Monthly Archives: October 2023

Philadelphia Area Weather Forecasts, Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Phillywx #Philadelphia #weather #PAwx

Forecast Change for Saturday Update

Updated Thu 11/02 @ 8:33 PM — It looks like a weak upper trough will affect our area on Saturday with more cloudiness than previously forecast, especially during the afternoon hours.

So while Saturday will be milder than Friday (Friday’s highs are expected to be upper 50s, just short of 60º in most places), the highs on Saturday will only reach about 61º-(62)º in some places, somewhat less than previously forecast due to the increased cloud cover. These highs may be different by Saturday.

NBM high temps for Saturday (Click on image for a larger view.)
Forecast Change for Saturday

Updated Thu 11/02 @ 6:21 PM — A quick update; I haven’t reviewed today’s models in detail. It appears on the latest GFS that a weak cold front tries to approach on Saturday with an increase in clouds. This was not previously forecast. I’ll update later with some new models.

Canadian RGEM shows cloudiness on Saturday. (Click on image for a larger view.)
Thursday through Sunday Outlook

Updated Wed 11/01 @ 8:27 PM — The cold weather expected tonight is being somewhat hyped on TV/radio, in my opinion. It’s going to be cold, but it’s already November and guess what, it sometimes gets pretty cold in November.

In case you missed it, here’s the low temperatures expected tonight (with an uncertainty of ± 2º) —

Today’s 18z NBM low temperatures forecast ( 2º increment contours are labeled)

There isn’t much else to talk about regarding the weather. A sprawling area of cold high pressure will affect our weather for the next several days, with moderating temperatures over the weekend.

Next chance for rain is Monday into Tuesday.

Wednesday Outlook

Updated Tue 10/31 @ 7:53 PM — Some showers may move in later tonight (Tuesday), especially from the city eastward.

The cold temperatures expected for tonight have been well-advertised. Here’s the 18z NBM model forecast lows—

NBM low mean temperatures for Tuesday night into Wednesday morning. Some uncertainty with the above standard deviation around ± 2ºF (Click on image for a larger view.)

Clouds linger in the morning. There’s uncertainty with the degree of clearing we get on Wednesday. The GFS, HRRR, HREF all show low clouds until noon to 2 PM. The Canadian models have clearing with partly to mostly sunny skies. I’m going with the Canadian models with more sun than clouds mid day into the afternoon.

Tuesday Wednesday Outlook Update

Updated Tue 10/31 @ 9:47 AM — Perhaps you’ve noticed that the forecasts have been less than consistent in recent days? There have been larger than average spreads in the model forecasts since the weekend. Luckily, the model forecasts have begun to come closer together.

The models guidance forecasts low pressure development in the western Atlantic and the position and track are close enough for some moisture and clouds to be thrown back towards us later today, Tuesday and early Wednesday.

I get a kick out of everyone thinking the European model (ECMWF) is so great. The ECMWF runs on Sunday and Monday had the low pressure system too far away to affect us. Kudos to the Canadian model (and to some extent our GFS ) that had forecast the low consistently closer to the coastline.

Sometimes I wonder if the TV forecasters put too much stock in the ECMWF forecasts. (They’re paying thousands of dollars a year to get the ECMWF; maybe they figure they have to use it?)

The ECMWF model data has been available for free as a reduced resolution and limited parameter format for about a year and a half. It’s been very nice to have, but so far, I haven’t been that impressed with its forecasts. Our GFS, GEFS combined with the Canadian models have provided better guidance for these forecasts, in my opinion.

For longer term forecasts, the NAEFS (which combines the GEFS and Canadian Ensembles) has been more useful for these forecasts.

Tuesday: Some cloudiness moves back in from the east about 4 PM today and some showers are possible late Tuesday night into Wednesday morning.

Wednesday: Some cloudiness and even some widely scattered showers linger from Philadelphia and eastward into mid afternoon Wednesday as a weak surface low develops near us.

Today’s latest 06z NAEFS forecast for Wednesday 2 PM shows a low pressure in the western Atlantic with a weaker low over our area Wednesday. Slow clearing Wednesday, especially from Philadelphia and eastward.
Forecast Update

Updated Mon 10/30 @ 5:23 PM — The question for this week is how much will the developing low pressure system in the western Atlantic affect our area?

It appears now that the answer is that some clouds may spill up into our area later on Tuesday and some shower activity is possible in eastern areas on Wednesday, especially early Wednesday morning. Here’s the latest NAEFS—

Today’s 12z NAEFS model has some showers possibly brushing the Philadelphia area early Wednesday. Not all models are on board with this forecast. The ECMWF has the system further eastward. The latest GFS is close to the NAEFS.
Monday Update

Update Mon 10/30 8:30 AM — The fog many areas are encountering this morning was forecast by last night’s models, but not yesterday afternoon’s models. It appears that we’ll move from fog to mostly cloudy today. The front is moving slowly, even more slowly than forecast yesterday, with the actual front now expected to move through early to mid afternoon (instead of this morning.)

