Posted Sunday 07/20/25 @ 10:25 AM — A review of the models further advances the overall forecast trend noted last night: any showers/storms will be very widely scattered. The NAM-NEST continues to show a few isolated pop-up storms this evening around 8 PM, yet the models show any time after noontime is fair game for an isolated pop-up storm.
Radar at 10:30 AM with superimposed RAP model parameters (Click on image for a larger view.)12zNBM hourly rain accumulation at 7 PM tonight. (Click on image for a larger view.)
It should be noted that the GDPS with “AI Spectral Nudging” model has a forecast similar to my own inclination that the wind shift earlier in the day may significantly limit storms near Philadelphia. We’ll see.
Models used for this forecast: GDPS-AI, NBM, RRFS , NAM-NEST
Sunday Forecast Update
Posted Saturday 07/19/25 @ 9:10 PM — A warm front finally moves north of our area around daybreak Sunday. Showers before daybreak move out for partly sunny skies for much of the day. Heat and humidity returns, with highs 89-90º in the city and 87-88º northwest. It will become windy later in the afternoon.
A cold front moves through late afternoon. Only some of the ingredients are coming together for storms. While there’s plenty of jet stream energy nearby, we won’t be in an area where the jet causes much upward vertical motion. Additionally, the winds shift to the west before the actual front moves through. Historically, that often means very widely scattered storms; many areas may not see much of anything due to the infusion of drier air.
Storms that do form will be few and far between but could be severe with heavy rain and will most likely occur north and west of the immediate area. The chance of storms continues through 10 PM.
The NAM-NEST shows a few heavy hitters about 8-9 PM—
18z NAM-NEST forecast PRATE at 9 PM. (Click on image for a larger view.)
Models used for this forecast: NBM, RRFS , NAM-NEST
Originally Posted Fri 5:50 PM —The stalled front just to our south that kept considerable cloudiness in our area on Friday, especially southern sections, will return as a slow moving warm front on Saturday through Saturday night.
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. Approximate position of warm front is drawn in (red) (Click on image for a larger view.)
Warm fronts are not distinct like cold fronts and this front’s position and movement were not forecast accurately by the models for Friday. We’ll see if the model guidance is better over the weekend.
Saturday Forecast
Low clouds with widely scattered showers in the morning hours. More widespread heavier thunderstorms likely about 2-5 PM in the afternoon, especially near Philadelphia.
NBM high temperatures: Blue Bell, PA 83º Philadelphia, PA 85º Moderate – Above Average Uncertainty (based on standard deviation): ± 2.5º
Sunday Forecast
Cloudy early morning, with clearing by early afternoon, if not earlier. A cold front moves through about 3-4 PM with more showers and thunderstorms. Heavier activity currently predicted for NJ.
NBM high temperatures: Blue Bell, PA 90º Philadelphia, PA 92º Average Uncertainty (based on standard deviation): ± 1.5º
Models used for this forecast: NBM, RRFS , REFS, ECMWF-AI, HRRR, NAM-NEST
Posted Friday 07/18/25 @ 10:49 AM — It appears that the front didn’t clear our area and stalled somewhat north of the expected location. More clouds possible today. I’ll give a complete weekend forecast early this evening.
Very Hot Thursday- Unimpressive Storms
Posted Thursday 07/17/25 @ 11:44 AM — With all the heat and humidity forecast (high temps near 95ª, dew points near 74º), you’d think we would have the perfect set up for afternoon and evening thunderstorms as a weak front moves through. That’s not the case today and the question is why?
The answer often seems to depend upon the jet stream winds, which in our area will be curved in an anticyclonic direction, causing downward motion. Additionally, any lift from being near a favorable region of jet streak will not occur; the jet is far to our north and we are in a non-favorable region.
12z RGEM jet stream level winds (250 mb) direction and speed (color) Anticyclonic flow with slight favorable cyclonic cuve off shore. (Click on image for a larger view.)
The REFS shows isolated, very light, fast moving showers this evening —
REFS 06z forecast for 9 PM. Little in the way of showers or storms. (Click on image for a larger view.)
There’s also a question about the timing of these showers, with some models showing 6 PM and others later. Case in point is the RRFS and another experimental version called the RRFS-MPAS
RRFS comparison with RRFS-MPAS, another experimental model. The next incarnation of the RRFS may be the RRFS-MPAS. (Click on image for a larger view.)
Hot Wednesday
Posted Wednesday 07/16/25 @ 9:30 AM — A hot very humid flow of air remains over our area. A disturbance in Ohio this morning will move towards our area today sparking showers and thunderstorms.
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. Disturbance in Ohio moves towards us. Upper air contours are not particularly supportive of severe storms.
The latest models show isolated showers developing in western sections as early as 3-4 PM with a significant increase in scattered activity between 6 PM and 10 PM. Some of the heaviest activity may occur as late as 1 AM Thursday morning.
