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Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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Wednesday Forecast Update

Posted Wednesday 05/13/26 @ 7:37 AM —The latest RRFS (06z) shows the approaching showers today moving in as two separate lobes; the first one, scattered and light swings through about 6-7 PM.

The second more organized line of showers moves through between 8 PM and 11 PM. The second line of showers is expected to weaken, for the reasons described below as it moves eastward. Total rainfall still expected to be light, although the latest RRFS shows 0.40″; most models still in the 0.25″ or less range.

06z RRFS forecast “simulated radar” at 6 PM. Two lobes of rain move through, the first very light and scattered.

An upper air low with a pool of cold air aloft will bring variable cloudiness with a chance of scattered instability showers during the afternoon hours on Thursday. Not all areas will see showers.

So Little Rain. Why?

Posted Tuesday 05/12/26 @ 8:02 PM — In recent months, we haven’t seen much in the way of typical Springtime thunderstorms. Most of the time, we’ve been an an anticyclonic upper air flow (clockwise curvature) which produces descending air flow.

Wednesday is another example where a large area of rain somewhat diminishes in intensity as it approaches.

This is a good example of another reason- our position in relation to the position of the jet stream (and more specifically, the constricted part of the wind stream resulting in higher speeds called the ‘jet streak’)

Our position on Wednesday evening will put us in what’s called the Left Entrance Region of the nearby jet streak. This region has vertical descending flow which works against precipitation.

GFS winds at 250 mb showing, the level of the jet stream. Philadelphia is in the Left Entrance region. The precipitation will reduce in intensity as it moves through this area. (Click on image for a larger view.)

A textbook representation is here—

The Left Entrance region has descending flow. (Click on image for a larger view.)

By the way, the jet in “jet stream” doesn’t get its name from being at the level that jet airplanes fly (34,000-38,000 feet).

The “jet” in jet stream refers to fluid dynamics terminology where constriction in flow causes an increase in velocity of flow. You’ve all seen a garden hose where the nozzle constricts flow causing a jet of water. Air is a fluid too. The upper air winds flow in the valley between air masses. Where that valley constricts causes the jet streaks.

Wednesday Forecast Update

Posted Tuesday 05/12/26 @ 5:46 PM —A cold front will approach and move through our area Wednesday evening. During the daytime hours, Wednesday will have mostly sunny skies through high cirrus clouds in the morning, then periods of clouds in the mid afternoon with the sun breaking out again about 4 PM. It will be windy and times.

Rain with this system will occur behind the front and wind shift. The wind shift occurs about 8 PM (Blue Bell) and the rain showers move in between 8 and 9 PM. Rainfall had originally looked to be impressive, now looks to be on the light side. Total rainfall about 0.20″ The rain is no longer expected to linger into Thursday.

18z RRFS Precipitation Rate at 9 PM Wednesday. (Click on image for a larger view.)

An upper level low will linger over our area Thursday and Friday. A mix of sun and clouds is expected, but with an upper low in May, one can’t rule out a scattered shower.

For weather nerds—

—”Effective on August 31, 2026, at 1200 Coordinated Universal Time (UTC), the National Centers for Environmental Prediction (NCEP) will discontinue the North American Mesoscale (NAM) Forecast System, the Short-Range Ensemble Forecast (SREF), the High-Resolution Ensemble Forecast (HREF), the High Resolution Window (HiresW), and the NAM Model Output Statistics(NAM MOS). These legacy systems are being replaced by the Rapid Refresh Forecast System (RRFS).

The RRFS and REFS production implementation and the retirement of the legacy models will occur on the same day.“

Tuesday-Wednesday Outlook

Posted Monday 05/11/26 @ 8:05 PM — We’ll finally get some sunny skies on Tuesday, as high pressure builds in behind the system that exited on Monday.

Yet another dip in the jet along with a surface low over the Great Lakes will push a cold front slowly through on Wednesday.

18z GFS forecast for Wednesday at 4 PM. Showers on the doorstep of our area ahead of a cold front. (Click on image for a larger view.)

The models continue to have some showers along this front linger into Thursday as the upper jet digs deeper towards the south. It look like this jet dip will have temperatures less chilly than the one with us on Monday.

Originally Posted Mon 9:40 AM —This week will be an active week weather-wise, as an atypical dip in the jet stream and cool air collides with May moisture and warmth from the south.

