Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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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—

Latest (18z) GFS shows the clipper like disturbance that will bring light snow Thursday morning. (Click on image for a larger view.)

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”/

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