Category Archives: Philadelphia Weather Outlook


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Friday and Weekend Outlook
Fri 05:27 PM Forecast Review — Not as much shower activity as forecast. Heavier activity was further east—

Update Fri 05/05 @ 11:13 AM — Despite the sunshine this morning, an upper air disturbance will bring scattered showers later this afternoon—

Satellite Water Vapor shows upper air disturbance to move through later Friday afternoon. (Click on image for a larger view.)

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The big upcoming change will be the transition to an upper level ridge. Temperatures will move back towards the low 70s by the weekend—

NAEFS Max Temperature forecast for late Saturday. Also visible are the 250 mb (jet stream level) contours showing the ridge that will slowly move towards us. Temperatures near 70 on Saturday and above 70 on Sunday are expected.

Thursday will be similar to today, Wednesday, although there should be noticeably fewer scattered sprinkles and perhaps some more breaks of sun.

A similar forecast for Friday.

The weekend looks to be dry with more sun!


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Wednesday Update -Continued Blocked Pattern

Update Wed 05/03 @ 9:37 AM —The blocked pattern will slowly transform into another blocked pattern over the weekend. By the weekend, our part of the country will be under more of an upper ridge than the current upper low. Until that time we’ll continue with unsettled weather: clouds, some sun, scattered showers, and cool temperatures.

Today, Wednesday, we’ll likely have the most widespread shower activity, but both Thursday and Friday may have a few late afternoon showers in some areas.

Here’s what things look like by Sunday

NAEFS forecast for Sunday at 2 PM. The red line, (540 thickness line) captures the general configuration of the jet stream. Notice the extreme extent into northern Canada indicative of a blocked pattern. The Northeastern US will now be under more of the upper ridge. The weather should become drier and milder. (Click on image for a larger view.)

Tuesday – Continued Unsettled Weather

Update Tue 05/02 @ 8:16 AM — The upper air low has moved to north of NY State and it is expected to precess and end up over our area Wednesday and Thursday.

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Previously Posted Mon 12:00 PM —

This Week’s Weather Overview

The blocked pattern will slowly loosen over the next several days. The upper low still over the Great Lakes will continue to rotate disturbances over our area, giving us periods of clouds and some showers intermixed with some sun.

Water Vapor Satellite image Monday morning shows the low pressure system still over the Great Lakes, rotating disturbances over our area. This may continue through Thursday. (Click on image for a larger view.)


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Wednesday Forecast Update

Update Wed 04/26 @ 9:48 AM — Some showers are moving in a bit earlier than forecast by most of last night’s models (with the exception of yesterday’s NAM-NEST). Additional scattered showers expected especially mid to late afternoon and evening. There will be breaks of sun at times today.

(It’s interesting that the two most recent NAM-NEST model runs did not show the showers this morning, but yesterday’s 18z run did. Such is the state of precipitation forecasts.)

According to the latest HRRR (12z) most of the showers will develop during the late afternoon and evening and most of the activity (not all) will be a bit north of the city—

Today’s 12z HRRR total precipitation forecast for Wednesday into Thursday. (Click on image for a larger view.)

Wednesday Forecast Update

Update Tue 4/25 8:28 PM — No significant change in the forecast from the forecast made this morning (below). Inspection of this afternoon’s models suggests that a warm front type situation will be part of the trigger for the showers Wednesday. The showers will be scattered and there may be breaks of sun with any showers. Any sunshine will increase instability promoting the possibility of low level thundershowers.

It still looks like plenty of rain this weekend, as early as Friday, but there are timing differences now between the GFS and the ECMWF. So it may rain Saturday or Sunday or both days. Or some of the rain may be late Sunday, with rain-free periods during the daytime. Too soon to nail that down.

Update Tue 04/25 @ 10:14 AM — It does appear that some scattered showers and low-level thundershowers will develop during Wednesday afternoon. Clouds move in late morning and showers/thundershowers can be expected from about 1 PM to 8 PM.

The scattered showers will develop dynamically in-place, rather than moving in from the west as a upper air wave moves through.

Here’s the latest HRRR cloud/radar forecast—

Today’s 12z HRRR forecast for 4 PM Wednesday afternoon. Clouds (dark) simulated radar and accumulated precip (green contours) (Click on image for a larger view.)

FYI, the models continue to forecast a fairly rainy weekend, now including Saturday and Sunday. Updates later.

Focus on Wednesday and after

Update Mon 4/24 10:27 PM — This afternoon’s models still have some showers on Wednesday but the timing ranges from starting in the early afternoon or early evening in our immediate area.

This weekend’s weather still looks very ‘interesting’ as heavy rain is looking more likely on Sunday from a stalled surface low enhanced by a highly amplified jet flow. Unsettled weather is possible late Friday and even some part of Saturday from the same system.

Update Mon 04/24 @ 8:57 AM — Last night’s models are signaling a change in the forecast as early as Wednesday. An upper air low pressure system north of the Great Lakes will rotate a disturbance through our area on Wednesday —

Satellite WV image Monday morning with superimposed RAP model jet level windstreams (250mb) . (Click on image for a larger view.)

Here’s the latest NAEFS forecast for Wednesday showing weak low pressure over us—

Today’s 06z NAEFS (Click on image for a larger view.)

The disturbance is weak, but enough to set off some showers on Wednesday. So what appeared to be a quiet few day of weather has changed into something less straightforward.

Today, Monday, and Tuesday will be the nicest days of this week. (Today will have some periods instability cloudiness develop in the afternoon.)

Additional disturbances and a developing trough (labeled “The weekend” above) will affect us Thursday through the weekend.

Quiet and Cooler

Previously Posted Sun 6:50 PM —

An upper air disturbance brought some clouds late today, Sunday, after a beautiful, mostly sunny day in the immediate PHL area. Areas east of us in NJ had some lingering clouds from the system that gave us the rain last night.

Following this disturbance, cooler air will be moving in for much of the first half of this week. Not much happening Monday through Wednesday, as high pressure and cool temperatures control our weather. Mostly sunny, some instability cloudiness at times.

We may get affected by a disturbance moving to our south on Thursday, with a chance of some showers.

Things look ‘interesting’ for next weekend. A highly amplified upper trough will develop and the models are showing a coastal secondary low that may develop and linger. There’s fairly good agreement among the ECMWF and GEFS models for this to occur. Cool, breezt and very rainy weather is expected. The time frame, depending on the model, is as early as Saturday, possibly lingering through part of Monday. I’ll keep an eye on this during the week.

Here’s the current NAEFS forecast for next Sunday—

Today’s 12z NAEFS forecast for next Sunday. Cold air plunges south. The red 540 thickness line captures the general contour of the jet stream. Secondary low formation in the Virginias may linger as the upper pattern takes on a negative tilt. (Click on image for a larger view.)