Monthly Archives: August 2025

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx #PhillyWx #Hurricane

Friday & the Weekend Outlook

Posted Thursday 08/21/25 @ 8:11 PM — High pressure builds in for Friday and Saturday. Sunny skies!

12z GDPS with “AI Spectral Nudging” forecast for Saturday at 11 AM High pressure in control! (green=1 hour accumulated precipitation; black/grey= cloud cover) (Click on image for a larger view.)

The very breezy, gusty conditions we had today will diminish on Friday into Saturday. An approaching cold front will bring breezy gusty conditions back on Sunday.

18z NBM wind meteogram forecast for Blue Bell, PA (Click on image for a larger view.)

Sunday will see an approaching cold front with increasing cloudiness. Showers likely by evening.

12z GDPS with “AI Spectral Nudging” forecast for Sunday 6 PM (Click on image for a larger view.)

My regular Weekend Weather Forecast will be posted late Friday afternoon.

Thursday Forecast

Posted Wednesday 08/20/25 @ 7:30 PM — The wind flow has shifted from easterly to northeasterly and this will eventually pull in some drier air aloft and at the surface on Thursday, clearing some of the cloud deck.

There’s not much consensus among the models when the clouds will lift and thin. Some models show clouds thinning for (maybe) some sun as early as noon or 1 PM. The two Canadian models maintain low clouds into late afternoon .

The latest model blend (NBM) leans toward mid to late afternoon, supported by the latest REFS and RRFS.

So I’m leaning towards this forecast: the morning will be cloudy with even some light sprinkles areas far north west and northeast of the city.

Clouds gradually thin mid to late afternoon. Look for gusty winds in our area that diminish gradually towards evening.

12z REFS cloud cover at 4 PM (Dark blue is low cloud cover) (Click on image for a larger view.)
Wednesday Forecast Update

Posted Wednesday 08/20/25 @ 2:34 PM — The showers have developed as expected. Additional energy expected to move in during the next several hours, both from an approaching cold front and the outflow of Hurricane Erin—

Visible Satellite 2:21 PM with drawn approximate location of the cold front. Erin moving north at this time. (Click on image for a larger view.)

The latest available hourly RRFS model shows the potential/possibility of heavier showers or even some thunderstorms about 3-6 PM.

Current Radar 3:30 PM (Click on image for a larger view.)

15z RRFS SWEAT INDEX. at 3 PM Low range, but suggests possibility of showers and even a few thundershower in the colorized regions. Not a sure thing. Hopefully, it won’t interfere with the Phillies game in progress. (Click on image for a larger view.)

As for Thursday, it currently looks a mostly cloudy day with some lingering showers early morning hours. I’ll update later today.

Posted Tuesday 08/19/25 @ 7:09 PM — The sunshine returned Tuesday afternoon, but it won’t last as a stalled front to our will try to move north as a warm front with additional moisture from the outflow circulation of Hurricane Erin. Expect scattered showers during the morning hours on Wednesday.

A cold front will try to move towards us on Wednesday afternoon, pushed by high pressure that will also take Hurricane Erin out to sea on late Thursday into Friday. Expect additional showers and possible thunderstorms Wednesday afternoon and evening.

18z Canadian RGEM model forecast for 3 PM Wednesday. Several areas of energy and moisture will converge on us, as discussed above.

As I’ve said several times over the past week, hurricanes seem to influence model forecast accuracy, so expect some changes and surprises. Updates tomorrow.

Models guiding this forecast: HREF , RRFS , REFS, ECMWF-AI, RGEM

Tuesday Forecast Update

Posted Tuesday 08/19/25 @ 11:15 AM — Latest ECMWF-AI model has little change in the path of Erin, with closest approach now 230 miles form the NC Outer Banks.

06z ECMWF-AI Single forecast Thursday 8 AM, now back to 230 miles from the Outer Banks. (Click on image for a larger view.)

Also, unchanged is the rapid turn away towards the northeast. By 2 AM, it’s 422 miles from the NJ coast—

06z ECMWF-AI Single forecast 2 AM Friday morning. 422miles from the NJ coast (Click on image for a larger view.)

