Daily Archives: Monday, December 25, 2023 8:46 pm

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx

Some Sunshine on Friday

Posted Thursday 12/28 @ 5:41 PM — It appears that things will clear out enough on Friday that we’ll see some sun. Indeed, a fair amount of sunshine is expected on Friday. (Yes, this is a change in the forecast from yesterday, where clouds were going to remain.)

The upper trough moves over us on Saturday with a reappearance of clouds, at least for a part of the day—

Today’s 18z GFS shows upper trough line (blue line drawn) and the clouds (blue/maroon shading) over us at 3 PM. (Click on image for a larger view.)

Thursday 9:55 AM —Forecast Review — We had between 1.5″ and 2.7″ of rain in our area over the past day or so. As is almost always the case, the exact areas of the rain maxima were different than any of the model forecasts, but the models gave a good idea of what range of rainfall to expect.
MRMS rain gauge & radar-based measurement of rainfall received over the past day. Color shading is in inches. Line contour numbers are in mm (25.4 mm= 1 inch) (Click on image for a larger view.)

A weather pattern change appears likely around January 4th through 6th. I’ve updated my Winter Outlook.

Tonight Through Friday

Posted Wednesday 12/27 @ 9:49 PM — The heavy rain expected really began in earnest about 8 PM. Some areas to our west have already had significant rainfall.

Here’s the current rainfall (MRMS measured) that fell through 9 PM—

MRMS rain gauge & radar-based measurement of rainfall received. Color shading is in inches. Line contour numbers are in mm (25.4 mm= 1 inch) (Click on image for a larger view.)

Here’s the additional rainfall expected from now through 6 AM, according to the latest HRRR—

Tonight’s 00z HRRR total rainfall from 7 PM tonight through 6 AM Thursday. (Click on image for a larger view.)

Posted Wednesday 12/27 @ 5:43 PM — Heavy rain and gusty winds are still expected, beginning this evening and tapering before daybreak Thursday. The rainfall totals are still in the general range as shown earlier.

Ordinarily following a storm, clearing moves in, but that’s not the case with the current system. A large upper low is still to our west and clouds and intermittent light showers are possible Thursday and even Friday.

Latest GFS (18z) forecast for Friday shows low pressure (Black L) far out in the Atlantic by Friday, but the upper air low (Blue L) lingers to our west. Clouds and areas of showers still likely through Friday. (Click on image for a larger view.)
Wednesday 7:43 AM — Forecast Review — None of last night’s models successfully predicted the early batch of rain that moved through this morning. (The ECMWF, available about 2:10 AM, was the closest.)

Whenever the early part a model forecast fails, it always puts the remainder of the forecast into a low confidence situation. So, we’ll have to proceed with this in mind for today and lean on the ECMWF. See the Wednesday Forecast Update below

Wednesday Forecast Update

Posted Wednesday 12/27 @ 8:09 AM — Relying on the ECMWF model, it continues to have a bullseye of heavy rain over the Philadelphia area by late evening —

00z ECMWF forecast for 1 AM Thursday morning. Heavy rain over our area with low pressure off the Delmarva coast. (Click on image for a larger view.)

The ECMWF shows rain throughout today of varying intensity, with greatest intensity this evening and overnight.

Total rainfall will be heavy, around 1.5-2″ with locally higher amounts—

12z ECMWF total rainfall through Thursday morning. (Click on image for a larger view.)

The ECMWF also has an area of showers that moves through as late as Thursday afternoon.

Heavy Rain Being Forecast… Again

Posted Tuesday 12/26 @9:09 PM — Tonight’s early models are delaying the heavy rain until Wednesday evening. Rainfall during the daytime will likely be light.

Posted Tuesday 12/26 @ 6:05 PM — It looks like the rain will move in from the west, beginning about 9-11 AM Wednesday morning.

Here’s the NBM blend showing ‘simulated radar’ at 11 AM—

19z NBM shows ‘simulated radar’ at 11 AM. The rain moves in from the west. (Click on image for a larger view.)

Posted Tuesday 12/26 @ 5:06 PM —As posted this morning, a secondary low is expected to develop off the coast Wednesday. Monday night’s models were leaning towards a much more westward rainfall maxima, but today’s models (and the NBM- model blend) have maintained heavier rainfall in our area for Wednesday.

The latest 18z HREF, HRRR and NAM-NEST are showing fairly heavy rainfall—

Tuesday’s 18z HREF model shows a band of heavy rainfall by late Wednesday night. As is often the case, the models rarely get the placement of the heavy rain band correct, but it’s a good guide to know what’s possible in the area. (Click on image for a larger view.)

I don’t know the exact hydrology situation in our area, but I think the rivers and streams are already on the full side from the heavy rains of about a week ago. If we get this level of rainfall, expect some more flooding.

Forecast Update

Posted Tuesday 12/26 @ 9:15 AM —Last weekend’s forecast had low pressure moving in from the west. Over the past day or so, the models are showing a secondary low will develop off of the coast—

Current (Tues morning) satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine violet contours) and superimposed MRMS radar. I’ve drawn in the cold front and warm fronts, along with the the expected position of the coastal low (blue L) and its expected path. (Click on image for a larger view.)

With the secondary low formation, the energy of this system is expected to “jump” over us, possibly reducing the total rainfall in our area on Wednesday.

Several models have backed off considerably on the rainfall in our area, but the latest 12z NBM still shows about 1″ of rain (this may change) —

Today’s 12z NBM shows a generalized 1 inch or so in our area. (The GFS has backed off from this amount. ) (Click on image for a larger view.)

Additionally, rain that had been expected to begin Tuesday evening is now expected to begin Wednesday morning.

Additional rain possible on Friday.

Week Overview

A low pressure system in the nation’s mid section will move east and spawn a secondary coastal low on Wednesday—

Today’s 18z NAEFS forecast for Wednesday night. Secondary coastal low will bring rain Tuesday evening through Wednesday. (Click on image for a larger view.)

Before that time, we’ll have cloudy conditions, with early fog on Tuesday. Then continued mild, highs 51-53º.

About 0.8 to 1.1″ of rain is expected from Tuesday evening (about 8 PM) through Wednesday.

Thursday will be another mild, transitional day. Windy!

Another coastal low is expected on Friday—

Today’s NAEFS forecast for Friday at 7 PM. Additional rain that clears out for a dry New Years Day Weekend. (Click on image for a larger view.)

As mentioned in my recently updated Philadelphia Winter & Snow Outlook, a pattern change is strongly signaled for the first week in January— look for much colder and likely stormier weather towards the end of next week.