Posted Wednesday 05/14/25 @ 5:28 PM —Following some scattered showers Thursday morning, many of us may finally see some sun Thursday by mid afternoon. Unfortunately the upper air will be cold and thermal instability will lead to scattered showers and thundershowers between 2 and 6 PM. A mix of clouds. sun and scattered storms sums it up for the afternoon.
Friday, a strong impulse rotating around the upper low near the Great Lakes will result in potentially some strong fast moving storms here around noon. Here’s the RRFS, which suggests a bow-echo formation around noon—
12z experimental RRFS simulated radar forecast for noon Friday.
The potential for damaging winds Friday with this impulse needs to be watched.
Wed 5:20 PM —Forecast Review —The experimental RRFS did better than the HRRR today regarding its forecast for lingering moisture and rain around Philadelphia this afternoon.
Posted Wednesday 05/14/25 @ 12:14 PM — Opted against going to the afternoon Phillies game. The morning models support a dry slot developing, and current radar/water vapor imagery shows dry slot advancing northward—
Satellite Water Vapor with superimposed composite radar at noon. (Click on image for a larger view.)
The hold-out for improving conditions is the experimental RRFS. It still shows some showers during the afternoon game. (Perhaps we’ll see why it’s still ‘experimental.)
14z experimental RRFS simulated radar at 2 PM. Could this be right?? (Click on image for a larger view.)
What about the Phillies Games today?
Posted Wednesday 05/14/25 @ 9:10 AM —It does look like there will be a relative ‘dry slot’ develop beginning about 2-3 PM today in the city. The latest HRRR (11z) shows rain exiting to the northwest.
Here’s the simulated radar forecast for 1 PM. There may be some showers at the scheduled game start—
11z HRRR simulated radar forecast at 1 PM As we always see, exact placement of forecast rain is more frequently than not inaccurate. (Click on image for a larger view.)
The latest HREF model, which is a statistical ‘ensemble’ of the HRRR, NAM-NEST and RAP models shows minimal rain for the hour preceding 2 PM —
06z HREF forecast 1 hour prior rainfall at 2 PM. (Click on image for a larger view.)
Additional rain is expected during the evening, but the models are leaning towards it being somewhat west of the city.
As for total rainfall so far for this event, this has been the most significant widespread rainfall we’ve had in a long time. Here’s the latest 48 hour total estimate—
MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch) (Click on image for a larger view.)
Wednesday Forecast Update
Posted Tuesday 05/13/25 @ 5:19 PM — The weather is unfolding as forecast, with a stream of increasingly heavy showers moving up from the south southeast around a closed upper low to our west.
The latest ECMWF-AI model has rain continuing tonight and becoming quite heavy by early morning Wednesday. Some thunderstorms may be in the mix on Wednesday. A total of almost 2″ of much needed rain is forecast for our area through midnight Wednesday.
ECMWF-AI forecast for 2 PM Tuesday. Heavy rain over our area. (Click on image for a larger view.)
It looks like some rain/showers continue on Thursday as the upper trough axis moves over us. Thunderstorms also possible.
Tuesday Forecast Update
Posted Monday 05/12/25 @ 5:56 PM — The forecast appears to be generally on-track with rain expected in the immediate Philadelphia area on or before 5 AM Tuesday morning.
Here’s the current satellite 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. Upper level low (L) will gradually move north-northeast. Bands of rain will rotate up over our area at least until late Wednesday and likely into Friday. (Click on image for a larger view.)
Light showers to start, then bands of heavier rain move in later in the morning and continuing, with some breaks, through the afternoon and Tuesday night. Rain tapers off somewhat on Wednesday, but thundershowers will likely develop during the afternoon.
Total rainfall around 1 inch possible in and around Philadelphia, by Wednesday morning. more west of the city.
Posted Monday 05/12/25 @ 7:40 AM — This week’s model forecasts continues to evolve, with a shift towards some heavier rain in the immediate Philadelphia area by Wednesday. (I’ll have to see it to believe it.)
For today, sunshine through high cirrus clouds, with clouds thickening between 4 PM and 7 PM from southwest to northeast. Rain begins here before daybreak Tuesday, but much of the rain will be showery in the morning.
The chance of more moderate to heavy rain waits until later on Tuesday and most likely Tuesday night into Wednesday morning.
06z REFS (experimental( shows only light rain likely here at noon on Tuesday. At least initially, the heaviest rain will be west of our area. (Click on image for a larger view.)
Originally Posted Sun 8:17 PM —Monday will have sun through high clouds, then increasing cloudiness by 4PM. High temps similar to Sunday, near or just above 80º. Rain begins after midnight early Tuesday morning.
An active weather week ahead, where another upper level closed low will bring good chances for rain from before daybreak Tuesday through Friday.