Unless there are changes, I’ll be tapering down my coverage of Erin, as it doesn’t pose a direct threat to our region.

As for our weather today (Tuesday), the there are interesting differences between the HRRR and the RRFS (experimental) The HRRR maintains low clouds here; the RRFS has clouds breaking for sunshine after noontime. I’m leaning towards some sunshine this afternoon, as the HRRR has a tendency to overestimate low clouds in our area.

Posted Tuesday 08/19/25 @ 7:58 AM— Several models are promising breaks in the clouds, and some sunshine in some areas, especially this afternoon.

No change with Hurricane Erin’s predicted track at this time, although the ECMWF-AI was again slightly more westward with its track last night.

Hurricane Erin Update

Posted Monday 08/18/25 @ 9:58 PM — The latest ECMWF-AI forecast continues its trend of a closer approach to the Outer Banks of NC. Now about 217 miles. Each model run has brought it slightly closer to the US coast before making a rapid turn to the northeast.

18z ECMWF-AI Single model forecast for 8 AM Thursday. (Click on image for a larger view.)
Tuesday, Wednesday, and Hurricane Erin

Posted Monday 08/18/25 @ 5:58 PM — We’ll continue to have an easterly flow of cool, moist air Tuesday and likely again on Wednesday. With the stalled front to our south, areas south of the city will continue to have a chance of showers. From the city and northward, there’s a chance of light drizzle or light sprinkles, but mostly cloudy covers it. Most of each day will be rain-free.

Again, with a nearby hurricane in the picture, the models often seem to have issues and forecasts can be a bit off.

As for Hurricane Erin, there has been a subtle change with the ECMWF-AI model, with the latest having the storm about 225 miles away from the North Carolina outer banks. The trend has been for a slightly more westward track. The standard ECMWF model is even somewhat closer, about 200 miles or so. Our own GFS-AI model has been consistently 50-75 miles further eastward with its track position. Overall, there’s pretty good agreement, as hurricane forecasts go.

12z ECMWF-AI model forecast for 8 AM Thursday. (Click on image for a larger view.)

Posted Monday 08/18/25 @ 9:46 AM — I don’t know about you, but listening to the weather forecast yesterday, I would have guessed that today was going to be a beautiful day with cool temperatures. Well we have the cool temperatures, but it’s a cloudy day with light rain and drizzle in many areas.

The light rain is due to horizontal moisture convergence: low level moisture associated with winds moving in opposing directions causing lift. Here’s the current RAP model winds at about 5000 feet and 10,000 feet, showing opposing winds—

12z RAP model barbs showing winds at 5000 feet (yellow) and 10,000 feet (green) . With moisture, light rain and drizzle has resulted.

The latest experimental RRFS shows low clouds and a possibility of drizzle and light rain through 3 PM, with drier air moving in from the northeast, allowing clearing—

11z RRFS at 3 PM. Drier air moving in from the northwest. Blue- low clouds; green- “conditional probability of rain” (conditional probability of rain can suggest more precip than actually falls. It’s really used for determining precipitation type in winter, but I find it useful for to suggest the possibility of very light precip like drizzle.) Click on image for a larger view.)

As for Erin, the ECMWF-AI model is consistent with Hurricane Erin moving about 250 miles from the NC outer banks. Here’s it’s predicted position at 8 AM Thursday—

06z ECMWF-AI forecast for Hurricane Erin Thursday at 8 AM (Click on image for a larger view.)

As I’ve said many times in the past, it’s been my observation that hurricanes near the coastal US seem to affect model forecasts in seemingly unrelated areas. This is not a scientifically proven fact, just an observation I’ve made over the many years I’ve been forecasting. I don’t know why this happens, but today’s (Monday) weather is a case-in-point— I know that yesterday’s models didn’t show drizzle/showers this morning.


I think we’ll see more of these forecast issues this week with Hurricane Erin. Anyhow, let’s see if the showers really last into this afternoon.

Initially Posted Sun 11:16 PM —Cooler temperatures but plenty of clouds this week. Additionally, there’s the chance of showers especially south of the city. Indirect effects of Hurricane Erin also will come into play by Wednesday. More info Monday morning.

