Posted Wednesday 10/01/25 @ 8:10 AM — The cloud cover associated with the tropical systems has departed and high pressure will bring cool weather and sunny skies.
06zGEFS shows high pressure to our north and a cold front approaching in the upper Plains with some cloudiness in the upper Midwest. The red line is the 500-1000 mb ‘thickness line’. Indicating a cold air mass in Northeastern Canada. (Click on image for a larger view.)
A ‘dry’ cold front in the upper Midwest will bring some cloudiness mixed with sun Thursday and Friday Continued dry conditions!
Posted Tuesday 09/30/25 @ 8:46 AM — The outflow moisture of Imelda will continue to cause cloudiness in our area through much of Tuesday. Rain associated with this moisture flow is hitting a rather strong high pressure ridge over us (1022 mb) and is dissipating to our south.
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. Water Vapor Imagery reveals emitted water vapor infrared temperature, NOT reflected light from clouds as is the case with visible satellite imagery. (Click on image for a larger view.)
The mid and high level cloudiness should thin out to just sunshine through high clouds mid-afternoon Tuesday.
Originally Posted Sun 5:51 PM —This week will be another good time for local meteorologists to take a vacation. High pressure will build down from the northeast, blocking the tropical system’s movement northward towards us and giving us dry conditions through at least next weekend!
Until that time, high pressure system builds southward with a front late Tuesday, and moisture outflow from the tropical systems will bring significant cloudiness both Monday and Tuesday. Skies clear Wednesday with temperatures cooling down.
Here’s the European AI model (ECMWF-AI) forecast for early Wednesday, with colorized high pressure (blue) and low pressure (pink)—
12z ECMWF-AI surface forecast for 2 AM Wedmesday with colorized surface pressures (Click on image for a larger view.)
The statistical ensemble model, the NAEFS, shows zero chance of rain here through Oct 7th!
Posted Sunday 09/28/25 @ 9:24 AM —The northward and westward extent of yesterday’s rain was greater than forecast, with the heaviest band of rain through our area rather than just east of the I-95 corridor. Here’s the MRMS rainfall 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) (Click on image for a larger view.)
As for Tropical Depression #9, the European AI model (“ECMWF-AI-Single”) has it likely becoming a Tropical Storm or even a Category 1 Hurricane, but the model has joined the other models with it moving east and away from us.
ECMWF-AI 06z forecast for Tuesday. Likely at least a Tropical Storm, but its track is now eastward. (Click on image for a larger view.)
“Poor Predictability”
Posted Saturday 09/27/25 @ 5:20 PM — I began yesterday’s Weekend Weather Forecast with a prominent comment about poor predictability as a result of tropical systems in the mix. We’re seeing this first hand— the rain has advanced faster than any of the model predictions from Friday, or for that matter, this morning.
Radar and RAP model low level Omega about 5 PM. Rain is moving in faster than forecast, although the RAP model shows downward vertical motion (Omega – yellow dashed contours), which inhibits rainfall. What had been forecast for 10 PM and later is likely earlier. (Click on image for a larger view.)
The models are consistently showing rain to end around daybreak and skies to clear for sunshine Sunday morning. I think we can still trust this.
Saturday Rain Update and Tropical Disturbance #9 (Imelda)
Posted Saturday 09/27/25 @ 8:21 AM — Last night’s model guidance has shifted the areas of rain to east of the I-95 corridor, i.e, South Jersey. Only light showers are forecast for Philadelphia and northwestward with totals < 0.10 inches and many models showing < 0.05″.
It looks like the Phillies game won’t be affected with such light rain arriving likely after 10 PM.
Latest REFS (06z) 1 hour rainfall at 10 PM (Click on image for a larger view.)Latest HREF (06z) 1 hour rainfall at 11 PM (Click on image for a larger view.)
As for the tropical disturbance expected to become tropical storm or hurricane Imelda, most models have the storm approaching the coast of South Carolina and then moving out to sea. In marked contrast is the ECMWF-AI model (which did so well with hurricanes last season) which has the storm in a track that would affect us by next weekend—
Latest ECMWF-AI (06z) shows the storm blocked initially, then moving up into the Mid Atlantic. This is an outlier forecast, but that said, the ECMWF-AI did very well last hurricane season. (Click on image for a larger view.)
Previously Posted Fri @ 5:11 PM — —Following a beautiful day on Friday, the weather will become unsettled (and perhaps somewhat poorly predictable) as low pressure in the Tennessee Valley moves off to the coast south of our area bring clouds and rain.
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. Water Vapor Imagery reveals emitted water vapor infrared Temperature, NOT reflected light from clouds (Click on image for a larger view.)
