Posted Saturday 05/24/25 @ 9:37 AM — Last night’s forecast still holds. The sunshine this morning will be mixing with clouds by about noon. There’s a chance of an isolated, fast moving light shower about 3-5 PM. although most areas are expected to remain dry.
Current Water Vapor with Radar shows the disturbance expected to move through—
Water Vapor, radar composite and RAP model vertical motion around 9:15 this morning (Click on image for a larger view.)
Previously Posted Fri @ 6:19 PM — —The main low pressure system that affected our weather over previous days is slowly moving away, but the first upper low and a second upper low will bring cool weather and unsettled skies at times through the first portion of the Memorial Day Weekend. Monday should be mostly sunny.
(An upper low can be thought of as a cold circulation of air aloft.)
ECMWF-AI forecast for Sunday at 8AM 1 and 2 are upper low pressure systems. The red lines are called 540 thickness lines, and they demarcate colder average temperatures of the lower half of the atmosphere. In winter, areas north of the 540 thickness lines are cold enough for snow. In late spring, that cold air causes instability to develop with sunshine heating the ground. (Click on image for a larger view.)
We had widely scattered fast moving showers late this afternoon, and with cold air aloft, that possibility still exists for Saturday afternoon. Sunday, any instability showers should remain far north of the Philadelphia area.
It should be noted that any really warm weather isn’t forecast for our area this weekend or even for the coming week.
Saturday Forecast
Sunny in the morning. Instability cloudiness develops before noon and lasts several hours into the afternoon. A few, isolated showers move through at that time. Most areas will be rain-free. It will be windy during the mid-day hours.
NBM high temperatures: Blue Bell, PA 63º Philadelphia, PA 66º Uncertainty:average (based on standard deviation): ± 1.9º
Sunday Forecast
Partly sunny, although the experimental RRFS shows considerable cloudiness south of the city and in Southern NJ. Less windy but still breezy.
NBM high temperatures: Blue Bell, PA 68º Philadelphia, PA 70º Uncertainty: average (based on standard deviation): ± 1.8º
Memorial Day
Sunny. Winds subside further but still breezy. High thin cirrus clouds move in late afternoon.
NBM high temperatures: Blue Bell, PA 68 72º Philadelphia, PA 74º Uncertainty: low (based on standard deviation): ± 1.4º
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