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#Philadelphia #weather #PAwx #PhillyWx #Hurricane Erin

Forecast Review —Last night’s second round of thunderstorms brought much heavier rain than had been forecast to some areas—
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.)

Sunday Update

Posted Sunday 08/17/25 @ 5:15 PM — Latest RRFS has likelihood of the strongest storms within the concentric circle -green areas—

19z RRFS Calculated Severe Weather Index > 240 (Click on image for a larger view.)

Much of the city is outside of these areas. Some areas may receive little or no rainfall.

Posted Sunday 08/17/25 @ 5:01 PM — Storms have developed to our northwest.

RADAR 4:48 PM with superimposed RAP model 700 mb wind streams and low level Omega (vertical motion) in yellow. Storms moving into areas of negative Omega (upward motion) should keep them intact. Second line may encounter some decrease in activity. (Click on image for a larger view.)

Highest likelihood of the strongest storms will be south and west of the immediate Philadelphia area.

Sunday Thunderstorms

Posted Sunday 08/17/25 @ 9:54 AM —A cold front will move through the area this evening into tonight. Thunderstorms (scattered) are expected to develop ahead of the frontal passage late this afternoon and especially 6 PM to 9 PM near Philadelphia. Here’s the current situation at 9 AM—

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 thunderstorms will be scattered with the greatest density of storms southwest of our area—

08-17-25 12z NBM Thunderstorm Coverage density (bluish boxes) and Hourly Probability (color shading) Overall thunderstorm probability is about 18%/1hour or less over most of this map area, and I generally consider 18% or more as reasonable chance. Areas in the bluish boxes, it’s most likely. (Click on image for a larger view.)

Regarding thunderstorm severity, here’s the Severe WEATher index (SWEAT) calculated on HRRR data—

12z HRRR severe weather index (SWEAT). Highest values, greatest color, are in the 250 range, suggesting strong but not severe storms.

As for Hurricane Erin, I’m sticking with the ECMWF-AI model forecast, which continues to show a larger storm with the latest run taking it slightly west of previous tracks. The overall track has been similar over time, with an east-west variation, to slightly westward—

08-17-25 06z ECMWF-AI forecast for Thursday 8 AM. Closest track to NC outer banks so far. (Click on image for a larger view.)

Originally Posted Fri 5:28 PM —Some storms have fired up to our south and west Friday afternoon—

Radar Fri 4:30 PM with superimposed RAP model omega and 700 mb windstreams (10,000 feet) (Click on image for a larger view.)

A weak, slowly approaching cold front will bring the chance of widely scattered thundershowers Saturday and Sunday afternoons. Exact placement and timing still to be determined but mostly far west and north of the city.

12z ECMWF-AI model forecast for 2 PM Sunday. Cold front to our northwest. Tropical disturbance (Hurricane Erin) to slowly move northwestward. (Click on image for a larger view.)

The latest ECMWF-AI model forecast for Hurricane Erin takes it still closer to the coastline of NC late Wednesday. This AI model did very well with last year’s hurricane predictions. It’s been back and forth regarding the track of Erin. Here’s the latest—

12z ECMWF-AI model forecast for late Wednesday night. Closest forecast track so far. (Click on image for a larger view.)

Typical August temperatures and humidity for this weekend.m Afternoon scattered to isolated storms mostly north and west of the immediate PHL area.

Saturday

Partly to mostly sunny. Chance of a widely scattered thunderstorm mainly west of our area. Heat index 91 for PHL

18z Model Blend (NBM) high temp Saturday low uncertainty Contours are 1º increments (Click on image for a larger view.)
Sunday

Partly sunny. Chance of a widely scattered thunderstorm mainly west of our area late afternoon. Heat Index 96º Philadelphia.

18z NBM high temp Sunday. Low uncertainty. Contours are 1º increments (Click on image for a larger view.)
Philadelphia Area Weather Forecasts

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx #PhillyWx

Friday Storms

Posted Friday 08/15/25 @ 10:26 AM — A quick update. Today’s storms will be south of the city and west. Here’s a combined thunderstorm probability and thunderstorm coverage density map—

12z NBM thunderstorm probability (green yellow) and boxes (high density likelihood) at 6 PM (Click on image for a larger view.)