Despite this forecast, it’s not entirely clear whether the needed heavier rain will be in and around Philadelphia, or continue a long standing pattern for the heavier rain to remain to our far north and west. Some models are showing heavier banding right over Philadelphia, but others (Canadian and various runs of the German ICON model and the RRFS) show the heaviest rain to our far northwest.
Today’s 18z GFS model forecast total rainfall through daybreak Wednesday. Greater accumulations by Friday. (Click on image for a larger view.)
Some models show just intermittent showers on Tuesday, while others are forecasting periods of moderate, continuous rainfall here. Too early to be sure.
There will also be the possibilities of thunderstorms during the week; the higher dew points, available moisture aloft and instability will set the stage for these storms, especially late afternoons and evenings.
Fri 12:04 PM Update — The Trump Administration is withholding National Science Foundation funds. The MRMS and live Satellite Water Vapor data I obtain from Unidata and post on this site will become (what is termed) “temporarily unavailable”, as all staff are being suspended this weekend.
There’s a long list of unthinkable things happening. How bad will things get??
Fri 9:44 AM —Forecast Review —The models did quite poorly with this rain event: the onset of the rain in Philadelphia was off by almost 8 hours and the axis of heavy rain was far northwest of the city, not where forecast and, not where it’s needed.
While the models did have a band of heavy rain northwest of the city, the delay on eastward movement last night seriously cut into potential rainfall near the city and in NJ.
Here’s the preliminary MRMS rainfall totals (called Pass 1) as of 9 AM—
MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch) (Click on image for a larger view.)
Additional rain to our west was originally forecast to move in during Friday. The latest RRFS has this rain falling apart as it approaches the city. (Can we believe it?) Here’s current radar at 9:45 AM—
The tendency for rainfall to stay to our northwest and to dissipate near the city and eastward does not portend well for our area or NJfor the coming summer.
The tendency towards dry conditions (drought) still seems to be in play and it’s a trend that began late Spring 2024.
Thursday Forecast Update
Posted Thursday 05/08/25 @ 7:47 AM — The big question for today— when will the showers move into our region? Most models show isolated or widely scattered showers and thundershowers as early as 3-5 PM, mostly in northern and western suburbs.
The ECMWF-AI still maintains dry conditions for most of us, except northern suburbs) until at least 5 PM. However, statistical ensemble models (NBM, HREF, REFS) all show a good chance of some widely scattered or isolated showers as early as 2-3 PM.
The latest hourly RRFS (10z) captures the isolated nature at 4 PM—
Today’s 10z RRFS simulated radar/cloud forecast for 4 PM – Isolated/scattered showers/thundershowers. As is always the case with models, the exact location is often inaccurate, but it captures the general trend. (Click on image for a larger view.)
The NBM (model blend) shows a similar configuration. By 5 PM, the NBM shows this hourly probability of any rainfall—
06z NBM probability of any rainfall (>0.01″) at 5 PM. I generally find that any hourly probability greater than 18% (green shading) is a good bet.(Click on image for a larger view.)
Showers and thunderstorms are likely this evening and tonight into much of Friday.
Thursday-Friday
Posted Wednesday 05/07/25 @ 5:29 PM — Another upper low pressure system will approach our area Thursday into Friday, and will exit by early Saturday.
Current satellite water vapor image 5PM 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. Upper low approaches during the afternoon Thursday.
The big question (read uncertainty) is when the rain associated with this system will arrive here. Most models have some showers and thundershowers in northern Bucks county and Lehigh county by 12 PM – 3 PM Thursday afternoon. Less certain is the possibility of some showers and thundershowers developing in and around the immediate Philadelphia during the afternoon.
The ECMWF-AI model keeps the showers out of our immediate area until late afternoon, becoming likely between 6 and 8 PM. The experimental RRFS shows some isolated showers developing as early as 3 PM, becoming increasingly likely as the afternoon unfolds.
A mix of sun, clouds, and sunshine through clouds is a good bet for the afternoon with a chance of these scattered/isolated showers.
Experimental RRFS forecast time lapse gif (Click on image for a larger view.)
Also a good bet is for showers and thunderstorms developing as early as 5 PM but very likely between 6 PM and 8 PM Some of these will move in from the west, but many will spontaneously develop in place.
Rain continues Thursday night and through much of the day Friday, likely ending Friday evening. A decent rainfall (0.8″ – 1.5″) is expected, with heaviest rain expected north and northwest of the immediate Philadelphia area.
The weekend looks good!
Wed 9:12 AM —Forecast Review — I just wanted to post the 72 hour total rainfall from our most recent weather maker—
3 day rain totals MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch) There was quite a range over small distances. Many areas received very little rain. (Click on image for a larger view.)
Forecast Review & Wednesday Update
Posted Tuesday 05/06/25 @ 5:47 PM — Sunshine did develop by noon and showers did break out from daytime heating, but mostly just to the west of the immediate Philadelphia area; many areas saw no rain, just periods of cloudiness —
Radar loop from 12:54 to 1:35 PM today. (Click on image for a larger view.)
