Posted Sunday 05/11/25 @ 9:40 AM — A beautiful Mother’s Day. The weather couldn’t be finer. Light winds, low humidity. Highs near or above 80º
05-11-25 12zNBM forecast high temperatures for Sunday (Click on image for a larger view.)
Enjoy the great weather today, because the AI models are forecasting several days of rain, potentially heavy, from before daybreak Tuesday through Friday.
Clouds move in during the afternoon Monday and rain begins shortly after midnight early Tuesday morning.
Posted Saturday 05/10/25 @ 9:22 AM — Enjoy this wonderful (but somewhat cool and, initially, rather windy) weather that we’ll have through Monday. The models are consistently forecasting rainy weather from before daybreak Tuesday through early Thursday—
06zECMWF-AI Single forecast for Tuesday at 8 AM. A slow moving system associated with another closed upper level low (Blue L) will move in before daybreak Tuesday. (Click on image for a larger view.)
Previously Posted Fri @ 5:33 PM — —Low pressure that gave us the rain on Friday will depart Friday night. High pressure builds in on Saturday. The pressure gradient between the departing low and the building high will cause windy conditions on Saturday.
NAEFS forecast for Saturday at 2 AM. High pressure will be in play over the weekend, keeping the developing low pressure system near the Gulf of Mexico Gulf of America Gulf of Mexico to be suppressed to our south. (Click on image for a larger view.)
Saturday Forecast
Sunny and windy.
NBM Winds and gusts
NBM high temperatures: Blue Bell, PA 71º Philadelphia, PA73º Uncertainty (based on standard deviation) : above average ±2º
Sunday Forecast
Sunny and less windy.
NBM high temperatures: Blue Bell, PA 75º Philadelphia, PA 78º Uncertainty (based on standard deviation): above average ±2º
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.
A note for those of you who are visiting this site from “X“.
Due to the highly unbalanced political environment on X, I’ve considerably cut back auto-posting there. To receive ALL of my forecast postings, I recommend following me on mastodon.social or bluesky.social
Posted Sunday 05/04/25 @ 9:40 AM — So far, the rain from this closed upper level low has not played out as forecast. Following the rain/thunderstorms last evening that just clipped the Phillies game, there hadn’t been much additional rain.
Some rain redeveloped this morning, just clipping the Broad Street Run, which wasn’t accurately predicted by any model.
Here’s the amount of rainfall from last night—
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) 24 hour totals. It seems that the number is a bit low for the area around Citizen’s Bank Park. (Click on image for a larger view.)
Currently two bands of rain exist, west and east of the city—
Composite Radar with superimposed RAP model 700 mb (10,000 ft) wind streams (green lines) and Omega (vertical velocity) low level and high level. The Positive Omega area (?) may induce more showers. (Click on image for a larger view.)
The latest RRFS is a good match for the current radar. It has more storms moving through 2=5 PM—
06z RRFS forecast for 5 PM (Click on image for a larger view.)
The models often correctly predict the overall general pattern with closed upper lows, but it’s also common for the details regarding rain banding and timing to be not very accurate.
Forecast Update
Posted Saturday 05/03/25 @ 5:39 PM — Some fast moving isolated storms are developing just west of the city and in NJ, in areas having the greatest CAPE value. It’s a close call, but this evening’s storm development may remain away from Citizens Bank Park this evening.
Latest hourly RRFS forecast for 9 PM tonight (Click on image for a larger view.)
Some rain will likely move in later this, with the heaviest precipitation still just west of the immediate Philadelphia area.
For the Broad Street Run, the models still keep a relatively rain-free period from 7 AM Sunday through about 11 PM. Rain then moves in from the south, with the heaviest bands again just west of the city.
Still A Difficult Forecast
Posted Saturday 05/03/25 @ 10:26 AM — The effect of the upper low has yet to affect the Philadelphia area. The upper winds are still anti-cyclonic in curvature, inhibiting rainfall. This morning, I’ve waited to see the latest models to post this forecast.
Here’s the latest water vapor image showing most of the immediate area rain free and no obvious triggers downstream—
Water Vapor with superimposed radar 700 mb wind streams and low level Omega (Click on image for a larger view.)
This morning’s NAM-NEST, HRRR and RRFS keep any showers/storms to our far west, with sunshine through cirrus clouds for much of the day. Showers move eastward, just reaching the immediate Philadelphia area by late evening.
On the otherhand, the ECMWF-AI model along with the ECMWF and GFS have showers/storms moving in during the late afternoon and somewhat heavy activity this evening.
Here’s the GFS with showing rain developing as early as 5 PM—
06z GFS forecast for 5 PM Saturday
A difficult forecast with such wide differences in model guidance. The AI model has been most consistent and the closest to reality, so I’m leaning towards its forecast of showers developing here late afternoon.
As for Sunday’s Broad Street Run, the models suggest a break in the rain from 7 AM to 11 AM, but heavier rain moves in from the south by 11 AM to noon. I’ll update later this afternoon.
Includes forecast for Sunday’s Broad Street Run
Previously Posted Fri @ 5:34 PM — —The much advertised cut off upper level low takes shape late Saturday and will continue to affect our weather through Monday and possibly Tuesday.
The initial slug of energy is dropping south at this time and will close off as an upper level low over the Ohio/Mississippi Valleys.
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. Potential Energy (violet contours) will continue to drop south and a closed upper low (Blue L) will develop by later Saturday. A secondary surface low developing around Maryland will bring showers and thunderstorms Saturday evening to our area. Moisture from the south will continue to bring showers and thunderstorms through Sunday. (Click on image for a larger view.)
A secondary surface low developing around Maryland will bring showers and thunderstorms Saturday evening into early Sunday morning.
The exact position of the upper low will determine if we are in the heavy rain bands expected to develop. The final position is not a certainty.
Some widely scattered showers and thunderstorms are breaking out a this time (Friday late afternoon) and there are indications that additional scattered showers/storms may develop this evening. Not all areas will see rain tonight.
Saturday Forecast
Depending on the model, rain and shower may begin as early as 2 PM around Philadelphia, but rain and thunderstorms look definite by Saturday evening.
NBM high temperatures: Blue Bell, PA 86º Philadelphia, PA 87º Uncertainty (based on standard deviation): low ± 1.4º
Sunday Forecast
Rain tapers off around daybreak Sunday, leaving the morning hours potentially rain-free or just light drizzle, but additional ran is expected to develop around noon into the afternoon.
NBM hourly rainfall for 8 AM Sunday. Scattered very light drizzle, less than 0.01″. Heavier precipitation to our west. (Click on image for a larger view.)
NBM high temperatures: Blue Bell, PA 75º Philadelphia, PA 78º Uncertainty (based on standard deviation): average ± 2.4º
Center City: Winds light- moderate (8 mph) with gusts to 12 mph from the SE. Increasing during the afternoon hours.
Potentially accumulation 1-2 inches of rain is possible by Tuesday with locally higher amounts.
