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Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#Philadelphia #weather #PAwx #PhillyWx

Saturday Evening Thunder

Posted Saturday 06/28/25 @ 5:31 PM — On the surface, tonight should offer some fairly heavy thunderstorms; there’s plenty of CAPE (exceeding of 3000 joules/kg) and plenty of precipitable water (2.1″) and an advancing cold front as a trigger.

That said, the model forecasts aren’t all that impressive with the severity of the storms. There’s little to no jet stream energy aloft (the jet flow is far north of our region). Vertical wind shear, the ingredient needed to prevent thunderstorms from collapsing on themselves, is very low. Helicity is low-range elevated in the Allentown area, not so much here.

19z RRFS simulated radar/1hour rainfall at 9 PM. The heaviest activity ins near Allentown. (Click on image for a larger view.)

The latest Canadian RGEM just became available and it has some heavy rain closer to the city. This is in contrast to the US models and the AI model—

18z RGEM total rainfall through 1 AM Sunday morning. (Click on image for a larger view.)

So expect some showers and thunderstorms. Severe is always possible, but I don’t think so. Always follow the NWS advisories for these sort of things.

Sunday continues to look sunny and dry.

Saturday Update

Posted Saturday 06/28/25 @ 9:19 AM — The warm front is just to our west and clouds are beginning to breakup in S Jersey and in central PA.

Warm front will move through later this morning. Clouds break up for some sun by 11 AM to 1 PM. No rain expected until later this evening.

Showers and thunderstorms ahead of the next front (a cold front) will move in to western sections as early as 6-8 PM and continue through midnight.

Weekend Forecast

Posted Friday 06/27/25 @ 7:36 PM — Friday‘s rain was unexpected and not forecast by many models. Let’s see if we can do better with Saturday and Sunday.

Saturday: The front that went through late Thursday giving us the cool temperatures and easterly flow on Friday will return as a warm front and pass through between 10 AM and noon. The warm front will be preceded by showers and maybe some thunder, but there’s a wide range in model guidance regarding how widespread the showers will be.

Following the front, temperatures will rise into the mid and maybe upper 80s with dew points in the highly uncomfortable range of lower 70s. Many areas will see some sun by noon or sun filtered by clouds.

We may luck out with a dry period between noon and 5 PM. The atmosphere will be highly unstable and CAPE values will be close to extremely high levels of 4000 joules/kg. Shear will be very low, so pop-up storms will collapse on themselves and be short-lived and highly scattered. Pop-up storms will become more numerous, certainly by 5 PM and afterwards.

18z GFS forecast for midnight Saturday night. Cold front moving through with showers and storms. (Click on image for a larger view.)

A line of storms will move through Saturday night around midnight into the hours before daybreak Sunday morning.

Sunday will be mostly sunny and less humid. A period of cloudiness is expected around noon, but the sun should return filtered through high cloudiness in the afternoon. Lower dew points in the mid 60s with highs in the low to mid 80s.

Friday and the Weekend Preview

Originally Posted Fri 9:29 AM —The backdoor cold front that pushed through on Thursday has become stationary to our west and our south.

Fri 10:11 AM – Sorry folks, something was clearly wrong with the data I was looking at an hour ago for this forecast. Showers have broken out just west of the city. The RRFS that I was looking at showed nothing all day! Maybe that’s why they call the RRFS experimental”
Nexrad Radar at 10:11 AM (Click on image for a larger view.)

The RRFS forecast for the same time frame shows nothing!

00z RRFS Simulated Radar forecast for 10:10 AM. It shows nothing for our area! (Click on image for a larger view.)

What’s most disturbing- the NWS announced just yesterday that they’ll be retiring several models (NAM, HREF, HIRESW, NARRE) when the RRFS becomes operational in early 2026. See my RRFS page.

There is ongoing development of the RRFS where the plan is to take it from its current FV3 core to the MPAS (“model prediction across scales”) geometry. Here’s the highly experimental RRFS MPAS forecast for today which over-forecast the rain to our south, but captured these showers-

MPAS RRFS developmental model. For this, raw data is not available to the public. This is a NOAA graphic) (Click on image for a larger view.)

It’s not clear if the first operational release will be with the FV3 core or the MPAS core.

Originally Posted Fri 9:29 AM —

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. I’ve drawn in the approximate position of the stationary front.

As a result, Water Vapor Temperatures indirectly reveal height, called the “effective layer” of the water vapor in a region, therefore revealing the mid and upper level 3D contours of systems.. Colder temperatures (blue- grey – white) indicate higher altitude, sometimes revealing upward motion. Red- Orange indicates warmer temperatures and a lower height for this layer. There are several water vapor “channels”; the one I display here is called the mid level height channel.   (Click on image for a larger view.)

For today, Friday, a cool, stable easterly flow will keep skies cloudy, but the RRFS does have some sun filtered through high and mid level clouds early afternoon. Except for a quick possible sprinkle, the Philadelphia region should remain dry today.

Clouds return late afternoon and evening as the stationary front moves northeast as a warm front.

