THIS WEEK’S WEATHER | Philadelphia Weekend Weather & Storm Forecasts

Forecast: Friday through Sunday

Update Thu 11/24 10:35 PM —
Friday’s forecast essentially on-target. Most models have about 0.13” of rain, ending about noon or so, but several high resolution models just have a few showers with only about 0.05”. Windy in the afternoon.

No changes in Saturday’s or Sunday’s forecast at this time

Update Thu 11/24 @ 10:03 AM —
A front moves through on Friday. A storm brings considerable rain on Sunday.

Thursday’s water vapor/radar image shows systems to affect us over the next few days: a closed upper level low (L) with a cold front (1) moving through early Friday and the storm (2) that will affect us on Sunday.

Friday—The cold front comes in with some showers. This morning’s HIRESW has the rain ending about noon, earlier northwest of the city. Sunshine breaks out behind the front early afternoon. High 52-53º (Blue Bell, PA)

Saturday— Sunny. High 52-53º (Blue Bell, PA)

Sunday— Cloudy with rain. Rainfall ~1.0″. Very Windy! High 54-55º

Forecast: Thanksgiving through Sunday

Update Wed 11/23 @ 7:53 PM — Reduced rain totals on Friday
Update Wed 11/23 @ 6:06 PM —
The models have finally come together for a forecast for the weekend.

Thursday —Thanksgiving : Sunny, a brief period of cloudiness in the mid afternoon. High 53-54º

Friday— A cold front moves through in the late morning with showers. Rainfall light, about 0.06 inches. Clearing in the afternoon. High 52º

Saturday— Sunny and a bit cooler. High 51º

Sunday— Cloudy with rain. Rainfall 0.30″. Very Windy! High 55-56º

Evolving Forecast for Friday through Sunday

Update Wed 11/23 @ 9:59 AM —
The models continue to come together for the weekend forecast, but the forecast is now somewhat different than previous days.

With the exception of the GFS (which has a minimal of rain for us on Friday ~ 0.04″), Friday now appears to have light rain from the morning into early afternoon. The rain will be light (0.15-0.20 inches). Skies clear about 3PM with colder air moving in behind a cold front.

Saturday— Sunny but highs only in the low 50s, somewhat cooler than previously forecast.

Sunday— A secondary low pressure develops near NYC. Rain in the morning, possibly heavy. Showers in the afternoon.

06z GFS Forecast for Sunday at 7 PM. Secondary low forms near NYC. (Click on image for a larger view.)

Updated Tue 11/22 11:04 PM — Tonight’s NAM, GFS, and Canadian GEM models are more aggressive with some rain moving in from the south on Friday.

Weekend forecast comes into focus

Update Tue 11/22 @ 4:29 PM —
The models are finally coming together into a more unified similar forecast.

Thanksgiving Day – Thursday

Very nice autumn weather. Sunny and mild.

Friday Through Sunday

The models are coming together.
Friday: The latest GFS keeps us cloudy, with the entire area dry on Friday. However, several other models including the model Blend (NBM) have light showers just south of the city. The current Canadian RGEM captures this—

Canadian RGEM model northern extent of rain on Friday. (Click on image for a larger view.)

So there’s some uncertainty about the northern extend of showers on Friday.

Saturday appears to be very nice and sunny.

Sunday: It seems the scenario that had been expected on Friday will now occur on Sunday. A secondary low (coastal) low form on Sunday with the potential of moderate to heavy rain ( 1 -2″). Very windy.

GFS forecast for Sunday 7 AM. (main axis of rain off the coast?) (Click on image for a larger view.)

Continued Uncertainty in Weekend Forecast

Update Mon 11/21 @ 8:19 PM —
Today’s models have backed off on the heavy rain for Friday, with the Canadian, German ICON, and ECMWF all having some light rain moving through. Our GFS model has no rain for Friday. The GFS was the first to pick up on the change in jet pattern and I wouldn’t be surprised if we see a fairly dry Friday.

Temperatures will remain mild from Thursday through Sunday.

The model Blend (NBM), which is supposed to sort these things out, has a very high probability of rain on Friday. Go figure.

Unfortunately, there’s a chance possibility of rain on Saturday and rain is likely Sunday. The models are still divergent, except now for Sunday.

GFS model forecast for Sunday—

Todays’ 18z GFS rain/cloud forecast for Sunday 1 PM (Click on image for a larger view.)

In summary, the forecast for the period Friday through Sunday remains below average confidence.

Significant Change in Friday’s Outlook

Update Mon 11/21 @ 11:44 AM —
Since the weekend, I’ve indicated that there were extreme differences in the model forecasts for Friday. Sometimes that might mean one is getting it right and one is getting the forecast wrong. But often, it means that NEITHER model was accurately modelling the atmosphere.

Last night and especially this morning’s latest GFS has moved towards a forecast that is totally different and it appears that the ECMWF is moving in that direction—

This morning’s 12z GFS model forecast for Friday at 4 PM (Click on image for a larger view.)

Basically, it shows a cut-off closed upper air low over Texas, very different than the sharp dip in the jet flow from previous models, especially the ECMWF.

It now appears that low pressure with rain may not move in until at least Saturday, with just clouds on Friday. Movement of closed lows are very difficult to forecast. I expect further changes in the coming weekend forecast. Stay tuned.

Previously Posted Sun 6:40 PM —

Following Sunday’s [colder than] January-like temperatures and colder wind chills, this week will see moderating temperatures. High pressure will assure dry conditions through Thursday. Major uncertainties creep into the forecast by Friday around a developing storm.

Thursday—Thanksgiving Day

A beautiful day with sun, light winds and moderating temperatures.

GEFS forecast for Thurday morning, High pressure parked over us with dry conditions. (Click on image for a larger view.)

Uncertain Friday Storm

The models have been in lack of agreement with a potential coastal storm expected to develop on Friday.

The ECMWF has been the most consistent with a sharp plunge of the jet stream and a secondary coastal low pressure development—

ECMWF forecast for Friday 4 PM showing heavy rain and a developing coastal low. The red 500 mb contour shows the dip in the jet stream. (Click on image for a larger view.)

The “ensemble models”, specifically the NAEFS (a combination of the US and Canadian Global statistical models) had a similar forecast until this past day or so. It now shows a much less amplified jet flow and delayed development of the low—

NAEFS forecast for Friday. Note that the **red** thickness line looks nothing like the ECMWF’s and the low pressure is down in South Carolina.

Looking at it from a different perspective, the ensemble forecasts show forecast uncertainty in the form of increased standard deviations.

Here’s the latest GEFS pressure with pressure standard deviation—

The GEFS forecast for Friday shows mean low pressure center in South Carolina, similar to the NAEFS. Notice the darker purple shading along the coastline indicating high standard deviation in pressure and high uncertainty. (Click on image for a larger view.)

Here’s the Canadian GEPS, also showing high uncertainty along the coast. It’s low center is closer to the ECMWF forecast above —

Canadian Ensemble Model (GEPS) shows similar low center to the ECMWF forecast, but high uncertainty even further northeast. (Click on image for a larger view.)

The bottom line is that the forecast for Friday is extremely uncertain at this time, with a high likelihood of rain.

The latest NBM precipitation probability (which combines over 25 different models) for it’s forecast shows high chance of rain throughout the area—