Tag Archives: Heat Wave

Philadelphia Area Weather Forecasts

THIS WEEK’S WEATHER

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#Philadelphia #weather #PAwx #PhillyWx

Friday & the Weekend Outlook

Posted Friday 07/03/26 @ 7:26 PM — We hit an official high of 102º, this time captured by the RTMA at 4 PM

RTMA 20z High Temperatures. Fine Contours are near-surface (2m) temperatures in 1º increments. (Click on image for a larger view.)

Right now, it appears that both the 12z and 18z runs of the RRFS have over-forecast thunderstorms again. That said, a line of storms has developed to our north, moving towards northern parts of our area. The caption explains why these storms may fall apart or weaken as they approach—

7 PM Radar and RAP model 700 mb wind streams. The dashed yellow line is where the RAP model is showing the Omega (vertical motion) turning downward which may cause the storms to weaken or dissipate as they approach. (Click on image for a larger view.)
Friday Update

Posted Friday 07/03/26 @ 10:21 AM — First I want to address the discrepancy between my reported high of 102º yesterday and the actual official 103º high for Thursday. I emailed the NOAA Mount Holly office inquiring about the RTMA and the online 5 minute data showing 102º and the reported 103º official high discrepancy. Here’s their reply—

“The hourly and 5 minute observations are reported in whole degrees Celsius; the website which displays it then converts the observation to Fahrenheit. However, the temperature sensor measures in Fahrenheit. Consequently, there can be rounding errors in the multiple conversion processes with the observations.  The preliminary daily climate summary is based on the sensor data, not a maximum or minimum of the observations.”

This seems more than a bit overly convoluted and I’m not sure I really understand why any errors resulting from multiple ºF<—>ºC convertion rounding wouldn’t be compensated for on their websites. But that’s the official story.

The official high temperature is posted online here daily about 5:30 PM.

For today, Friday, the NBM, which has done so well past days, is forecasting these high temperatures—

This morning’s 12z max Temp (Model Blend- NBM) for Friday. Highs are reached usually between 3:30 PM and 5 PM. (Click on image for a larger view.)

As for rainfall today and this evening, the ECMWF-AI model keeps any showers far north away from our area, as does the latest HRRR. The RRFS, which in its current first release version, has a known bias to over-forecast convective precipitation, is predicting widely scattered showers here at about 8 PM- midnight. We may be learning that our newest model has some issues. Then again, the Canadian HRDPS is also forecasting showers. I guess we’ll see.

Friday Forecast

Posted Thursday 07/02/26 @ 7:57 PM — For Friday, I’m sticking with the Model Blend (NBM model) which has performed the best over the past three days. (The ECMWF has been forecasting max temps that are too high. The new RRFS model has been very disappointing regarding its temperature forecasts which are too low by as much as 2º-4º!)

The high temperatures being forecast for Friday are expected to be about 1º higher than Thursday—

Today’s 18z Model Blend (NBM) high temperature forecast for Friday. Likely to occur about 4-5 PM. (Click on image for a larger view.)

As for the chance of rain, the RRFS shows a cluster of thunderstorms moving through between 7 PM and 9 PM Friday evening. Not all areas will receive rain. I’ll need to look at the water vapor image Friday morning to have greater confidence in this rain forecast.

Thu 7:45 PM —Forecast Review —Here are the RTMA high temperatures at 4:45 PM Thursday.
RTMA Temperatures recorded at 4:45 PM Thursday. The RTMA temps are confirmed 6 hours later by the URMA model, not yet available. (Click on image for a larger view.)

Thursday Forecast Update

Posted Thursday 07/02/26 @ 10:18 AM — Today’s forecast high temperatures remain unchanged from yesterday. Actual Temps, not Heat Index.

This morning’s 12z NBM 12 hour Temp Max Actual Temperatures Fine Contours are near-surface (2m) temperatures in 1º increments. (Click on image for a larger view.)

Yesterday’s ECMWF and NBM forecasts were spot-on. Here’s today’s ECMWF Thursday’s high temperature forecast—

This morning’s (06z) ECMWF forecast high temperatures (5 PM) for Thursday. Fine Contours are near-surface (2m) temperatures in 1º increments.(Click on image for a larger view.)

Note that the ECMWF shows ‘cooler’ temperatures at the shore; this is an artifact and the result of the model I access is somewhat lower resolution and each grid point covers a wider area that spans over the ocean and over the land.

The Phillies are playing this afternoon!

Citizens’ Bank ParkBlue Bell
Actual Temp 2 PM100.199.3º
Heat Index 2PM 112.4º108.9º
Wet Bulb Globe Temp92.591.3

There appears to be no chance of thundershowers or rain today.

Forecast Update

Posted Wednesday 07/01/26 @ 5:30 PM — Today’s high at PHL airport was 97º. (The Model Blend forecast for today was for 97.7º± 2.2º The ECMWF forecast was for 97.1º) Dew points were in the mid 70s, making it feel much hotter.

For Thursday, the Model Blend forecast high is for 103.3º. The ECMWF forecast high is now ‘only’ 102.8º. Dew points will be lower, closer to 69º.