For today, the front that went through yesterday with the fast moving storms (we had some hail at the start) is still poised to bring some light rain to areas just east of I-95 today into the Jersey Shore.

Water Vapor image at 10 AM with superimposed RAP model MSL pressure (black contours), GFS/NAM potential vorticity contours (yellow & violet fine contours) with MRMS Radar and surface wind barbs, The dip in the jet stream is visible, as is the position of the rain over the coastal area. (Click on image for a larger view.)

Varying degrees of mid-level cloudiness is forecast for today but “mostly cloudy” captures it . There will be gradual clearing, perhaps about 5 PM, from the northwest. A few showers this evening are possible near Allentown as an upper air disturbance moves through.

Tuesday looks quite nice!

As for rainfall, here’s what the area received on Sunday evening/night. The rain was very localized- There were sharp gradients with areas receiving 0.25″ and a mile or so away, the totals were 0.75″—

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.)
Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

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Sunday Forecast Updates

Posted Sunday 05/10/26 @ 9:37 AM — This morning’s models show showers (and some thunderstorms) developing in western sections as early as 3 PM but won’t likely move into the city until 6-8 PM. The thunderstorms will likely not be severe and will occur mostly in southern Chester and Delaware counties.

As the front stalls a wave of low pressure will bring additional showers around midnight. The latest Model Blend (NBM) has the showers exiting early Monday morning, except in S. Jersey.

Dry and clouds gradually clearing on Monday.

12z Model Blend showing clouds (dark) and showers at 12 noon Monday. Our area will be dry and gradually clearing. (Click on image for a larger view.)

Updated Saturday 05/09/26 @ 9:42 PM — Much of our area had very little rain today. As for Sunday, the models are increasingly forecasting a line of showers to pass through with a cold front during the mid to late afternoon into the early evening. This front is expected to stall as a wave of low pressure rides up with showers into Monday.

Unlike Saturday’s weather which was driven by upper air disturbances and “forcing”, Sunday’s weather will be more surface frontal-based and should be forecast better by the models.

Saturday Forecast Update

Posted Saturday 05/09/26 @ 11:00 AM — This morning’s high resolution models have become available. The NAM-NEST model seems to be most aligned with what is happening on radar and satellite imagery, although its rain shield still lags about 1-2 hours.

Rain is already falling in western sections and will likely continue. The trigger for increased rainfall is shown in the image below and it will cause rain mostly west of our area to start.

Theta level thermodynamic upper air trigger identified. Path of this trigger identified with white arrows. (Click on image for a larger view.)

The NAM-NEST has rain moving in about 1 PM to the immediate PHL area, if not sooner. But where this system will cause heavier rain varies and is uncertain.

One thing this morning’s models have in common- rainfall about 7-8 PM in Philadelphia, possibly affecting the Phillies game. None of the models have particularly heavy rain in-store for us, except a few locations in NJ. The doughnut (or donut) hole of little precipitation still shows for parts of the immediate PHL area with this system. No matter how you cut it, today’s forecast remains low confidence.

Saturday Known Unknowns

Posted Saturday 05/09/26 @ 9:10 AM — With this upper air driven system, the models are having difficulty with the forecast today— none of the models from last night remotely match the current MRMS radar—

MRMS radar image at 8:30 AM. None of the models match this precipitation pattern at all. (Click on image for a larger view.)

I’ve even run the 11z RAP and RRFS, models from just 7 AM this morning; again neither captured the forward advance of the rain into Lancaster/Berks counties.

Additionally, surface pressure change, another parameter I like to match to determine the model best capturing ‘reality’ all come up short. As a result, the forecast for today is extremely uncertain. Rain will move in…actual timing may become clearer with the 12z models (which take direct upper air measurements with weather balloons.)

For now, expect showers. Will we get higher amounts in NJ as forecast last night? Will there really be some moderate level thunderstorms about 4 PM? It’s unclear.

Sometimes the best forecast information is knowing that we just don’t know.

Previously Posted Fri @ 7:05 PM — —Our weather on Saturday will be influenced by strong upper air disturbances with an assist from the position of the jet stream.

Water Vapor image at 5 PM with superimposed RAP model MSL pressure (black contours), GFS/NAM potential vorticity contours (yellow & violet fine contours) with MRMS Radar and surface wind barbs, The stream of radar echos from the upper air disturbance in Oklahoma/Missouri will move and induce precipitation in our area on Saturday. Click on image for a larger view.)