The upper low is finally moving out to the northeast and Wednesday looks to be partly to mostly sunny with sunshine continuing through cirrus clouds in the afternoon. No rain!
Another upper level feature, an upper trough with an upper low center brings rain late Thursday afternoon or evening.
Today’s 12z ECMWF-AI forecast for Thursday at 8 PM. (Click on image for a larger view.)
A surface low is forecast to develop on Friday near the mid-Atlantic area bringing more rain on Friday, likely clearing out early Saturday.
Despite all the clouds and showers over the past three days, many areas have not received all that much rainfall—
Posted Tuesday 05/06/25 @ 8:54 AM — Some other models coming in — the hourly HRRR continues with a break out into sunshine later this morning. The hourly RRFS has some cloudiness lingering around the city with spotty showers breaking out by noon.
Self-Destructive Sunshine
Posted Tuesday 05/06/25 @ 7:38 AM — An area of showers moved through before daybreak. Sunshine is expected in many areas between 11 AM and noon.
7 AM water vapor satellite image with superimposed RAP model 700 mb wind streams and vertical velocity pressure (omega). Drier, low level warm air (orange) will move up into our area while cold air associated with the upper low remains in place aloft. This unstable condition will result in showers developing around area 1 and these will move into our area between noon and 4 PM. (Click on image for a larger view.)
The sunshine will result in heating at the surface while cold air aloft associated with the upper low will combine into a thermodynamically unstable situation, what I like to call “self-destructive sunshine”. (Despite the atmosphere being in therapy for years, it still reverts to this self-destructive behavior) Showers and thunderstorms, some with locally heavier rainfall area possible from noon through 4 PM and perhaps later as well.
00z REFS (experimental) forecast for 3 PM. Prior 1 hour mean rain combined with statistical spread. Not all areas in green will be receiving rainfall. (Click on image for a larger view.)
It’s looking better for Wednesday, with sunshine and less chance of showers.
Tuesday Forecast Update
Posted Monday 05/05/25 @ 5:59 PM — As expected most areas around Philadelphia received very little rainfall today, although a few bands of heavier rain went through Chestnut Hill and areas north and west of the city. Fairly close to the precipitation “hole” forecast earlier this morning.
Unfortunately, the very heavy total rainfall predicted by the AI models for with this closed low have not materialized for many areas. Here’s the current preliminary MRMS estimate of actual rainfall for the past 48 hours—
MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch) (Click on image for a larger view.)
There’s a promising development for Tuesday‘s weather as several models are forecasting a dry slot and significant some sunshine from late morning Tuesday through about 4-5 PM.
For the daring who wish to plan outdoor activities on Tuesday, the chance of a scattered shower during the time period above is still vdry possible, as the HRRR has spotty localized showers/storms developing due to the daytime heating.
Here’s the REFS forecast hourly rain with statistical spread—
12z REFS mean rain and statistical spread forecast for 3 PM Tuesday. Sunny where it’s not raining. (Click on image for a larger view.)
So cloudy in the morning, then sunshine by late morning with areas of localized rain/thunder is the forecast for much of the day.
Rain returns Tuesday night into Wednesday morning, again spotty coverage.
Monday
Originally Posted Mon 10:17 AM —This week’s weather continues to be influenced by an upper level low pressure system in the Ohio Valley. Areas of vorticity (disturbances) rotate around this low, inducing 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. Upper level low with areas of potential vorticity wrap around this low and will bring periods of rain and thunderstorms through Tuesday. As you can see, the rain usually occurs a certain radius (600 miles) from the center. (Click on image for a larger view.)
A close-up shows areas of ‘potential vorticity’ (violet contours) inducing showers and storms. Think of these as eddy currents around a main circulation and they usually develop a certain radius from the center (~600 miles or so) —
Periods of rain and thunderstorms are expected through Tuesday. As has been the case this weekend, exact timing and placement of these disturbances are difficult to forecast accurately and each model has their own solution.
The latest ECMWF shows much of the activity staying to the east and west of Philadelphia today—
06z ECMWF forecast for 5 PM. Philadelphia lies on a ‘gap” of little precipitation today according to this model. (Click on image for a larger view.)
A high resolution ensemble model, the REFS (experimental) supports this forecast of minimal rain here in the city during the daytime Monday. The REFS is a composite model (an ensemble or group of 12 model runs) consisting of time-lagged versions of the RRFS and the HRRR. It attempts to average out the model run variations and errors that routinely occur.
Experimental 06z REFS showing the same ‘gap’ in the precipitation shield over Philadelphia today. (Click on image for a larger view.)
Just because the models show a ‘gap’ in the rain unfortunately does not guarantee that the immediate PHL area won’t see any rain during the daytime hours on Monday. We could use the rain!!
These upper air lows and their associated precipitation are rarely forecast accurately.
Stay tuned.
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