The latest RRFS is still forecasting some storms for early this evening just northwest of the city. (The HRRR is not.) The HRDPS has a few scattered storms, mostly in Bucks county and north. Current water vapor suggests the HRDPS might be correct.

5 PM Water Vapor (mid level) and Radar with 700 mb wind streams. (Click on image for a larger view.)
Wednesday Forecast Update

Posted Wednesday 07/01/26 @ 9:55 AM — Last night’s models continue with the similar temperature forecasts as previous runs; the trend is for somewhat hotter Friday and Saturday than Thursday!

A comment asked about heat relief from rain/thunderstorms.

Water Vapor image shows any upper/mid-level ‘triggers’ (upper level potential vorticities) for storms and Philadelphia doesn’t lie downstream in the upper air path of these disturbances—

Water Vapor image at 9 AM with superimposed RAP model MSL pressure (black contours), 500-1000mb Thickness (yellow contours) , GFS potential vorticity contours (violet fine contours) with MRMS Radar and surface wind barbs, (Click on image for a larger view.)

So there doesn’t appear to be any upper air triggers. However, there may be some triggers for storms at lower levels of the atmosphere.

Regarding pop-up storms, some may develop about 4-7 PM to our far west, according to Q-vector 700mb analysis of the RRFS data—

The Q-vector divergence over Philadelphia suggests that any storms that develop to our west may fall apart as they enter the area of divergence. (Click on image for a larger view.)

At even lower levels (850 mb) , there does seem to be a thermal energy trigger that may set off some storms—

06z RRFS theta-E 850mb frontogenic forcing at 6 PM may support pop-up storms about (Click on image for a larger view.)
Commentary:

As mentioned recently, over past months, I’ve been working with Claude AI and ChatGPT (mostly Claude) to develop Q-vector analysis and Frontogenic Analysis tools derived from the new RRFS model. I’ve been built them into my automated model download post-processing These tools are “derived parameters” and aren’t part of the model, but they are routinely used by meteorologists. Regular use is new to me and it will be a learning experience going forward when to apply these tools.

I’m gradually moving some of these post-processing tools to my downloads of the HRRR and Canadian HRDPS models.

Despite its newness, I’m not convinced that the RRFS is the best forecast model at this time for my efforts. As an example, the RRFS is forecasting high temperatures today that are way too cool (-3-4º) compared to the HRRR, HRDPS, ECMWF, ICON, and GFS. There are known deficiencies with the RRFS and they are being addressed with a RRFS v2 that is not yet available.

So, in supplying these possibilities of thunderstorms here late this afternoon, based on these tools and the RRFS, I’m stepping out into new forecasting territory.

It’s Going to be Over 100º.
Why are they still talking about the Heat Index?

Posted Tuesday 06/30/26 @ 8:04 PM — I suppose this falls into the category, “Opinion – Commentary”.

I know the TV weather entertainers like to talk about the Heat Index. It’s often (but not always) higher than the actual temperature and it can make a typical 96º-98º summer temperature forecast sound very ominous. This week we have a high temperature forecast that is actually in the low 100s and it’s already ominous. It really doesn’t need any additional hype.

Use of the heat index this week is actually a disservice since it fails to distinguish this true health-threatening 103º forecast from a heat index of 103º-105º that you hear about at some point almost every summer.

Let me put this another way. Were you going to do a major work project outside on Thursday (Will the Phillies really play?) knowing there will be a temperature of 103º, but now, knowing the heat index is 110º, you’ve decided you’re not going to do the project? Let’s get real!

I’m posting this heat index map for Thursday. Has it just changed your mind about outdoor activity from the previous maps with actual forecast temperatures?

19z NBM “Apparent Temperature” aka Heat Index at 3 PM Thursday (Click on image for a larger view.)

There’s a new way of imparting dangerous temperature conditions that takes into account temperature, humidity/dew Point, wind and Exposure to heat of direct sun. It’s called the Wet Bulb Globe Temperature. It’s got the right idea, but the way it’s scaled in degrees F, it likely will never catch on.

Here’s the Wet Bulb Globe Temperature for Thursday—

Model Blend WBGT (wet bulb globe temperature) at 2 PM Thursday. It just doesn’t sound ominous enough, even though a 92 reading is considered extreme. But it’s a better measure of the impact of summer heat and sunshine. (Click on image for a larger view.)
Heat Wave Update

Posted Tuesday 06/30/26 @ 5:37 PM — Tomorrow, Wednesday will be a ‘normal’ very hot day for the first day in July. Highs 96º-97º. Not so for the rest of this week.

It should be noted that the model extreme heat forecasts have been extremely consistent over many days, leading to higher than average certainty. Here’s the latest available ECMWF forecast high temperature forecasts (around 4:30 to 5:00 PM each afternoon)—

12z ECMWF high temperature forecast for Thursday at 5 PM. Fine Contours are near-surface (2m) temperatures in 1º increments. (Click on image for a larger view.)