Thursday Thunderstorms
Thu 6:06 PM —Forecast Review — Today’s thunderstorms occurred mostly south and east of Philadelphia, generally in the range of the NBM forecast posted earlier today.
Radar at 4 PM Thursday. (Click on image for a larger view.)

Posted Thursday 08/14/25 @ 9:36 AM — An upper short wave/trough caused yesterday’s storms. A weak ‘cold’ front moves through later this afternoon. The front really represents only a boundary for a slight drop in dew points pushed in from the north—

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), 700 mb wind streamlines and superimposed MRMS radar. (Click on image for a larger view.)

Low level winds causing low level moisture convergence will result in thunderstorm formation today, likely not as widespread coverage as yesterday.

Here’s the NBM thunderstorm coverage forecast for 6 PM. (compare to yesterday’s forecast coverage)

12z NBM Thunderstorm Coverage. Black box outlines most likely area for thunderstorms (Click on image for a larger view.)

Storm Timing: early afternoon for coastal NJ, especially Atlantic and Cape May counties. 4-8 PM for the boxed area above.

Hurricane Erin Update: Thursday’s ECMWF-AI has the storm again curving further away from the US coast.

Hurricane Erin

Posted Wednesday 08/13/25 @ 8:36 PM — A quick note about Hurricane Erin. The latest ECMWF-AI model just became available and it takes Erin the closest to the coast of North Carolina than any previous model run. Something to keep an eye on.

18z ECMWF-AI model shows hurricane Erin 250 miles off the coast of NC at 8 PM Wednesday. Something to watch. (Click on image for a larger view.)
Wed 8:05 PM —Forecast Review — We had the heavy rain and but I wouldn’t characterize the overall storms this evening as severe. Just strong. The high numbered SWEAT Index graphic I posted earlier today from an official weather data server doesn’t seem to jive with the actual weather observed.

I spent several hours today with ChatGPT with the goal of calculating my own SWEAT index graphics from HRRR data. I used a formula posted on the College of DuPage Meteorology which gates the calculation for low vertical shear days (like today.) The result I have has values under 250, quite different from the very high 380 values from this morning’s graphic. This calculation feels more inline with the severity of the actual storms that passed through early this evening—

Same 12z HRRR model run with recalculated SWEAT index forecast at 7 PM. These values are more in line with the severity of the storms that passed through. (Click on image for a larger view.)

We’ll need a few more thunderstorm days to see if the SWEAT Index has any added value, or if it’s use is just due to its nifty acronym.

Severe Weather Threat Index (“SWEAT” INDEX)

Posted Wednesday 08/13/25 @ 10:37 AM — I’m going to try something new today— The SWEAT INDEX, a calculated empirical predictor of severe weather storms. While my usual reliance of vertical shear, helicity, CAPE and ‘lifted index” works pretty well, those parameters are suggesting little in the way of widespread severe weather later today.

The SWEAT index combines some of these parameters into a composite score and includes wind speeds (kinematic parameters).
> 300 Severe Thunderstorms likely
> 400 Tornadoes likely

According to this morning’s HRRR (13z), the SWEAT Index is in the high 300s, especially near the city—

13z HRRR based SWEAT Index at 7 PM Several areas of high 300s suggests more widespread severe weather than I previously indicated. (Click on image for a larger view.)

So, we’ll see if this index is useful today.

Wednesday – Thunderstorms

Posted Wednesday 08/13/25 @ 9:26 AM — As described in my previous post, thunderstorms are expected to break out late Wednesday afternoon and evening. The latest models show storms to be more widespread than previously forecast, although the widespread severity threat is expected to be relatively low. Current radar shows the activity in western PA—

Radar ~8:30 AM with superimposed RAP model omega and 700 mb wind streams. (Click on image for a larger view.)

Timing: While a few isolated storms are possible western sections, most of our area won’t see any activity until after 4 PM, moving from west to east. Peak activity in our area will be 6-9 PM, especially 7 PM. Localized heavy rain is possible along with some strong wind gusts. (As the NWS always says, there’s the risk of “dangerous lightning”, I guess that’s in contrast to some lightning variety that isn’t dangerous.)