The concern over predictability is based on the existence of two tropical systems on course to approach South Carolina.
NAEFS forecast for Sundat at 5 PM (Click on image for a larger view.)
The moisture outflow and the warm center highly localized energy of these tropical systems often seem to play havoc with model guidance. So I expect to see shifts in the forecast over the next good number of days.
Saturday Forecast
High clouds in the early morning are quickly replaced by lowering, thickening clouds by late morning and through the afternoon. While a few scattered sprinkles are possible as early as 3 PM, heavier rain appears to wait for the end of the Phillies game Saturday evening. Rainfall will be highest from South Philadelphia and southeastward.
Light winds increasing in the afternoon.
NBM high temperatures: Blue Bell, PA 79º Philadelphia, PA 79º Above Average Uncertainty (based on standard deviation): ± 2.4º
Sunday Forecast
We’ll go with an optimistic Sunday forecast, but it wouldn’t surprise me if there is more cloudiness. Rain ends before daybreak, clouds break for sunshine. (The uncertainty in the NBM temperature forecast below captures the uncertainty in the cloud cover.)
Light winds.
NBM high temperatures: Blue Bell, PA 78º Philadelphia, PA 79º Above Average Uncertainty (based on standard deviation): ± 3.4º
Posted Friday 09/26/25 @ 9:09 AM — As always, the model forecast placement of rain and the actual rainfall coverage only match in a general way. Additionally, while some areas received large amounts of rain, some areas, over the past three days, received relatively modest amounts.
Here’s the MRMS final totals over the past 72 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.)
This weekend’s forecast will be tricky. A low pressure system to our south will move up towards us, blocked by high pressure to the north. Most models have come together with a forecast of light rain beginning as early as 3-4 PM Saturday, with much of the rainfall coming towards and after midnight.
How long the rain and clouds linger into Sunday is in question. Clouds and showers are expected to exit about 1-3 PM in northern sections, but clouds may linger from the city southward.
The model guidance is clouded by two tropical systems in the eastern Atlantic, whose moisture outflow and energy will undoubtedly affect the accuracy of the model forecasts over the next several days into next week.
ECMWF-AI forecast for Sunday at 2 PM. (Click on image for a larger view.)
Check back later this afternoon for my regular Weekend Weather Forecast.
Thursday Storm Update
Posted Thursday 09/25/25 @ 7:12 PM —So far, radar is not showing the weakening of the storms as forecast by the HRRR and RRFS. 7:43 PM — Signs that the line of storms is now weakening.
Posted Thursday 09/25/25 @ 5:38 PM — The last slug of rain is to our west. Despite the line of storms showing on radar right now, the model forecast is for significant weakening as it passes through Philadelphia.
Current Radar with RAP model vertical velocity pressure (yellow contours) and 700 mb wind streams (green contours) (Click on image for a larger view.)
Another image from a highly rated iOS app, “WeatherFront” —
High resolution radar image. (Click on image for a larger view.)
Here’s the latest available RRFS (19z). Its forecast has changed considerably since early this morning: Storms weaken considerably as they pass through the city—
19z RRFS forecast simulated radar at 9 PM. The line of storms weakens even more over the next hour.
The Phillies game may be minimally impacted, if we’re lucky.
Posted Thursday 09/25/25 @ 11:39 AM — As forecast several days ago, the heaviest precipitation with this stalled system has, so far, moved to our west. Another band of heavier precipitation developed in NJ. Here’s the current MRMS total precipitation received estimate up to this morning —
MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED through early this morning. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch) (Click on image for a larger view.)
The models are still showing areas of instability to develop over our area with the potential for areas of heavy rain during the afternoon. It appears that the final major band of rain will move through our area between 6 PM and 9 PM, according to the latest RRFS and HRRR. This is ahead of the actual front that moves through before daybreak on Friday.
RRFS 1 hour precipitation at 7 PM (Click on image for a larger view.)
It should be noted that the HRRR is less robust with this final push of rain than the RRFS.
Friday looks to be sunny and quite nice.
Thursday Forecast Update
Posted Wednesday 09/24/25 @ 9:17 PM — The model forecast guidance for the next several days has changed. It appears that the heaviest rain will now occur between now and Thursday evening.
There’s a chance of severe thunderstorms (localized high winds, heavy rain) between noon and 4 PM Thursday. The heaviest activity will be north and west of the city.
Here’s the RRFS showing areas of strong upward motion—
18z RRFS at 3 PM with 700 mb RH and DZDT upward vertical velocity centers. (Click on image for a larger view.)
