Forecast update Sunday 8 AM
Warm air retuning in the mid levels of the atmosphere is causing this layer of altostratus this morning. Light snow has developed with this pseudo-warm front feature. None of the models had picked up this feature. These clouds should dissipate by late morning. High temperatures 42-44.

The current pattern can be summarized as cold air remaining in Canada with small dips in the jet bringing colder air over the Northeast, which then alternates with a warmer southwesterly flow of moisture bringing rain.

Low pressure will move off the coast tonight and weak high pressure will build in. The flow will remain cyclonic from the departing low through Saturday afternoon.   Saturday will be fairly windy and cooler.  Highs about 48-50.

Fri 11PM Update: Saturday high temps 45-47

Winds 15 mph with gusts.  Following some sun early, the cyclonic flow will cause some altocumulus cloudiness during the mid day hours into the afternoon, then will dissipate as winds subside.

For Sunday, weak high pressure will provide mostly sunny skies and more seasonably cool temperatures.  Highs 42-44.

High clouds move in late in the afternoon on Sunday as the next wave of warmer air and rain approaches for late Monday into New Years Eve. (Bravo to the new FV3-GFS which predicted this bad weather for New Years Eve well over a week ago, although it was originally predicting a snow storm two weeks ago. )

Things chill down toward the end of the week, as the GFS has the possibility another brush with rain or light snow Thurs evening.



All models are on-board with a low pressure system that moves in late Thursday into Friday.  Rain, possibly heavy (more than 1″), is very likely for much of Friday.  It will  be very mild. High temperature near 60.

Saturday dries out and it will be colder and closer to seasonable.  There may be instability clouds.

There’s a wide range of model forecasts for Sunday, as a low pressure system tracks to our south. We may get some light precip Sunday if the low tracks a bit further north.  Type will depend on timing, but mostly like very light rain showers.

The storm I mentioned a week ago for New Years  Eve/Day is on track to be a rain-storm.

Right now, I don’t see much in the way of snowy winter weather.

There’s no giant slug of cold air predicted to descend into the central US.

Cold intrusions will repeatedly occur in the North Eastern section of the US that will effectively block storms coming up the coast.

These cold intrusions will alternate with mild air and wet flows. Things would have to change big-time for my colder-than-normal climate forecast from late November to become a reality.





Forecast update Sunday 11pm
Too warm for snow tonight and the NAM has reduced the QPF in Phladelphia to zero.

First, will the Eagles win again with Foles?  Yes.

That said, let’s turn to tonight’s forecast which may be a more difficult call. A front will pass through tonight between midnight and daybreak.  Weak low pressure is predicted to move along the front.  Each model run has increased the intensity of this wave of low pressure.

The models are consistent with low temperatures being above freezing (36 degrees in Blue Bell).  The latest NAM has several critical temperatures of the atmosphere supporting snow, except for the lowest level.  QPF values are now 0.07 inches water.  Currently the models have a general consensus of light snow with a slight grassy coating in the areas in blue –

Current Model P-type prediction after midnight Sunday.

If there is further strengthening of the low, dynamic cooling may move this area of snow closer to the city.  Right now, the areas shown are the best bet, but it wouldn’t surprise me if there’s some very light snow falling closer to the city.  Since it will occur before daybreak, we’ll have to see if anything is left on grassy surfaces.  This is the best we can do this year for a white Christmas.

Of more interest is the period around New Years Day.  For almost a week,  the GFS and FV3-GFS have been showing some sort of major storm (rain, ice or snow) in the Jan 1-2 timeframe.  Something I’m watching. 


Forecast Update Saturday 11pm
Little change in the forecast for Sunday; Mostly sunny with high 44. A cold front moves through after midnight Sunday. A weak impulse will bring light rain showers before daybreak Monday.

Some of the thermal profiles suggest this may fall as light snow showers or flurries, especially northwest of the city. There’s some evidence that this impulse may strengthen slightly, so stay tuned.

UPDATE Sunday 7 AM:  Last night’s NAM shows it will be a bit too warm in the immediate PHL area for light snow tonight. (Despite the upper atmosphere having temperatures that support snow.)  Here’s the current precipitation type (PTYPE)  forecast for this evening into Monday. We would have to get much more intensification and dynamic cooling for any accumulation here.

NAM forecast
NAM forecast 11 PM tonight

High pressure will slowly build in this weekend behind the departing low pressure system that brought the heavy rains today.

On Saturday, according to the GFS, there will a significant amount of low level instability cloudiness – stratocumulus should be prominent with occasional breaks of sunshine possible, more sun possible later in the day.  It will be windy with gusts approaching 30 mph.  High 45

The high builds in Saturday night and begins to depart on Sunday as another secondary cold front moves through Sunday night. Sunday will be mostly sunny and seasonably chilly, but still above average. Winds will be lighter, out of the southwest.   High will be about 44.  (Average high is 41.)

The secondary front moves through Sunday night with light rain showers possible. UPDATE Fri 11 pm : There’s a slight chance of it falling as snow showers or flurries after midnight.

Monday, Christmas Eve, will be mostly sunny and colder.  High 41.

The medium range outlook through New Years maintains the pattern of near average temperatures with some warmups and rain.  Little possibility of snow, although the long range shows cold air building in Siberia, Greenland and the polar areas. Expect a change to colder and stormer conditions the first week in January.