The model blend (NBM) creates a thunderstorm coverage parameter (1-4) with 4 being the highest. The NBM shows very dense thunderstorm coverage at 7 PM in these areas—

Today’s 12z NBM thunderstorm coverage at 7 PM. Numerous storms (4) expected at 7 PM in red areas. (Click on image for a larger view.)

This weather event doesn’t merit a ‘severity table’. The latest HRRR shows low shear and helicity. However, there’s always the possibility of a surprise severe storm.

Models guiding this forecast: HRRR, REFS, NBM, RAP

Active Weather Wednesday

Posted Tuesday 08/12/25 @ 8:02 PM — An upper level short wave will move through during the afternoon and evening hours on Wednesday.

An increase in humidity Tuesday night will result in fog Wednesday morning. There may even be some light patchy drizzle in some areas around daybreak.

Clouds should break late morning for hazy sunshine through a layer of high cirrus clouds. With daytime heating and the moisture in place, the approaching shortwave will result in showers and thunderstorms as early as 2-4 PM. These will be scattered and it won’t be possible to pinpoint the exact locations in advance. Showers and storms will become more numerous by early evening. Some of these storms will likely result in pockets of heavy rain.

The Canadian RGEM and the RRFS along with the HRRR are forecasting areas of unusually heavy rainfall ( 3.7″) with storms right around the immediate Philadelphia area through much of the evening. Some strong dynamics move through about 9 -10 PM with possible severe weather.

The RRFS forecast at 10 PM—

12z RRFS forecast Simulated Radar, Shear Vectors, Helicity at 10 PM.

We’ll have a better handle on severity aspects Wednesday morning. I’ll update after 8:45AM with the new data.

Monday Update
Mon 8:37 PM —Forecast Review — Today’s high temp at the airport was 88º.

Posted Monday 08/11/25 @ 9:20 AM — I’ve been hearing much about temperatures reaching 90º here today. That certainly may happen, but most models have us in the upper 80s, with only a small area reaching 90º. Add the low humidity and the apparent temperature (heat index) will be in the upper 80s just about everywhere.

06z NBM forecast 12 hour high temperatures for Monday. Very small area reaches that temp. (Click on image for a larger view.)

Factor in the low humidity, and the apparent temperature is only in the upper 80s—

06z NBM Apparent Temperature (heat index) for Monday.

Tuesday will likely reach 90º in more areas. Wednesday‘s high temps may be damped down to the low 90s due to cloud cover, but Wednesday’s humidity will be noticeable with the heat index in the mid 90s.

Regarding the likely western Atlantic hurricane next week, it appears it will be blocked from moving westward. Current AI model tracks suggest a movement to the north, then northeast away from the coast.

Originally Posted Sun 9:52 PM —An increase in heat and humidity will be in store for the Philadelphia area this coming week, especially noticeable Tuesday and Wednesday. An approaching cold front will bring some showers and storms sometime late Wednesday afternoon or evening. Temperatures in the 90s will be widespread by Tuesday and Wednesday, with heat indices significantly higher.

The new dry period we’ve recently entered may not be broken by the showers late Wednesday, as most of the shower/storms are currently forecast to stay to our north and west. That may change, especially since the front may linger nearby on Thursday.

Long time followers of this blog know that I’ve seen a relationship between hurricane development near the continental US and model forecast accuracy of seemingly unrelated weather far from the storm.

This may become a factor as we approach the weekend and beyond. A tropical system, likely a hurricane will approach the southeastern US. Track of the storm is uncertain, but the European AI model was outstanding last year. It currently is forecasting a hurricane (AL97) approaching Florida or South Carolina by next Monday evening or Tuesday.

Many models are forecasting this storm’s formation and there’s disagreement in the eventual track, even between the current GFS-AI and ECMWF-AI models. Here’s the ECMWF-AI—

Today’s 18z ECMWF-AI Single forecast for NEXT MONDAY at 8 PM (Click on image for a larger view.)