10 PM

Radar at 8:10 PM with RAP model vertical motion (Omega- green contours.) In addition to having no wind shear, these storms are moving into areas of negative Omega (downward vertical motion- white boxes around negative green dashed contours.) The storms should collapse, but not before some areas get some quick downpours. (Click on image for a larger view.)

Today’s HRRR was unusually poor in forecasting the current storms. The experimental RRFS did better. Continuing with the RRFS, it is shows additional scattered short-lived storms through 3 AM as the backdoor cold front slowly moves through.

Thursday will have a mix of clouds and sun. Isolated to scattered storms are expected to begin to pop about 3 PM into Thursday evening. More details tomorrow.

Wednesday into Early Thursday

Posted Wednesday 06/25/25 @ 8:30 AM — As described yesterday, the heat dome will retreat late Wednesday as cooler high pressure slowly noses in from the northeast (backdoor cold front). Before that time, temperatures will again approach high levels, between 98º to 100º in and near the city.

Dew points will be higher today at peak temperature time, and I’ll have to resort to the Heat Index temperatures to capture the full impact. (I avoid posting heat index values since I think they’re often used to unnecessarily sensationalize, exaggerate heat episodes on TV broadcasts. Maybe not so today.)

06z NBM “Apparent Temperatures”, aka Heat Index. (Click on image for a larger view.)

As for thunderstorms, high heat, high humidity and several short waves ahead of the approaching backdoor cold front will create conditions ripe for thunderstorms with heavy rain. The latest HRRR is least on-board with storms here, but the AI models, the NAM-NEST, Canadian RGEM,and the RRFS show the possibility of isolated storms (especially in the wind shift/convergence zone in NJ) as early as 2PM.

Increasingly thunderstorm coverage expected late in the afternoon and especially in the evening hours (6 PM to midnight) where they should become more numerous. Not everyone will see a storm, but those that do will likely see some heavy rain. Severe winds, always a possibility, are not expected at this time.

To convey the scattered nature of the storms, here’s another model parameter that attempts to predict the strength of individual thunderstorm cells at 7 PM.

06z NBM Estimate “Radar Echo Heights” at 7 PM. Anything over 30,000 feet can be a very strong thunderstorm cell. Here, there are some above 40,000 feet. The exact predicted location of these storms cannot be taken literally with these models. (Click on image for a larger view.)
Wednesday- Heat Early Afternoon Reduced by Thunderstorms

Posted Tuesday 06/24/25 @ 7:57 PM — Disturbances moving across the upper heat dome are expected to bring showers and thunderstorms to our area Wednesday afternoon and evening. Some may be strong.

Water Vapor image Tuesday evening shows upper air disturbances poised to move towards us on Wednesday afternoon and evening as the heat dome retreats somewhat allowing them to reach us. (Click on image for a larger view.)

High temperatures of 99º and possibly 100º in parts of the city will be reached early, before 2 PM.

Thunderstorms develop and move in from the northwest and develop in place as early as 2 PM and continue into the evening. Impressive thermal instability (negative 8 “lifted index” and near 40 K index) and high CAPE values approaching 3000 J/Kg could make any storm strong to severe.

I’ll update Wednesday morning.

Tue 4:30 PM —Forecast Review — Another hot day, where the AI models were more accurate with their temperature forecasts than the very advanced NBM statistical Model Blend. (Although the Model Blend did fine when you factor in the standard deviation.)

Here’s the RTMA temperatures at 3:45 PM—

RTMA temperatures at 3:45 PM today. Black contours are 1º increments (Click on image for a larger view.)

Here’s the official readings from the NWS Philadelphia Airport KPHL—

Highest Heat Index occurred at about 2 PM with the dew points being the highest. (Click on image for a larger view.)

Heat Continues

Posted Tuesday 06/24/25 @ 9:26 AM — Many models have similar high temperature forecast for today. The latest Model Blend (NBM) just became available. Here’s its forecast highs with degree of uncertainty (standard deviation) for today, Tuesday—

12z NBM High Temperatures (not heat index or apparent temps) (Click on image for a larger view.)

Here’s the latest ECMWF-AI model high temperature forecast—

06z ECMWF-AI High Temp forecast (Click on image for a larger view.)

Here’s the new Canadian CMC GDPS-AI forecast—

00z CMC GDPS-AI model high temps for Tuesday (Click on image for a larger view.)
Heat Recap

It really felt hot today! Temperatures seem to have only reached 99º in this area, but dew points were 70º (and where highs were only 96º, e.g. Blue Bell, the dew points were closer to 74º and 75º

Here’s the RTMA @ 3:45 PM, capturing the area’s temperatures—

RTMA 15 minute repeat model. Actual with interpolated temperatures. (Click on image for a larger view.)

The NWS has modernized their website for KPHL airport, now showing temperatures every 5 minutes—

(Click on image for a larger view.)

Of interest is the localized area of showers that developed, as predicted by the new Canadian GDPS-AI along with the wind convergence zone—

Radar at 4:35 PM Wind convergence liine with showers in southern Cape May county, as predicted by the Canadian AI. (Click on image for a larger view.)

It’s interesting that the experimental RRFS did not predict these showers in Cape May. (The NAM-NEST did predict these as well.)

For Tuesday, here’s the latest ECMWF-AI temperature forecast—

ECMWF-AI Single Temperature Forecast for Tuesday(Click on image for a larger view.)

Stay cool!

Originally Posted Mon 9:22 AM —Yesterday’s high temperatures were diminished by the unexpected and unforecast clouds and showers.

The heat will be here for sure today.

Continuing with the “experiment” from yesterday, here is the new Canadian GDPS-AI along with the Model Blend (NBM) forecast high temps for today. likely about 3-4 PM—

00z Canadian GDPS-AI high temps for Monday (Click on image for a larger view.)

Here’s the Model Blend (NBM) high temps—

06z NBM forecast high temps. Contours are 1º increments. (Click on image for a larger view.)

The “Open Data” version of the ECMWF-AI single is very similar to the above.

Just to see if the Canadian has any unique insights, it is forecasting some thundershowers to develop in NJ at 3-4 PM due to the development of a moisture convergence zone—

CMC-GDPS Moisture convergence zone thundershowers.

Stay cool!