Because this weather is being forced by upper air disturbances and jet stream induced lift, placement of the rainfall maxima will be difficult to predict accurately. I’ve reviewed the HRRR, HRDPS, ECMWF-AI, GFS, NBM, REFS and RRFS to get a handle on the rainfall for Saturday.

The model blend (NBM) is the best bet here for accumulated rainfall—

18z NBM total rainfall forecast by Sunday morning. (Click on image for a larger view.)
Saturday

Low clouds will predominate on Saturday. While some light showers are possible in the morning, most areas may not see any precipitation. The action starts about 2-4 PM as rain and moderate level embedded thunderstorms develop. Some models have an abrupt end around 6 PM; others keep showers around until at least 8 PM. Highs: Blue Bell- 67º Philadelphia 69º ±3.5º (Large standard deviation!)

Sunday

Clouds early, the mostly sunny for much of the day. A very widely scattered shower or thunderstorm is possible late afternoon; most areas will not have any showers. Most likely areas are Delaware and northern Bucks County. Highs: Blue Bell- 79º Philadelphia 81º ± 2.5º (Average standard deviation!)

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx #PhillyWx

Friday Update Saturday Outlook

Posted Friday 05/08/26 @ 10:44 AM — This morning’s NBM has returned the chance of showers moving in from the west about 4 -6PM Friday afternoon. This morning’s RRFS keeps the showers north – in Bucks county

Posted Friday 05/08/26 @ 8:19 AM —The showers that were a possibility for this afternoon are no longer in the forecast. Any showers today will be in the evening and likely far north of our area.

For Saturday, there’s a divergence in the model forecasts, with some having some showers beginning in the 11 AM-1PM time frame and others holding off the main shower activity until mid to late afternoon.

One thing the models do have in common is a possible ‘doughnut hole’ in the precipitation field over Philadelphia. The NAM-NEST captures this best—

Friday

Posted Thursday 05/07/26 @ 7:40 PM — As captured in this morning’s water vapor image, there are significant disturbances up stream from us that will bring chances of showers through the next several days.

A disturbance in northwest Pennsylvania will bring some light showers to us after midnight tonight (early Friday morning)

Radar image at 7:30 PM Thursday evening. These showers will stay mostly north of our area, but the RRFS shows some shower activity near the city about 1-2 AM. (Click on image for a larger view.)

There’s a bit of a question mark regarding Friday. Another disturbance is forecast by the RRFS and REFS to move through during Friday afternoon and evening with additional showers. Timing is important as they may move in as early as 2 PM or as late as 4-6 PM. I’ll update Friday morning.

Yet another disturbance, this one stronger, will bring clouds and showers on Saturday, mostly late morning into late afternoon.

Thursday-Friday

Posted Thursday 05/07/26 @ 10:43 AM —The sun has broken out from the city westward, but some showers and clouds may linger in southeastern NJ until mid afternoon.

For the rest of today (Thursday), expect sunshine.

A series of disturbances are rotating around an upper air low pressure system in Canada. Areas of moisture will combine with these disturbances to bring showers, perhaps later on Friday afternoon/evening and again on Saturday (mostly north of the city). Timing and exact placement of the showers are ‘up in the air’ right now.

Water Vapor image at 10 AM with superimposed RAP model MSL pressure (black contours), GFS potential vorticity contours (disturbances) (violet fine contours) with MRMS Radar and surface wind barbs, Areas just north of the Delaware Valley are in the main track of most of these upper air disturbances. (Click on image for a larger view.)
Thu 10:39 AM —Forecast Review —The showers developed along a band just northwest of the city between 9 PM and 11 PM last night. Additional showers moved through overnight. Here’s the 2 day accumulated rainfall. The precipitation banding did not develop in the location predicted by the models.

Wednesday Forecast Update

Posted Wednesday 05/06/26 @ 9:03 AM — The forecast rain is just west of Philadelphia at 9 AM and will be moving into the city by 10 AM. Latest REFS models have brought down the precipitation to the range forecast by the majority of other models – about 0.20″ with a few locally higher amounts in Southern NJ and far west in Berks County (Forget the high totals from last night’s posted ‘experiment’.)

