Update Fri 5/23 8:51 AM — The latest RRFS model runs keep most of the day dry with clouds and breaks of sun. Scattered showers become more numerous after 5 PM into the early evening.
Posted Thursday 05/22/25 @ 5:43 PM — Rain continues to rotate around the main area of low pressure. This system will eventually move off to the northeast over the next 48 hours.
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. Low pressure moves off to the northeast over the next 48 hours. (Click on image for a larger view.)
Friday: Relatively cold air aloft will move in behind the system during the first two days of the weekend. Looking at the best match for Friday’s forecast, it appears that we may see some sun at times Friday morning. Cold air aloft will cause enough instability for clouds to form with scattered showers in the afternoon, especially mid to late afternoon.
It will be windy and gusty. Cool! High near 65º
Saturday: Cold air aloft will result in considerable low clouds (stratocumulus) especially by later morning and during much of the afternoon. There’s still a chance of light scattered showers. It will be windy and gusty. High near 65º
Sunday: More sun, but still periods of some cloudiness. Still breezy/windy. High near 67º
Monday: Mostly sunny. High near 68º
Thursday and Friday
Posted Wednesday 05/21/25 @ 7:43 PM — A secondary coastal low is expected to form tonight and will bring heavier rain tonight into Thursday.
Today’s 18zGFS forecast for 8 AM Thursday. Secondary low develops off the NJ coast. (Click on image for a larger view.)
A line of heavier rain is moving through Central Pennsylvania this evening and will arrive here later this evening.
Thursday will have periods of rain on and off during the day and into the evening hours.
Friday will still be cloudy, windy and chilly with a chance of showers, especially in the afternoon.
The weekend will feature gradual clearing. Plenty of clouds on Saturday and even Sunday may have some cloudiness develop mid day due to instability. Monday looks to be the best day. It will be chilly and windy much of the weekend; it won’t feel like the beginning of summer.
Wednesday Thursday Forecast
Posted Tuesday 05/20/25 @ 6:02 PM —Rain moves in from the southwest, reaching us between 7 and 9:30 AM on Wednesday.
YES, THE UNIDATA SERVER IS AGAIN UP AND RUNNING. BUT FOR HOW LONG? 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. (Click on image for a larger view.)
(The onset of the rain has moved later with each model run.) Wednesday looks rainy for most of the day, perhaps with some breaks at times. Heaviest rain likely during the afternoon hours.
A possible break in the action Wednesday evening, then more rain, on and off through much of Thursday. Thursday’s rain will likely be more showery and intermittent than Wednesday.
It will be windy and become gusty, especially Wednesday night into Thursday morning.
About 1 inch or more of rain is expected.
Improving conditions on Saturday through Monday. Saturday will be quite cold and windy.
The Trump Administration is withholding National Science Foundation (NSF) funds. The MRMS and live Satellite Water Vapor data I post here is from Unidata, funded by the NSF. The data has become “temporarily unavailable” as of this afternoon, as announced two weeks ago. The support teams for the servers have been furloughed, placed on temporary leave.
So, no more MRMS rain totals and fancy model superimposed water vapor until….
I don’t want to get political here. But what’s going on makes me angry and sad.
Tuesday Update
Posted Monday 05/19/25 @ 8:36 PM — We had much more cloudiness for much of the morning and early afternoon today than forecast by any model. Skies eventually cleared.
For Tuesday, the approaching system has slowed down and it appears that any rain will wait until daybreak Wednesday.
In the mean time, we’ll start sunny Tuesday morning, but we’ll be sandwiched between low clouds to the north and advancing middle level clouds to the southwest. So we’ll go with sunshine in the morning and sun through high cirrus clouds in the afternoon. It wouldn’t take much for the cloud forecast to be off tomorrow.
Today’s 18z GFS shows us sandwiched between low clouds and mid level clouds Tuesday afternoon. It wouldn’t take much for the cloud forecast to be wrong. (Click on image for a larger view.)
It should be much less windy on Tuesday compared to Monday.
Rain on Wednesday into Thursday. Over an inch is forecast. See this morning’s forecast immediately below for the weekend sneak peak.
This Week & Memorial Day Weekend
Originally Posted Mon 9:42 AM —Currently a low pressure system is gathering strength and moisture near Nebraska and will move eastward—
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. (1) indicated where a secondary low will be spawned. (Click on image for a larger view.)
A secondary low will develop along the eastern Mid-Atlantic from this system. Rain will move in, about 5 AMWednesday morning and linger in some form at least through Thursday evening and even Friday.
NAEFS model statistical “mode” version with overlaid GEFS model “bias-corrected” precipitation forecast for Wednesday_ (Click on image for a larger view.)
Over an inch of rain is expected with this system.
The big question is the Memorial Day Weekend. The AI model (ECMWF-AI Single) is showing a cut off closed low hanging back, bringing chilly temperatures and the chance of showers even through Saturday.
ECMWF-AI forecast for Saturday afternoon. Closed low off the New England coast with instability showers expected here.
Things look better for Sunday and Monday.
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 my forecast postings, I recommend following me on mastodon.social or bluesky.social
Posted Sunday 05/18/25 @ 9:21 AM — A beautiful day ahead with temperatures in the low to mid 70s, sunny skies and low humidity.
