Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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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.