Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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Fri 8:41 AM Forecast Review — The rain last night and this morning was best forecast by yesterday’s HRDPS/ICON/ECMWF last night. Here’s where the rain fell—
MRMS accumulated rain last night based on actual radar estimatesr/rain gauge measurements. Well predicted by the HRDPS yesterday. (Click on image for a larger view.)

Friday

Updated Fri 10/20 @ 8:41 AM —For today, periods of clouds and periods of sunshine in the immediate Philadelphia area and western suburbs today, Friday. Rain moves back in this evening likely after 6-8 PM.

A Changing Forecast

Updated Thu 10/19 @ 8:13 PM —Much of the forecast for this coming weekend has changed and there is unusually high uncertainty with the forecast.

A highly amplified jet stream, which had been expected to stay amplified through Sunday, is now expected to flatten rapidly.

As a result of this changing upper air flow, the models are having trouble with placement of the surface low pressure systems, with many models showing multiple areas of low pressure Friday into Saturday.

Case in point is Friday’s weather— There’s a wide range of forecasts, with our NOAA models showing a period of rain here Friday morning in the Philadelphia area

The Canadian RGEM and HRDPS, along with the ICON model have the rain staying east of the Delaware river, in NJ.

The ECMWF is closer to the Canadian/ICON and has the rain mostly east of Philadelphia, but just reaching into the city and immediate suburbs.

Today’s 18z HRDPS forecast rainfall through 2 PM Friday. Mostly east of the immediate PHL area. (Click on image for a larger view.)

With low confidence, I’m leaning towards the Canadian, German ICON and ECMWF models. which keep most of Philadelphia dry during the daytime Friday, especially west of the city. Things change at night as an area of showers is expected to move in from the west by most models, but the ECMWF has it mostly missing our area.

Most models have some rain moving through Friday evening and night, then considerable clearing late morning Saturday. Total rainfall will be light.

Some sunshine with instability cloudiness then develops Saturday afternoon with widely scattered showers possible, but most areas dry. It will be windy.

Sunny with instability cloudiness on Sunday.

I’ll take another at-bat with this forecast tomorrow. Hopefully, I won’t have striked out.

Faster Storm, Less Rain

Updated Thu 10/19 @ 8:50 AM — The coastal storm expected to form Friday into Saturday continues to be much less like last weekend’s rain storm.

A warm front moves through early Friday with showers. Some partial clearing/sunshine possible Friday afternoon, but scattered showers also possible after the front.

A less organized coastal low will bring some rain Friday night into Saturday morning. Most of the energy of this storm will ‘skip’ over our area and the intensification will be further to our north.

Canadian RGEM forecat for 5 PM Saturday. (clouds in black/grey) Some clearing in our area. Low pressure has multiple centers with main low north and east of previous forecasts. (Click on image for a larger view.)

Following some morning and early afternoon showers, some sun with windy/gusty conditions late Saturday afternoon.

Sunday looks to be sunny in the morning, instability cloudiness in the afternoon, with winds and gusts. Colder.

Faster Moving Storm

Update (revised) Wed 10/18 @ 6:17 PM — The system that will affect our weather is moving faster than earlier forecasts and cloudiness with this system will move in as early as Thursday afternoon. The rain follows around or before daybreak Friday. Some breaks of sun are possible Friday afternoon.

The large storm in the central US will move to the Great Lakes as a secondary low develops along the coast. Here’s the current water vapor image that displays mid and upper level features—

Current Satellite Water Vapor image with superimposed RAP model thickness lines (yellow), and superimposed MRMS radar. Low pressure will move to the Great Lakes while a sharp dip in the jet flow induces a secondary low off the coast. I’ve drawn in the forecast positions of the low pressure systems (Click on image for a larger view.)

Rain Friday, especially afternoon and evening. Light rain continues through Saturday with increasing wind and wind gusts. Some breaks of sun are also possible Saturday afternoon.

By Saturday morning the coastal low has formed—

Today’s 12z NAEFS (mode) version showing coastal low at 11 AM Saturday

While driving home this afternoon, I heard the term “heavy rain” Friday and Saturday in their forecast. I would use the term “moderate rain” since many models still show a total of less than 0.75 over two days in our immediate area. The rainfall on Saturday looks light with a few possible exceptions mid day.

