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Sunday Forecast Update

Update Sun 04/30 @ 4:24 PM — Areas of very heavy rain continue, especially in New Jersey, where large areas have received close to 5″ of rain since Friday. An additional inch or more is possible!

The latest HRRR shows some heavy downpours with thunderstorms possible between 8 PM and midnight ahead of a cold front passage.

The actual cold front will move through between 10 PM and midnight and periods of heavy rain will continue through the frontal passage.

Wind gusts may exceed 55 mph in some areas around midnight as a pressure gradient develops with the departing low pressure system.

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Previously Posted Fri 5:19 PM —


Spotty rain develops during the morning and becomes heavy during the mid afternoon. An additional 1.25 inches of rain possible. Milder. Windy and gusty.

High temperature 63.8.º sd 1.5º ( NBM model location Blue Bell, PA)

NBM forecast winds and wind gusts (with standard deviation) for location Blue Bell, PA Wings Field.


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Friday Update- Rain on the way

Update Fri 04/28 @ 9:28 AM — The latest radar has the rain somewhat closer to us than predicted by last night’s models. Nonetheless, the RAP is showing a bit of push-back of the rain advance, based on isentropic (thermal energy) analysis.

MRMS radar at 9:22 AM with superimposed RAP model 300K level Theta-E isentropes shows a bit of an energy wall to surmount for the rain to move into the PHL area, despite being just 40 miles away.

The models are still showing light sprinkles possible here about 11 AM, but the main area of rain moves in about 1-2 PM.

Rain: Parts of Friday, Saturday, Sunday

Update Thu 4/27 @ 9:08 PM — Several models have over two inches of rain falling Friday night alone.

Update Thu 04/27 @ 7:51 PM —The blocking pattern described below is setting up for this weekend and at least into Tuesday.

Rain moves in on Friday early afternoon, but some scattered sprinkles are possible during the morning.

The main slug of rain moves in from the southwest around 11AM to 1PM according to the latest GFS model—

This afternoon’s 18z GFS model forecast for 10 AM Friday. (Click on image for a larger view.)

On Friday, once the rain starts, it will become moderate to heavy at times.

The bulk of the rain will be departing on Saturday morning, but the models maintain some scattered showers around during the day, especially northeast of our area.

Heavier rain moves in on Sunday.

This weather will be generated by circulation around a upper level closed low pressure system near the Great Lakes that will remain almost stationary. Timing the breaks in the rain will be challenging as the impulses circulating the low are tough to model exactly.

We may get enough of a break Saturday afternoon to salvage the day.

Sunday doesn’t look as promising at this time.

Total rainfall may exceed 2 inches for the weekend.

I’ll try to nail it down in my regular Weekend Weather Forecast.

Update Thu 04/27 @ 10:01 AM — Updated Thursday forecast here.

Previously Posted Wed 5:24 PM —

Developing Omega Block Pattern

The upper air flow is moving towards a blocked pattern, specifically an “Omega Block” pattern. Unlike the recent “Rex” block named for the meteorologist who identified it, the “Omega” block is named for the Greek letter omega Ω because the wind flow resembles this letter.

Here’s the jet stream wind flow (at level 250 mb) for late Sunday. The Omega Ω resemblance is clearly visible—

Today’s GEFS 12z shows the forecast jet stream level (250 mb) wind flow for late Sunday resembling the greek letter Omega. (Click on image for a larger view.)

All blocked patterns result in stalled or slowly moving systems. In this case, we’ll be stalled in a cool, wet, cyclonic pattern with a closed upper low over the Great Lakes rotating disturbances towards us.

The developing block will bring rain to us by Friday afternoon. Periods of rain will be with us through the weekend and even through next Tuesday! There will be breaks in the rain and timing these breaks will be the challenge for this weekend’s forecast. It’s going to be a good weekend for ducks and flowers.

Rest of this week

Update Thu 04/27 @ 10:02 AM — The latest HRRR forecasts even more lingering cloudiness today. We may not see any sun in the city until after 2 PM.

This morning’s 12z HRRR cloud forecast (Click on image for a larger view.)

Update Thu 04/27 @ 8:35 AM — It looks like we’ll get some sunshine about 1 PM in the city and earlier west of the city. Clouds move back in about 6 PM. High temperatures about 61º but a high standard deviation of 3.5º based on the uncertainty in the cloud cover.

Update Wed 4/26 11:06 PM — Tonight’s models have clouds and light sprinkles lingering possibly past noon.

Before we get to this pattern, Thursday should start cloudy, but considerable sunshine expected around noon. A mix of sun and clouds for the afternoon. High will be near 65º


Friday will be cloudy and rain moves in about 2-4 PM from the west.


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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.)