THIS WEEK’S WEATHER

Sunday, December 3, 2023 9:21 pm GFS 2 Comments

#Philadelphia #weather #PAwx

Saturday- Changed Forecast

Posted Friday 12/08 @ 4:46 PM — I’m preparing the Weekend Weather Forecast, but I wanted to post a big change in the forecast for Saturday ASAP. The warm front is now expected to bring some showers Saturday morning with clearing after 1-2 PM. That’s a significant change than what had previously been the model guidance for just clouds and fog.

Details coming in my Weekend Weather Forecast

I’ve updated the Winter Snow Outlook for the next 10 days.

Friday Weekend Update

Posted Friday 12/08 @ 9:10 AM — A quick update. A few changes in the forecast outlined in my Weekend Preview below—

There will be heavy fog very early Saturday morning and especially Saturday night.

Rain ahead of the cold front on Sunday will begin with spotty showers in the morning. Rain and winds pick up after 2 PM (earlier far western suburbs). Rain becomes quite heavy Sunday evening and overnight with high winds expected.

Latest GFS (06z) total rainfall forecast for Sunday into Monday. (Click on image for a larger view.)

Check back for my Weekend Weather Forecast later this afternoon or early evening.

Friday and Weekend Preview

A warm front will try to move through tonight (Thursday night) and Friday will start cloudy. Clouds should break for some sunshine through high clouds by late morning or early afternoon. Highs should reach 48-49º Friday afternoon.

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine white contours) with superimposed MRMS RADAR. **Warm front in Ohio will gradually move north of our area Friday morning and some sunshine is expected with moderating temperatures.** **A second warm front now in Missouri will move through on Saturday with an increase clouds.** (Click on image for a larger view.)

A second warm front will slowly move through on Saturday. Saturday will be milder with highs 50º-53º but will become considerably cloudy in the afternoon.

The second warm front will be north of us Sunday and highs will reach the mid 60s!

A strong cold front approaches Sunday night. Heavy rain, high winds (45-50 mph) Sunday night. Thunderstorms possible.

NBM shows widespread heavy rainfall for Sunday into predawn Monday. (Click on image for a larger view.)

NBM Meteogram for KLOM Blue Bell shows wind gusts 40-50 mph Sunday evening (red box). The NBM tends to be conservative with wind gust forecasts. (Click on image for a larger view.)

Thu 11:25 AM Forecast Review — The snow flurries came and the surface temperatures rose above freezing early in the event, so little to no accumulation occurred in the immediate Philadelphia area and adjacent suburbs. Interestingly, the heavier band of precipitation occurred south of the city, not northwest of the city as forecast.

RADAR image 11:23 AM showing temperature line (32º) well north or our area and heaviest band of precip consistently to our south. (Click on image for a larger view.)

Light Snow Update

Posted Wed 12/06 @ 11:11 PM — Tonight’s models are suggesting that the light, accumulating snow will stay north of Doylestown. Only flurries closer to the city.

Light Snow Thursday Morning

Posted Wednesday 12/06 @ 5:17 PM — Today’s models are in fairly good agreement that a clipper type disturbance will move down in the jet flow Thursday morning, giving us some light snow, possibly a coating, dusting or up to 0.5 inches in the far northwestern suburbs—

Here’s the expected “snowfall totals” (I’m using quotes for a reason) —

18z HRDPS forecast accumulation of snow by noon. (Click on image for a larger view.)

These totals are in the range of model noise and anything less than 0.5 inches should be viewed somewhat skeptically. That said, I think these forecast amounts are possible, considering the consistency of the several models.

Temperatures are heading upwards and anything that sticks will likely melt by the end of the day.

Wednesday Forecast Update

Posted Wednesday 12/06 @ 10:38 AM — The snow flurry forecast for Wednesday has been on-track and additional snow flurries (non-accumulating) are likely early this afternoon.

10:30 AM: Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine white contours) with superimposed MRMS RADAR. An area of snow showers in the northwest suburbs will move in. **The clipper type disturbance for Thursday** is shown (white arrow) The yellow contours are thickness lines and the yellow arrows capture the increase in thickness expected as the airmass moves to the northeast on Thursday. (Click on image for a larger view.)