Radar with RAP model parameter overlay at 9 AM Wednesday. The fine green contours are areas of upward vertical motion. The dashed green lines are areas of downward motion. (Omega) Upward motion is at most 2-3 ubars/sec this is quite low. A good heavy thunderstorm might show 15+ ubar/sec. This explains in part the low rainfall totals expected. (Click on image for a larger view.)

Additional scattered showers expected during the afternoon. It won’t be raining all of the time. The actual cold front moves through between 5:30 PM and 8 PM with additional showers.

Changed Forecast

Posted Tuesday 05/05/26 @ 5:31 PM — The trend towards little rainfall continues. A divergent and anticyclonic upper air pattern along with somewhat low precipitable water values should keep any rainfall over the next two days to under 0.35″ according to most models.

Water Vapor image around 5 PM Tuesday shows streaky filaments with a anticyclonic (clockwise) curvature. This is not conducive to heavy rain. Scattered light showers are what this indicates, and it’s supported by the models. (Click on image for a larger view.)

According to the latest RRFS, these light showers move in from the west during the mid to late morning hours on Wednesday. Additional light scattered showers around 1:30 PM and again around 5 PM and 8 PM There may be breaks of sunshine between these multiple rounds of light scattered showers.

After midnight Wednesday into Thursday morning, additional scattered showers. There may be a rumble of thunder in some spots, mostly far south of the city.

Thursday may have some clouds and a widely scattered shower in the morning but many areas will see sunshine for the afternoon.

The most optimistic model regarding rainfall is the REFS (which may be wrong by a factor of 2, but I’m putting it out here as a experiment—

REFS statistical rainfall accumulation. (probability matched mean) contours are 0.2″ (Click on image for a larger view.)
Tuesday through Thursday Update

Posted Tuesday 05/05/26 @ 8:30 AM — Today’s forecast remains essentially unchanged. There’s a chance Philadelphia may see highs near 87º or perhaps 88º. Winds increase significantly in the afternoon and evening with gusts near 40 mph.

A cold front will move through on Wednesday. Precipitation appears to be on the light side for our area. Another low pressure system expected to form along the front was expected to bring additional rain on Thursday… that low is now forecast to exit to our south with precipitation just brushing Philadelphia.

06z ECMWF-AI forecast for 2 PM Wednesday. The second low near the Mississippi River was expected to move up towards us is now forecast to move to our south. (Click on image for a larger view.)

Thursday may turn out to be drier than originally forecast.

Tuesday Wednesday Update

Posted Monday 05/04/26 @ 8:04 PM — Two more things… Despite the mid 80s temperatures on Tuesday, dew points will be very low. Indeed, it will feel cooler than the thermometer temperature. The winds will further reduced the effect of the temperature.

The rain on Wednesday and Thursday will be light, not the sort of rainfall we really need. Combined rainfall totals for both days range from only 0.20″ to 0.35″

Posted Monday 05/04/26 @ 4:08 PM — We had two incredibly perfect Spring days Sunday and Monday. We’ll have a well-advertised taste of early summer on Tuesday. High temperature forecasts have increased since yesterday. Here’s the latest model blend high temperatures with a higher range indicating standard deviation—

18z Model Blend (NBM) forecast high temperatures. Fine white contours are 1º increments. Second number indicated mean plus standard deviation.(Click on image for a larger view.)

It will be windy on Tuesday, especially during the afternoon hours until early evening—

18z HRRR wind gusts at 2 PM. (Click on image for a larger view.)

Wednesday is looking rainy, as a cold front approaches from the west. The onset of the rain looks to be earlier— 7 AM western sections and between 8-9 AM around Philadelphia.

Some strong dynamics are forecast to move through about 10 AM. Despite this, no severe or even very strong storms are forecast at this time.

Originally Posted Sun 7:17 PM —Two significant weather events in store for us this week. Tuesday will feature a major warm-up with temperatures reaching the mid 80s!

18z NBM max temperatures on Tuesday. Fine Black Contours are near ground (2m) temperatures in 1º increments. (Click on image for a larger view.)

Yet another cold front will move in on Wednesday afternoon with rain. This front may stall with lingering showers into Thursday. The GFS has it lingering, the ECMWF has it moving off to the coast. Temperatures drop behind the front.

18z GFS forecast for Wednesday at 1 PM. (Click on image for a larger view.)

Details to follow during the week!