Due to the slow movement, another weak low pressure system is forecast to develop along the front and affect us with rain showers starting about 4-5 PM.

Monday

Previously Posted Sun 8:09 PM —

A cold front is to our northwest and its energy and associated rain is sliding up past us to our northwest—

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine white contours) and superimposed MRMS radar. Most of the energy will move up to our northwest missing us.
The upper trough visible over North Dakota will move eastward and be over us by Wednesday with much colder temperatures. (Click on image for a larger view.)

The actual front moves through our immediate area late morning on Monday. Ahead of the front, some additional showers are forecast to break out from the city and southeastward before daybreak Monday, but most of the energy with this front is moving up to New England.

Early morning sunshine will be replaced by cloudy conditions later in the morning and windy and gusty conditions as the front moves slowly through our immediate area.

By Wednesday, the trough will move moving over us, as low pressure develops in the western Atlantic.

GEFS forecast for Wednesday 5 AM shows the trough moving closer to us as low pressure develop too far off the coast to directly affect us. Temperatures in the 30s expected Wednesday night. (Click on image for a larger view.)
Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#Phillywx #Philadelphia #weather #PAwx

Sun 5:21 PM Forecast Review — None of the model forecasts were stellar today. In a general way, the heaviest rainfall was to our north as expected, but heavier showers moved through during the afternoon. As mentioned, there was much greater than average “spread” (uncertainty) with today’s models, so the poor forecast performance wasn’t a surprise.
Sunday Forecast Update

Updated Sat 10/28 @ 6:06 PM —There’s still a wide range of model forecasts for the showers on Sunday. The current trend ( with the exception of the HREF model) is for very little in the way of rain here in the immediate Philadelphia area, as forecast by the HRDPS this morning (see graphic below on previous update.) I’m still leaning towards very little rainfall Sunday.

Capturing the great uncertainty and low confidence is the NBM model which still shows an incredibly large standard deviation of 6.0º in its forecast high temperature for Sunday. (Typical sd is about 0.8º -1.5º for a 24 hour forecast.)

Sunday Forecast Update

Updated Sat 10/28 @ 9:30 AM — Following some heavier showers at daybreak Sunday, it appears that most of the rain/showers will be north and west of the city. Only very light, occasional, scattered sprinkles possible for the rest of the daytime Sunday.

06z HRDPS shows extremely light sprinkles for much of the immediate Philadelphia area (following some heavier showers early morning) (Click on image for a larger view.)

Previously Posted Fri 8:14 PM —

Friday’s media forecasts of “sunny with highs near 80” didn’t jive with reality – the clouds, light sprinkles and cooler temperatures. Things continue to change and there’s a change in the forecast for Sunday. First let’s see where we are—

Currently, cold air is diving southward into the Rockies and some push eastward is occurring on a somewhat stationary cold front north of us—

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine white contours) and superimposed MRMS radar.

The above water vapor image corresponds to this NAEFS weather map—

Friday afternoon’s NAEFS model depiction of current weather, corresponding to the water vapor imagery in the graphic above. (Click on image for a larger view.)

The question for the weekend forecast is how much push east and southward will the cold front to our northwest move?

Saturday

The models are still forecasting sunny skies and unseasonably warm temperatures. There seems to be a consensus on this. Some cloudiness may move in, especially north and west, late afternoon.

High temperature 81º ( NBM model— location Blue Bell, PA)
High temperature 82º ( NBM model— location Philadelphia, PA)
uncertainty – ± 1.4º

Sunday

The real question mark has been with Sunday’s forecast. Prior to today, it was felt that the showers associated with the front would stay to our north and west. That’s no longer the case.

Today’s models (with the exception of the Canadian HRDPS and RGEM) are forecasting low clouds, with showers as early as daybreak and light showers/rain throughout the day. But there’s higher than usual uncertainty. The temperature forecast standard deviation is 6.1º which is extraordinarily high; typical is 1º-2.5º and captures the low confidence in the current forecast.

Total rainfall is anywhere from 0″ to 0.5″

High temperature 62º ( NBM model— location Blue Bell, PA)
High temperature 65º ( NBM model— location Philadelphia, PA)
uncertainty – ± 6.1º

Sometimes, the best forecast is indicating that for Sunday, we just don’t know for sure.

I’ll update Saturday morning.