The greatest chance for snow flurries today is where POPs (Probability of Precipitation) are highest this afternoon—

HRDPS POPs (probability of precipitation) at 2 PM Wednesday. While 56% may not seem high, on an hourly basis, anything greater than 40% is a high. (Click on image for a larger view.)

Changing Focus to Thursday

Posted Tuesday 12/05 @ 7:55 PM — So there’s a slight chance of some flurries Wednesday. Of greater interest is a disturbance that moves down from the northwest while cold air is in place Thursday morning, at the same time the upper half of the atmosphere is warming. Some additional snow showers are seeming more likely, possibly with a dusting type accumulation in far northern areas.

ECMWF forecast for 7 AM Thursday morning. Very cold air at the surface with moisture associated with a clipper type disturbance **will bring the chance of some additional snow flurries/showers Thursday morning.** Temperatures are expected to warm during the day Thursday to well above freezing. Any (light) accumulation, if any, should melt. Areas far north of the city (far northern Bucks County) may see a light accumulation before it melts later in the day.

This clipper type disturbance is looking more pronounced in today’s models, than it had previously. I’ll keep an eye on it.

Snow Flurries Wednesday

Posted Tuesday 12/05 @ 5:03 PM — A system to our south combined with disturbances rotating around an upper trough will result in some light precipitation Wednesday, likely snow flurries or snow flurries mixed with light rain showers. No accumulation for most of the area; a dusting possible north of Doylestown on grassy surfaces.

The latest NBM shows extremely light flurries possible during Wednesday afternoon—

18z NBM shows extremely light blue (snow flurries) and light green (rain showers) at 2 PM. Compare to the previous forecast below (Click on image for a larger view.)

The latest HRDPS shows an area of light precipitation moving through about 1 PM. The 36.5º contour is highlighted; it’s the temperature where there’s a 50% chance of either rain or snow (an overly-simplistic, back of the envelope methodology).

18z HRDPS forecast at 1 PM Wednesday. (Click on image for a larger view.)

Additional snow showers possible on Thursday as warmer air returns aloft!

Update

Posted Monday 12/04 @ 5:42 PM —A cold front moved through and we have had some colder temperatures today. These slightly below seasonable temperatures will be with us through Thursday. (Average high is about 47º Blue Bell)

Latest NBM 18z temperature meteogram for Blue Bell, PA (Click on image for a larger view.)

Despite a weak disturbance moving near us early Wednesday morning, no snow or even flurries are expected here.

NBM conditional precipitation type for 7 AM Wednesday morning. 32º line is drawn as white. Blue is light snow, green light rain. (Click on image for a larger view.)

A warm up is expected Friday through Sunday, but it will be accompanied by rain late Saturday into Sunday.

Today’s 12z NAEFS forecast for Friday at 4 PM. Large upper ridge over us, but low pressure in the Texas panhandle will move towards us bringing rain late Saturday into Sunday. More cold air dropping down in Montana. (Click on image for a larger view.)

Previously Posted Sun 9:21 PM —

A changeable week, weather-wise, as an upper cold trough moves in by Wednesday, then replaced by an upper (warmer) ridge into the weekend.

ECMWF forecast for Wednesday afternoon shows a cold upper trough over us. A large ridge of milder weather to our west is poised to move in late in the week and into the weekend. That ridge will be less pronounced when it gets here, as some colder air is pushing southward from Canada. With the milder weather next weekend, expect some rain, likely Sunday. (Click on image for a larger view.)

For Monday, we’ll have considerable mid level cloudiness develop that will be with us for most of the day. Highs 47º-49º which is actually near our seasonal average temperature.

Our coldest day will be Wednesday.

Milder air by Friday and especially Saturday. Rain expected on Sunday.

I don’t see any pattern for snow in the coming week or two. As was the ‘diagnosis’ from last year’s winter, “there’s just not enough cold air”/

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

Friday, December 1, 2023 6:19 pm GFS

#Philadelphia #weather #PAwx

Sunday Forecast Update

Posted Saturday 12/02 @ 5:50 PM — Rain moves in after midnight tonight and tapers off during the early afternoon. It will remain cloudy with light winds. It should no longer be raining in Philadelphia at the start of the Eagles game. Light winds and temperatures about 52º at 4:30 PM.

