Posted Monday 06/30/25 @ 5:41 PM — Showers and thunderstorms moved in from the south between 2:30 PM and 4 PM.
Towering cumulonimbus clouds about 2:20 PM Looking southeast along the Schuylkill bike trail. (Click on image for a larger view.)Current Radar composite at 5:32 PM. Additional activity in Maryland may not make it here. The REFS shows some additional activity here about 2 AM Tuesday morning. (Click on image for a larger view.)
Very few of this morning’s models accurately predicted the timing and location of this afternoon’s storms. I always like to look back and see id any specific model did well. The HRRR, our best high resolution model, didn’t perform too well today, nor did the NAM-NEST. (The AI models don’t have the high resolution needed for thunderstorms.)
Looking back, the Canadian high resolution model (HRDPS) did reasonably well. The experimental REFS also did surprisingly well.
So with that, the REFS becomes “the model of the week”. It is currently forecasting showers/thunderstorms to return between 3 and 5 PM Tuesday afternoon. Some may be strong to severe. Stay tuned.
Monday Forecast Update
Posted Monday 06/30/25 @ 9:07 AM — Yesterday’s Model Blend (NBM) proved to be correct about the lack of showers this morning, and I’ll stay with the NBM for today’s forecast, with individual models showing a range of forecast times for showers and thunderstorms.
Pop up storms can occur anytime after 1 PM today with peak activity between 5 PM and 8 PM.
The latest NBM shows activity near the city picking up about about 1-2 PM just east of the city.
06z NBM prior 1hour rain forecast for 2 PM today. Location placement is almost never exact. (Click on image for a larger view.)
A considerable increase in coverage will occur later in the afternoon. Some areas may receive heavy rain in a short period of time.
Likely focus of heavier storms will be central areas of South Jersey in the mid afternoon and areas north of the city in the late afternoon.
Originally Posted Sun 8:50 PM —The ‘cold’ front that moved through Saturday has stalled just to our south and thunderstorms have been breaking out in South Jersey for much of Sunday, especially near the Delaware Bay and Cape May.
The front will return as a warm front on Monday. Large differences in the forecast exist for Monday morning. The AI version of the ECMWF and the latest GFS have scattered showers breaking out early Monday morning, while the regular ECMWF, the AI version of the Canadian GDPS keeps us dry until 2-4 PM.
The experimental RRFS has scattered pop-up storms in the morning; this may be an example of its tendency to over-forecast convective rain.
22z RRFS
The ECMWF-AI has so much precipitation and clouds in the morning that it keeps our high temperatures in the mid to upper 80s instead of the low 90s forecast by the drier forecasts.
The model blend (NBM) keeps us dry Monday morning as well, and I’m leaning towards its forecast. Its high temp in Philadelphia is 90.5º
It’s a tough call.
The models all come together by Monday afternoon with showers and thunderstorms popping up anytime between 1 PM and 4 PM and continuing into the evening. Peak time is about 6 PM according to the NBM.
Tuesday looks even more interesting, as a cold front moves into very moist and unstable air over the Philadelphia region. Stay tuned.
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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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.)
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.)
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.