I’m keeping an eye on the upcoming weekend and the Memorial Day Weekend. Here’s the current water vapor satellite with radar and select RAP model parameters superimposed—
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. Flow from the southwest will affect us by late Tuesday. (Click on image for a larger view.)
An unsettled period, late Tuesday night through Thursday with rain at times. Depending upon the exit of another cut off upper level low, things should improve by the Memorial Day Weekend, however, it’s looking rather chilly for May.
I also wanted to post the MRMS rainfall total for the heavy rains received this past Friday—
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.)
Saturday Update
Posted Saturday 05/17/25 @ 9:41 AM — The showers that developed (only forecast by some of last night’s 00z models) with the cold front this morning are moving through.
Current Radar with satellite water vapor and RAP model 300 mb vertical velocity pressure (omega) superimposed. (Click on image for a larger view.)
Sunshine should break out shortly. I wish the forecast for the rest of Saturday was clear cut, but it’s not. Several models have some scattered showers and thundershowers mid or later this afternoon. The latest HRRR which just became available keeps any scattered pop showers far west of our area. The Canadian HRDPS, which was correct about these showers this morning, still forecasts scattered light showers as late as 7 PM. We’ll have to see.
Posted Saturday 05/17/25 @7:35AM — I was off-duty last night but the models showed some light showers with this morning’s cold front.
Low pressure in Canada will move east and will drag a cold front across the area late Saturday morning. High pressure builds in for Sunday.
Saturday Forecast
Cloudy early morning. Clouds break for sun about 10 AM to noon. Somewhat breezy.
NBM high temperatures: Blue Bell, PA 83º Philadelphia, PA 86º
Sunday Forecast
Sunny and somewhat windy. Cooler.
NBM high temperatures: Blue Bell, PA 74º Philadelphia, PA 76º Uncertainty (based on standard deviation): above average
Posted Friday 05/16/25 @ 4:38 PM — The first short wave moved through around noon. It kept together better than I had forecast. A second short wave will bring showers and thunderstorms here, also a bit later than had been earlier forecast.
Here’s the current radar at 4:50 PM—
Radar with superimposed RAP model Omega (Click on image for a larger view.)
The HRRR and NAM-NEST have these storms dissipating before getting here between 8 and 9 PM. The RRFS and Canadian HRDPS have the storms moving into our area. I’m leaning towards the RRFS.
Posted Friday 05/16/25 @ 11:45 AM — There’s strong upper level support for the storms currently in western sections, but lower level support is lacking as it moves eastward at 11:44 AM. Let’s see which will prevails.
Radar with upper level RAP model Omega contours at about 11:44 AM (Click on image for a larger view.)
The models are showing less distinction between the original two short waves, now predicting showers and thunderstorms through the afternoon and early evening hours.
Posted Friday 05/16/25 @ 9:06 AM — A quick update. Latest radar shows a bow echo associated with the first of two short waves that will move across our area today.
The current area of fast moving heavy rain rain/ strong storms is expected to weaken as it moves into Philadelphia later this morning. Some rain still possible with this batch. Current RAP model shows moderate downward motion ahead of this impulse consistent with some weakening.
Current composite radar at 9 AM with superimposed RAP model 700 wind streams and low level Omega. Omega is negative in our area, indicating weakening. (Click on image for a larger view.)
Another impulse expected later this afternoon (2-6 PM) will be fueled by higher CAPE values and higher vertical wind shear. Some stronger storms expected with this second short wave. I’ll update around noon with newer models.
Revised ForecastPosted Thursday 05/15/25 @ 7:40 PM — After reviewing the afternoon (18z) models which became available after my previous posting, I have to revise my forecast.
It appears that there will be two waves of energy that pass through on Friday. The first, in the late morning, will be strong in far western areas, but several models have this rapidly diminishing in intensity as it encounters a weak mid-level ridge over Philadelphia.
A second impulse moves through between 2 and 5 PM, as noted below, which appears to have more mid-level support, causing some very strong storms as it moves into NJ. We’ll see how it all plays out. I’ll update tomorrow morning.
Posted Thursday 05/15/25 @ 5:15 PM — A potent disturbance will move through during Friday afternoon. Previously expected around noon, the latest models have it moving through about 2-5 PM, earlier to our far west.
The upper air will support strengthening as it approaches, but it’s forecast to hit a weak mid-level ridge right over Philadelphia. Some models (HRRR) have it weakening and moving to our north and south over this ridge. The latest RRFS (experimental) maintains some of its strength and has it further intensify as it makes it across the river into NJ.
So this disturbance (called a “short wave”) is not looking as intense as it was yesterday, but some of the thunderheads may reach 50,000 feet according to the RRFS with fast onset heavy rains and winds.
RRFS 700 mb (10,000 feet) level short wave and Relative Humidity (shading) at 10,000 feet. Not shown are CAPE values which are in the 1000-1500 joule level and areas of vertical shear which exceed 25 1/sec. All the ingredients for storm formation. (Click on image for a larger view.)
The weather clears for Saturday, but a few widely scattered late day thunderstorms still possible.
Thu 5:11 PM —Forecast Review — We had the sun break out about mid afternoon, as forecast. Some areas, especially in New Jersey have been seeing some showers and low level thunderstorms.
We’ve had quite a bit of rain over the past three days. Here’s a final summary, based on the MRMS—
MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED over the past 72 hours. . Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch) (Click on image for a larger view.)
Thursday through Friday
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.
The latest GFS shows this west-northwest trend—
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.