RRFS Calculated Severe Weather Index (SWEAT INDEX)
RRFS SWEAT index at 3 PM. Orange SX > 300 shows severe possible/likely (Click on image for a larger view.)
It looks like the front moves through late Thursday night and there will be clearing on Friday.
Wednesday Forecast Update
Posted Wednesday 09/24/25 @ 6:58 AM — Another wave of low pressure developing along the stalled frontal boundary will pull another warm front up into our area later Wednesday, along with showers and possible thunderstorms.
Most models show little rain activity before 4-5 PM, but the RRFS (experimental) underwent some recent revisions shows showers and thunderstorms popping up as early as 1-4 PM around the city. (I’m not sure if the recent RRFS revisions are moving the model in the right direction. This will be a good test.)
The rainfall tonight into Thursday may be quite heavy, with as much as 2+” of much-needed precipitation.
06z Canadian RGEM forecast total precipitation by Thursday. The experimental RRFS (not shown here) is similar, but with the axis of heavy rain nearer the city. (Click on image for a larger view.)
Another round of heavy rain again later Thursday afternoon into the evening. As mentioned on Monday, this final round of heavy rain may bring strong to severe thunderstorms Thursday evening.
Tuesday
Posted Tuesday 09/23/25 @ 8:20 AM — An indistinct warm front will slowly approach today, Tuesday, with the full effects not felt until Wednesday into Thursday.
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. Water Vapor Imagery reveals emitted water vapor infrared TEMPERATURE, NOT REFLECTED LIGHT from CLOUDS., I’ve drawn the approximate position of the warm front although this feature is not as distinct as drawn. (Click on image for a larger view.)
Current cloudiness should dissipate this morning and we should see increasing sunshine. High clouds move back in from the west and lower clouds move in in the far northwest suburbs by about 4 PM. Rain moves in this evening. As posted yesterday, most of the rain will shear off to our northwest, but the latest model guidance suggests a bit more rain may make it into Philadelphia.
Dew points (humidity) will increase today into the mid 60s and closer to 70º by Wednesday. Uncomfortably high dew points in the low 70s by Thursday.
Wednesday looks to be considerably cloudy with some light showers possible in the mid to late afternoon.
Thursday, especially Thursday evening will have somewhat heavy rain and possible severe weather.
Many models are showing total rainfall through Friday to be in the 1.5- 2.5″ range with locally higher amounts.
Mon 09:09 PM Update —While this is well into the future, there’s a strong signal for possible severe weather late Thursday afternoon into Thursday evening. I’m keeping an eye on this.
Posted Monday 09/22/25 @ 5:43 PM — A cold front will approach the area Tuesday, but it will stall somewhere near our area, likely just northwest of the immediate Philadelphia area.
Tuesday will have sunshine through high cirrus clouds. It will be warm, with highs in the low to mid 80s near the city.
Showers and thundershowers will develop in far western sections of Montco and Bucks counties about 6 PM Tuesday. Much of the rain and thunderstorm activity will slide just west of our area; much of the rain will miss us.
12z RRFS total rainfall through midnight Tuesday. Most of the rain stays to our northwest. (Click on image for a larger view.)
The above RRFS is very similar to the latest ECMWF and the ECMWF-AI forecasts. The Canadian RGEM and GDPS with “AI Spectral Nudging” models have a bit more rain closer to the city.
Waves low pressure form along this stalled front and move up into our area through Friday. Timing these wave exactly is difficult in advance, but here’s how it looks currently—
Wednesday looks to be mostly cloudy with a very low chance of light shower. A wave developing on the front will bring rain late Wednesday night into Thursdaymorning. Then another break in the rain is expected with just cloudy conditions Thursday afternoon.
Heavy rain and thunderstorms likely later Thursday evening and night.
Models guiding this forecast: GDPS-AI, NBM, RGEM RRFS , ECMWF-AI,
Friday may clear (?)
Originally Posted Sun 5:47 PM —Our beautiful dry weather will last through some of Tuesday. The models are in good agreement with a forecast of a cold front (1) coming through our area Tuesday late afternoon into the evening with showers and maybe a thunderstorm. The front hangs up just to our south and east as an upper low (3) closes off and a surface low sends waves of energy up along the front.
Today’s 12z ECMWF is in good agreement with the ECMWF-AI and the Canadian GDPS-AI models for a rainy Wednesday through Friday as a front hangs up near us. (Click on image for a larger view.)
We’ll have a good chance of rain from Tuesday night through Friday, likely with some breaks in the action. This is an evolving pattern and I’ll update with specifics during the week.