Friday—

12z ECMWF high temperature forecast for Friday at 5 PM Fine Contours are near-surface (2m) temperatures in 1º increments. (Click on image for a larger view.)

The above ECMWF might be too high, then again, maybe not. Here’s the latest Model Blend forecast Thursday—

18z NBM (model blend) 12 hour high temperatures. Fine Contours are near-surface (2m) temperatures in 1º increments. (Click on image for a larger view.)

The above Model Blend Max Temp uses advanced statistical techniques. With the previous version of the model blend (NBM v4.3) I found that the high temperature PLUS standard deviation was more accurate in situations like these—

19z Model Blend Temperature with standard Deviation added. Fine Contours are near-surface (2m) temperatures in 1º increments. (Click on image for a larger view.)

The new Model Blend (NBM 5.0) Max Temp may have corrected this cooler bias. We’ll see.

Historic Heat Wave Potential

Posted Tuesday 06/30/26 @ 9:24 AM — The models consistently point to the potential for an historic heat wave over the next several days. The ECMWF-AI and the GFS continue to show actual temperatures in excess of 106º. The Model Blend (NBM), recently updated to version 5, has a good track record in forecasting high temperatures. (The previous version 4.3 tended to run a bit low.)

Here’s the current water vapor image—

Water Vapor image at 9 AM with superimposed RAP model MSL pressure (black contours), 500-1000mb Thickness (yellow contours) , GFS potential vorticity contours (violet fine contours), Note the position of the 576 thickness line, the demarcation of severe heat and humidity. (Click on image for a larger view.)


Here are the Model Blend High Temp Forecasts Wednesday-Friday:

06z NBM 12 hour mean high temperatures (Click on image for a larger view.)
06z NBM 12 hour mean high temperatures (Click on image for a larger view.)
06z NBM 12 hour mean high temperatures (Click on image for a larger view.)

Please note that the high temperatures this time of year often occur between 4 PM and 6 PM.

Saturday will see temperatures still over 100º from the city eastward.

Severe Heat Wave through Sunday

Posted Monday 06/29/26 @ 7:50 PM — A heat wave with temperatures 90 or above begins on Tuesday and will last at least through Sunday. High Temperatures Thursday through Saturday may set records.

We start with highs 90-91º on Tuesday.

ECMWF Thurs Highs
GFS Thurs Highs
CMC GDPS Thurs Highs
NBM Thurs Highs

The above temperatures forecast for Thursday (and some models have it hotter on Friday) are rarely seen forecast for this area. These temperatures are actual forecast temperatures, not heat indices.

The higher resolution NBM makes it clear that it will be very hot, even at the shore. (The lower resolution models obscure this.)

Monday Forecast Update

Mon 7:48 PM —Forecast Review —A few showers developed in Berks and Upper Chester counties, but nothing like the forecast from this morning’s RRFS and this afternoons RDPS.

Posted Monday 06/29/26 @ 9:46 AM — The boundary of the very hot humid air has not quite made it into our area, as shown by current wind directions and dew points-

RTMA current conditions (9:30 AM) dew points and wind directions. I’ve drawn the approximate position of the true tropical boundary. (Click on image for a larger view.

This morning’s water vapor image shows an upper air disturbance near the Great Lakes that will swoop down into our area this evening, pulling the very hot air closer to us—

Water Vapor image at XX with superimposed RAP model MSL pressure (black contours), 500-1000mb Thickness (yellow contours) , RAP potential vorticity contours (violet fine contours). The position of the 576 thickness is still southwest of us. The purple contours near the Great Lakes indicate upper air energy that may cause a few showers/thundershowers Monday evening. (Click on image for a larger view.)

The latest RRFS shows this upper disturbance above will cause some showers/thundershowers again Monday evening in some areas only.

RRFS (06z) forecast 1 hour rainfall at 9 PM Monday evening. The RRFS does not have these showers making it into NJ. (Click on image for a larger view.)

The RRFS did pretty well with its showers forecast Sunday night, actually under-forecasting the rainfall. We’ll see if it can do it again.

Originally Posted Sun 8:39 PM — Parts of the area received much-needed rainfall this evening, significantly more than had been forecast.

With the expected heat and humidity building this week, there’s always a chance of a storm in the afternoon, but often times, this sort of heat in the upper atmosphere ‘caps’ the storms. So this was likely our last chance for significant [somewhat] widespread rainfall for much of this coming week.

The severe heat expected to build this week has been well-advertised. As mentioned, in all the years of forecasting, I haven’t seen this sort of heat forecast here for at least 20-25 years.

Here’s the AIGFS forecast for Thursday with upper and lower level high pressure heat domes over us—

18z AIGFS forecast for Thursday 2 PM. notice the position of the orange-gold 760 thickness line which extends into Maine. It also shows a thermal trough may develop along the coast near us. (Click on image for a larger view.)

Here’s the latest Model Blend Temperature forecast- Incredible heat!

Model Blend Actual Temperature,Heat Index, Dew Point with standard deviations. (Click on image for a larger view.)

This week’s weather promises to be interesting. Stay tuned for updates.