Vertical shear is low as is helicity. Severe weather not likely, but still possible. The atmosphere is quite unstable.
Total rainfall forecast range is quite large with the Canadian RGEM showing areas as much as 1″+ of rain while the NBM is showing less than 0.5″ near the city. Heaviest rain will occur near Reading and northwestward.
Tuesday Uncertainty
Posted Tuesday 07/15/25 @ 10:42 AM — Just enough time for a quick update. Uncertainty in today’s forecast. NAM-NEST shows low clouds hang tight in many areas this afternoon. I’m mentioning the NAM-NEST because it tends to do well with these sort of things. RAP and HRRR shows clouds break up to partial sunshine. Most models have no rain during the afternoon. Model Blend (NBM) also shows reduction in clouds in the afternoon. RRFS/REFS not available today. So a low confidence forecast for some clearing and most areas dry this afternoon.
Monday Eve Heavy Rain Update
Posted Monday 07/14/25 @ 4:21 PM — The expected batch of storms is developing as expected to our west. I’m still going with the REFS timing of it moving into the immediate Philadelphia area between 5 and 6 PM.
Radar 4:30 PM with superimposed RAP model vertical velocity pressure parameters (contours). Significant vertical motion support and the most intense storms will stay north, affecting northern Bucks county and above. (Click on image for a larger view.)
A change in the forecast, based on the latest REFS: the area of storms will exit about 2 AM from the immediate PHL area and will not linger into Tuesday morning.
A wide range of rainfall totals, as is often the case, is expected.
12z REFS rainfall totals this evening. The exact locations are rarely correct, but it gives an idea of the potential for localized heavy rainfall. (Click on image for a larger view.)
Posted Monday 07/14/25 @ 9:42 AM — This morning, I wanted to wait for the latest model runs before posting. As mentioned yesterday, a weak, slow moving upper level trough will move across our area and act as a trigger for somewhat more organized thunderstorm development today.
The timing has shifted somewhat later, now into the evening hours, moving in as early as 6 PM with maximum activity between 7-8 PM and midnight. However, it should be pointed out that the AI versions of the ECMWF and GDPS have widely scattered activity possible for most of the afternoon into the evening, especially western sections.
It may get hung along the Jersey coast with showers lingering here near the city well into Tuesday morning.
Heavy rain will be the ‘feature’ of today and tonight’s activity and the REFS is showing areas receiving as much as 1″-4″. Aside from the heavy rain and lightning threat, severe activity looks to be low.
A considerable amount of cloudiness is expected today from the city westward, but areas in NJ may see considerable sunshine. High temperatures will be in the mid 80s, not likely hitting 90º unless the cloud cover forecast is wrong. High humidity expected with dew points in the 70s. The NBM shows 88º for today.
While not a severe thunderstorm day, I thought I’d compile a severity table to give a sense of what elements are missing today for severity to be likely—
Severity Parameter (HRRR)
Sept 1 , 2021 Tornado Outbreak (example of highly severe)
I’m planning to use the RRFS going forward instead of the HRRR. Some of the parameters won’t be exactly equivalent. Due to the slow downloads of a still-experimental model being hosted on a non-production server, I don’t download all equivalent parameters at this time since downloads take 3-5 times more time. Additionally, the availability lag time for the HRRR is about 1.5 hours or less. The availability lag time for the RRFS is about 3-7 hours depending on the forecast range.
Severity Parameter (HRRR)
Sept 1 , 2021 Tornado Outbreak (example of highly severe)
4-1-23 Tornadoes (Example of severe) HRRR
Today 07-14-25 HRRR 12z
Today 07-14-25 RRFS 12z
Notes
CAPE Joules/kg
3500-4200 ⚑⚑⚑⚑
2100 ⚑⚑
2900 ⚑⚑
2900 ⚑⚑
western sections
Helicity m^2/s^2
1350 ⚑⚑⚑⚑⚑
655 ⚑⚑
161 ⚐ ⇩
330 ⚑⚐
Far Northwest
Vertical Shear 1/sec
40-46 ⚑⚑⚑
40-45 ⚑⚑⚑
18 ⚐ ⇩
18-22 ⚑
Precipitable Water
2.7″ ⚑⚑⚑⚑
0.83” ↓
2.3″-2.5″ ⚑⚑⚑⚑
2.2″-2.4″ ⚑⚑⚑⚑
Heavy rain potential
Lifted Index º K
minus 6º ⚑⚑
minus 9.3º ⚑⚑⚑
Minus 7.0º ⚑⚑
Minus 6.7º ⚑⚐
HRRR Hail inches
1.9 ⚑⚑
1.4 ⚑
1.8 ⚑⚑
Peak Wind Gusts mph
40-50 ⚑⚑
40-50 ⚑⚑
30mph ⚑⚑
25 mph ⚑
Storm Motion Shear Vector Alignment
Aligned – ~ 90º ⚑⚑
Almost aligned ⚑⚑
Not aligned ⚐ ⇩
n/a
250 mb Jet Stream Wind mph
63 ⚑
135 ⚑⚑
24 ↓↓
Max Updraft Velocity
116 m2/s2
North and Northwest
Max 850 DZDT
28 m/s
Buck County
Severity Parameters Impact: ⚑ indicates favors Severity⚐ indicates Possible Severity ↓ indicates works against Severity ⇩ Significant, but less impact
Originally Posted Sun 8:11 PM —This week’s weather will continue to be warm/hot and very humid through at least Friday. With the exception of Monday, most of the weather will consist of random areas of thunderstorms developing from small, localized mesoscale disturbances.