Sat 5:36 PM Forecast Review — Today’s high temperatures showed a wide range, as expected. The skies did clear about 11 AM to noon, except near the airport, which was reporting fog into early afternoon. PHL also had a relatively low max temperature.

RTMA measured and interpolated actual temperatures at 3 PM today—

20Z RTMA – actual measured and interpolated temperatures at 3 PM today. (Click on image for a larger view.)

Saturday Update

Posted Sat 12/02 @ 10:47 AM — Right after posting my update this morning, the 12z HRDPS became available. It’s showing 60º and mostly sunny in the afternoon. We’ll see.

Posted Saturday 12/02 @ 9:58 AM — This morning’s models still show low fog breaking for some sunshine through mid and high clouds between 11 AM and 1PM. (The latest HRDPS has clouds lingering until about 1:30-2 PM)

Here’s the HRDPS at 1:00 PM showing low clouds hanging tight in the low valleys near the rivers—

06z HRDPS showing low clouds near the river valleys at 1 PM **(Clouds are in grey. black)** (Click on image for a larger view.)

I don’t think many areas are going to see the 60º forecast by last night’s TV weather entertainers. This morning’s 12z NBM still shows a high of 57º with a very high standard deviation (high uncertainty) of ± 4º

So, there still remain a wide range of forecasts in temperature and cloud cover. The warm front is getting hung up near our area, causing the uncertainty.

Updates

Update Fri 12/01 11:22 PM — Tonight’s models suggest that high temperatures on Saturday are likely to be anywhere from 54° to 59°. Still high uncertainty. Clouds and fog may linger until 11 AM before skies brighten.

Posted Friday 12/01 @ 5:56 PM — The latest HREF and Canadian RGEM have become available and they capture the extreme uncertainty in tomorrow’s high temperature forecast—

18z HREF—

18z Canadian RGEM—

Previously Posted Fri 6:19 PM —

Two disturbances ahead of a cold front will affect our weather this weekend. One is bringing the rain we’re getting this afternoon (Friday). The next one will bring rain primarily Saturday night into Sunday.

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. I’ve drawn in the cold front and warm front. Area 1 is affecting us now. Disturbance 2 will bring rain on Sunday. (Click on image for a larger view.)

As posted last night, there’s an usually large range of model forecasts for the next few days; the most significant differences (and uncertainties) are around the the cloud cover/high temperatures on Saturday and the location of the secondary coastal low formation on Sunday.

Let’s tackle each one individually.

Saturday

A disturbance and associated warm front is moving through Friday evening and it is expected to exit by Saturday morning. There’s a wide range in the cloud cover forecast. The ECMWF forecasting sunshine breaking through has our high temperatures in the low 60s! Most of our models show areas and periods of considerable cloudiness and are in the 56º to 59º range. The NBM shows an incredibly high standard deviation of ± 5.0º, which captures the very high uncertainty of the forecast.

Here’s the summary—

Clouds break for periods of sunshine through high clouds with periods that are considerably cloudy. I’m leaning towards a compromise on temperatures. A below average confidence forecast. Saturday will be dry. Rain begins again about 11 PM Saturday night

High temperature 57º ( — location Blue Bell, PA)
High temperature 59º ( — location Philadelphia , PA)

Sunday

As the low pressure in the Great Lakes and its associated cold front spawns a secondary coastal low, we’ll have periods of rain much of Sunday with a taper off during the afternoon. There are huge model differences in the placement of the low, the amount of rain, and when the rain ends. It appears that the cold front will move through Sunday evening. I’m going with a compromise.

Rain ends about 4-7 PM.

High temperature 54º ( NBM model— location Blue Bell, PA)
High temperature 56º ( NBM model— location Philadelphia, PA)
uncertainty ± 3.5º (still high uncertainty)

Philadelphia Weekend Weather & Storm Forecasts

Monthly Archives: December 2023