Here’s the latest NBM model total rainfall through midnight Saturday. It’s increased somewhat from this morning’s forecast—

Today’s 18z NBM total rainfall forecast through midnight Saturday. (Click on image for a larger view.)

Updated Wed 10/18 @ 10:20 AM — One more thing. It appears that showers will arrive around daybreak Friday, earlier than previously forecast.

Weekend Forecast Update

Updated Wed 10/18 @ 10:01 AM — What had been looking like a repeat of last weekend’s soaker, now appears to be a light, on and off rainfall for Friday through Saturday with totals less than 0.5″ here in the immediate Philadelphia area and westward. Areas east in NJ will have somewhat heavier rainfall, but still not on the order of last weekend’s rain.

As mentioned as a possibility in last night’s update, the heaviest rain is likely to stay east over the Atlantic with the deep low pressure system intensifying further to our north, near New England. The latest NAEFS still maintains plenty of wind and wind gusts.

Here’s the latest NBM rain total forecast Friday through midnight Saturday

Today’s 12z NBM rainfall forecast for Friday and Saturday combined. (Click on image for a larger view.)

There’s differences in details with the models and I expect further changes in the forecast with this system.

Weather Update

Updated Tue 10/17 @ 7:39 PM — A quick update. Wednesday should have more sunshine and less cloudiness than today. No showers expected.

A large coastal storm will develop early Saturday morning. Some of the showers with this system may move in as early as Friday morning.

Despite the strength of this storm, our area may not receive anywhere as much wind and rain as areas in New England. The models differ with the speed and path of the storm, with several keeping the heaviest rain over the Atlantic. Too early to be sure. Nonetheless, Saturday will be another windy and rainy day, with partial clearing Sunday. Temperatures will be quite cool later Sunday with highs reaching only 50º or so.

Saturday morning forecast—

Today’s 12z NAEFS shows the coastal low (black L) with the upper air low (blue L) far from the surface low. The GFS and ECMWF have them closer together, meaning a speedier exit. (Click on image for a larger view.)
Tuesday through Saturday

Updated Mon 10/16 @ 4:50 PM — Once the upper low exits on Tuesday, high pressure builds in for Wednesday and the weather this week can be summed up in three graphics—

NAEFS forecast for 11 AM Wednesday shows a ridge of high pressure over our area with fair skies and light winds. A southwesterly and then southerly flow follows later Wednesday through late Thursday brings moderating temperatures. Some cloudiness moves in during the day Thursday. Thursday will become WINDY. (Click on image for a larger view.)

As the high pressure system move off, low pressure in the Gulf teams up with a deepening upper trough over the Great Lakes late Friday to create a significant coastal Nor’easter by Saturday morning. Friday will be mostly cloudy with light rain possible in the afternoon.

Today’s 18z NBM Temperature meteogram for location Blue Bell PA Tuesday through Thursday. (Click on image for a larger view.)

By Saturday—

NAEFS forecast for 8 AM Saturday morning. Rain, possibly heavy and windy! (If this were the middle of January, this could be quite the snow storm!) (Click on image for a larger view.)

The NAEFS did a superb job forecasting last Saturday’s rain storm over a week in advance. There’s every reason to think its forecast for this Saturday’s storm is good.

Previously Posted Sun 8:20 PM —

An upper air low pressure flow will affect our weather Monday through part of Tuesday. High pressure builds in on late Tuesday into Thursday.

A developing system, remarkably similar to this past weekend’s rainstorm, may affect our weather late Friday into next Saturday.

The current water vapor image shows the players for the coming week—

Sunday’s Current Water Vapor image with superimposed RAP model thickness lines (yellow), potential vorticity (fine white contours) and superimposed MRMS radar. The elongated positively tilted double upper level lows (blue L) will rotate disturbances through on Monday. The jet flow (white arrows) will bring large high pressure (black H) down into our area late Tuesday through Thursday. The system in the Pacific Northwest (red L) will dive down southward and reemerge as a coastal low for Friday into Saturday.
Monday

Weather will be similar to today (Sunday) considerable instability cloudiness, especially in the afternoon. A few sprinkles possible Monday late afternoon (4-6 PM), which should dissipate before the Phillies game.