Category Archives: Philadelphia Weather Outlook

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Phillywx #Philadelphia #weather #PAwx

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.

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx

Quick Update

Updated Fri 10/13 @ 12:00 PM — This morning’s GFS just available shows an earlier start time for the rain Saturday, different than last night’s models. The GFS shows accumulating rain as early as 8-10 AM Saturday moving west to east. (I tend to like the GFS timing with these things, but it’s not fail-proof.)

Today’s 12z GFS simulated radar and accumulated rain contours (green contours 0.01″ intervals.) forecast for 10 AM Saturday. (Click on image for a larger view.)

Update Fri 10/13 9:26 AM — Sunny and cooler Friday.

Model trends show a later start to the heavier rains Saturday. A late morning quick sprinkle can’t be ruled out but the main area of rain moves in about 1-3 PM and the heaviest rain during the evening and nighttime hours. Rainfall totals may be less than previously forecast in the immediate Philadelphia area, about 1” with higher amounts near the Jersey Shore and northwest of Reading. Windy at night.

Rain ends Sunday morning. Breaks of sun in some (but not all) areas Sunday afternoon. Windy.

I’ll update later today.

Friday through Sunday

Update Thu 10/12 10:25 PM — Friday looks to be sunny and cooler. Rain starts later on Saturday, moving in from west to east, between 11AM and 2 PM. A faster moving system will likely clear out by Sunday afternoon, with rainfall totals somewhat less than posted earlier.

Updated Thu 10/12 @ 6:04 PM —A large storm will develop as a secondary low forms off of the Delaware coast later Saturday.

Currently, water vapor imagery shows the seeds of the storm to affect us Saturday into early Sunday—

Water Vapor image 5:20 PM Thursday with superimposed RAP ), potential vorticity (fine white contours) and superimposed MRMS radar. Low pressure in Nebraska will move east and a secondary coastal low will form Saturday. (Click on image for a larger view.)

The above image corresponds to this NAEFS model map—

Today’s NAEFS map for the current time (5:20 PM) Thursday. (Click on image for a larger view.)

By late Saturday into Sunday morning , a strong coastal storm will have formed—

NAEFS forecast for 5 AM Sunday morning showing the coastal storm (Click on image for a larger view.)

Rain begins mid morning on Saturday and becomes heavier by the afternoon.

Total rainfall, based on the current NBM is 1.2″ to -1.4″—

NBM total rainfall through Sunday ranges from 1.2-1.4″ in our area. It should be noted that the ensemble forecasts and the ECMWF show lower totals, on the order of 0.6″ to 1″.

Rain ends sometime Sunday morning. but clouds linger with windy conditions. This is a change from previous model forecasts, which had showers lingering later.

The latest GFS shows some sun Sunday afternoon.

An interesting storm. Stay tuned for updates.

Thursday through Sunday

Updated Wed 10/11 @ 8:54 PM — Go Phillies! OK, back to the weather.

The models have us being in fair, dry weather Thursday through Friday, and the latest models have the heavy rain starting somewhat later, now during the morning hours Saturday instead of midnight Friday. It may clear later Sunday, but right now I wouldn’t bet on it.

Of interest is the current water vapor imagery which shows moisture moving up from the south along with some rain developing in the central US, ahead of the system expected on Saturday.

Current Water Vapor image with superimposed RAP model thickness lines (yellow), potential vorticity (fine white contours) and superimposed MRMS radar. Of interest is the moisture moving up from the south along with the area of disturbance ( red?) . The models have this being suppressed to our north and south, but you wouldn’t know that from this image. (Click on image for a larger view.)

It will be interesting if any cloudiness develops early afternoon Thursday from this questionable area. The GFS suggests this possibility, the NBM not so much. The Canadian HRDPS, not at all.

I’ve learned to never second guess the models, so at this time, some increased cloudiness on Thursday is just a thought experiment.

Wednesday and Beyond

Updated Tue 10/10 @ 5:21 PM — The closed, stationary upper low over Canada continues to rotate disturbances, but the models are forecasting those disturbance to have a trajectory further to our north Wednesday thorough Thursday.

As a result Wednesday and Thursday will have more sunshine than clouds and temperatures are expected to moderate with highs approaching 70º by Thursday—

NBM Model Temperatures/Dew points for Blue Bell, PA (Click on image for a larger view.)

It should be noted that the high temperatures start to decrease a bit on Friday. That’s due to increasing cloudiness ahead of a rather robust weather system that will bring rain Saturday and much of Sunday.

The NAEFS and GEFS has been forecasting a significant low pressure system to move in and re-develop a secondary surface low along the coast over the weekend Some heavy rain is possible with this system—

NAEFS forecast for Saturday 2 PM. Upper low (blue L) with surface low (black L) will transfer energy to a coastal secondary low. Heavy rain possible. (Click on image for a larger view.)
Tuesday Update

Updated Tue 10/10 @ 10:11 AM — Yesterday’s forecast of “a low chance of widely scattered showers” for today has moved into a more likely event.

The latest NBM shows very light showers moving through from now into mid afternoon—

Today’s 12z NBM total rain accumulation for today, Tuesday. (amounts less than 0.02″) The NBM still shows some breaks of sun during the day as well. (Click on image for a larger view.)
Tuesday

Updated Mon 10/09 @ 9:32 PM — Showers are moving through this evening as forecast, the result of a disturbance rotating around the upper low in Canada. Weaker disturbances will move through Tuesday, for a mix of sunshine and clouds. A low chance of a widely scattered shower Tuesday afternoon. About 2-3º warmer than Monday.

Current water vapor image shows the current upper air situation—

Water Vapor image with superimposed thickness values (yellow) and RAP potential vorticity (white fine contours/blue arrows) show the area of disturbances causing the clouds and showers. (Click on image for a larger view.)

Wednesday looks good.

Unfortunately, the weekend still looks quite rainy, but too soon to be sure.

Previously Posted Sun 8:21 PM —

An upper air low will close off and rotate disturbances around it, as high pressure builds in by late Tuesday into Wednesday.

Water Vapor Image with RAP model thickness and potential vorticity. Upper level low (L- Blue) will become almost stationary for much of the week. Some disturbances will develop and rotate around the low affecting our weather Monday night. .

Monday will be chilly and sunny, but instability cloudiness will develop mid afternoon (like today) as a disturbance moves in. Some showers possible Monday night, clearing on Tuesday.

By Wednesday the low is “closed” and remains stationary but high pressure builds in. Meanwhile, next weekend’s weather is brewing in the Pacific northwest—

NAEFS forecast for Wednesday morning shows closed low, with high pressure building in. The white arrows show disturbances originating in the Pacific that will cross the country and potentially give us a rainy weekend.