On Monday, a weak, slow moving disturbance and upper trough will create somewhat more organized storms, especially towards evening. Some of that activity has been developing in Central PA this afternoon and may move into our immediate area, especially western sections this evening—
Current (Sunday 8 PM) Radar with RAP model parameters Many models have this area falling apart as it approaches Philadelphia (Click on image for a larger view.)
The upper trough and diminishing jet streak is visible with the GFS forecast for Monday morning—
18z GFS model forecast for 8 AM Monday. This trough (dip in the red contours) and the jet streak will create more organized thunderstorms Monday afternoon and evening. (Click on image for a larger view.)
Following any morning showers, Monday should become partly to mostly sunny through high clouds. Showers and thunderstorms move in from the west between 4 PM and 9 PM, but may lose upper support as they try to move past the city into NJ.
12z REFS forecast for 8 PM Monday evening. The rain and storms may not make it into NJ. (Click on image for a larger view.)
The rest of the week looks just like typical summer weather. Highs near 90 and VERY humid with scattered late day storms, too difficult to predict in advance.
Updates during the week. Stay tuned.
Models used for this forecast: GFS GDPS-AI, ECMWF-AI , REFS, RRFS
Posted Sunday 07/13/25 @ 8:58 AM — A strong high pressure circulation at the surface will block most thunderstorm formation in our area today. The latest water vapor image with RAP model superimposed shows the high circulation clearly—
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. High pressure circulation at the surface (H) and upper level thickness ridge (yellow contour 573) will keep storms to our north and west today. (Click on image for a larger view.)
While many models are showing highs of ‘only’ 88º, I’m leaning towards the those that show nearer 90º. Dew points near or at 70º.
As has been the case, the experimental RRFS (and NAM-NEST) shows a few isolated showers northwest into Bucks and Montco around 6 PM. Likely over-done.
As a weak boundary moves towards us, a few showers and thunderstorms are possible in our area towards daybreak Monday.
Models reviewed for this forecast: GDPS-AI, NBM, RRFS , REFS, ECMWF-AI, HRRR, NAM-NEST
Saturday Forecast Update
Posted Saturday 07/12/25 @ 8:34 AM — Not much change from last night’s forecast posting. The Canadian GDPS with “AI Spectral Nudging” still has some pop up showers between 4 PM and 9 PM; most of the area will be dry. Here’s the total forecast coverage of the showers—
GDPS forecast Total rainfall coverage between 4 PM and 9 PM (5 hour time frame. ) The shading is 0.2″ to 0.7″. (Click on image for a larger view.)
Most areas will see temps 89º-91º The NBM has dew points in the uncomfortable 72º range. The GDPS-AI has dew points in the upper 60s at the height of the temperature range.
Sunday looks partly sunny and dry; no rain in our area until after midnight.
Originally Posted Fri 8:11 PM —A typical summer pattern will be with us through the weekend. Warm, humid each day. A chance of pop-up storms late afternoon, but most of the area will be dry.
Reviewing Friday’s forecast, some pop-up storms developed in South Jersey, but did not move far westward. As mentioned, the REFS has a tendency to over-forecast convective rainfall. Looking back, the new Canadian GDPS with “AI Spectral Nudging” was spot on with the today’s weather and I’ll lean on it for this weekend’s forecast.
Saturday Forecast
Cloudy with some fog in the morning. Clouds lift for sunshine by late morning through the afternoon. A few pop-up thundershowers possible about 7 PM
12z GDPS with “AI Spectral Nudging” forecast for 7 PM Saturday. Clouds are dark/grey and black contours.. Most areas will see no rain. (Click on image for a larger view.)
NBM high temperatures: Blue Bell, PA 89º Philadelphia, PA 90º Higher than average uncertainty (based on standard deviation): ± 2.4º
Sunday Forecast
Sunny. Some clouds far west. Showers possible far western sections in the evening.
NBM high temperatures: Blue Bell, PA 89º Philadelphia, PA 90º Average uncertainty (based on standard deviation): ± 1.5º
Models used for this forecast: NBM RRFS GDPS with “AI Spectral Nudging”
