#Philadelphia #weather #PAwx #PhillyWx
There’s a long list of unthinkable things happening. How bad will things get??
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—
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 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—
The NBM (model blend) shows a similar configuration. By 5 PM, the NBM shows this hourly probability of any rainfall—
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.
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.
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!
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 —
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.
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.
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.
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—
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—
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.
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—
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.
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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