Saturday will have clouds in the morning, possibly with some showers, sprinkles and fog, then breaking for sunshine and increased humidity and temperatures as the warm front moves north. Showers and thunderstorms possible Saturday night, especially after midnight.

Sunday becomes sunny and quite warm, humid. Details will depend on the actual position and movement of the warm front.

Despite the unsettled and changeable weather this weekend, I don’t see much in the way of widespread substantial rain around Philadelphia

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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!

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#Philadelphia #weather #PAwx #PhillyWx

Sun 1:05 PM —Forecast Review — Not a forecast to be proud of— Both last night’s and even this morning’s models failed to forecast this “mesoscale complex” with strong upper air support dropping south along the edge of the mid level heat dome.

I was out on a hike this morning and looked at the darkening skies to our north. I downloaded new data to my iPhone remotely and and saw the radar in no way matched the model forecasts from even two hours earlier. I remotely uploaded the “Update” below at 10:43 AM.

Here’s the current radar with the hourly RAP model superimposed—

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. Orange contours are jet stream level vertical motion. Strong UPward areas right over Philadelphia suburbs into NJ will enhance showers/storms. The blue shading is the strong upper air vertical motion of the Mesoscale Complex.
  (Click on image for a larger view.)

This morning’s HRRR (12z -8AM EDT) shows the a diminished mesoscale complex moving far north of us at 1 PM—

12z HRRR forecast for 1 PM Sunday. Rain stays well to our north. Completely under forecast the rain and position. (Click on image for a larger view.)

The HRRR is one of our best “CAM” models. (CAM = “Convective Allowing Model”, the sort of process occurring that this model is designed to capture. ) Clearly, there was some input upper air data problem at the very least.

Our newest CAM model, the experimental RRFS showed a bit of activity with a 14z (10 AM) model run. Still quite disappointing.

14z (10 AM EDT) hourly RRFS (experimental) did somewhat better, but still not a good forecast. (Click on image for a larger view.)

Even this morning’s ECMW-AI model, just available, didn’t get this right—

This morning’s 12z ECMWF-AI model kept the rain far north of our area. (Click on image for a larger view.)

As I’ve said as recently as last week, these modeling errors and forecast ‘surprises’ is one of the things that I find fascinating about meteorology.

Update Sun 6/22 10:43 AM — The large area of rain to our far north was forecast by the models to be further north than currently on radar. They were also forecast to dissipate. They are the result of hot humid air pushing northward in the mid levels of the atmosphere.

The latest HRRR still shows it dissipating as it approaches.

Posted Sunday 06/22/25 @ 9:14 AM — The Canadian Meteorological Center (CMC) announced their release this week of their advance into AI weather models with the release of an experimental global model based on the GDPS. (GDPS-AI) . The CMC describes this new model as “spectrally nudged” towards an AI model. (Weather models use wave-form physics (spectral methods) to recreate the atmosphere structure and dynamics.)

The GDPS-AI is run twice daily, 00z and 12z and becomes available four hours and 45 minutes after it is run. It forecasts out 240 hours. The nice thing about this AI model: it has hourly forecasts through forecast hour 84, in contrast to the European and US AI models that forecast at 6 hour intervals.


My experience with the Canadian models has found their regular models tend to bias towards the high side of temperatures in this region during the summer months.

So let’s do an ‘experiment’. Let’s see if their AI version is forecasting temperatures that are too high today?

00z GDPS AI model temperature forecast for 5 PM today, Sunday. (Click on image for a larger view.)

Here’s the latest NBM model, just available. Its highs are significantly less, but the temperatures have a wide uncertainty (standard deviation) of ± 2.2º (The NBM (“model blend”) is a weighted average of 30-40 models)

Today’s 12z NBM high temp forecast. Contour lines are 1 degree increments. The Temps are statistical means, ±2.2º (Click on image for a larger view.)

It wouldn’t surprise me if the NBM is more accurate today, assuming adding 1º-2º to to its forecast.

Originally Posted Sat 10:28 AM —The main weather feature of this weekend and much of next week will be a building upper level ridge, bringing very high temperatures and humidity to the Philadelphia region. The building ridge is visible on this morning’s water vapor image—

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 based on TEMPERATURE, NOT REFLECTED LIGHT from CLOUDS.
As a result, Water Vapor Temperatures indirectly reveal height, called the “effective layer” of the water vapor in a region, therefore revealing the mid and upper level 3D contours of systems.. Colder temperatures (blue- grey – white) indicate higher altitude, sometimes revealing upward motion. Red- orange indicates warmer temperatures and a lower height for this layer. There are several water vapor “channels”; the one I display here is called the mid level height channel. (Click on image for a larger view.)

The expected temperatures based on the European AI model (the ECMWF-AI Single) were posted yesterday. When the temperatures here are hitting 100º, I don’t see that much value, (and only hype) in giving the heat indices.

According to the latest NBM, high temps will hit 90º today, Saturday near the city and 97º on Sunday.

When the temperatures get this hot, there’s always the possibility of an isolated thundershower popping up. A trigger is often needed for these to occur. The models show some storms in far northeastern PA later this afternoon.

Yet with the upper atmosphere being so hot, the upper level heat in many ways inhibits convective rainfall.