The 1 AM run of the NAM reduced the QPF considerably and we are back to the original forecast from Friday of 1-2 inches with sharp cut off north of Philadelphia.
Light snow ends shortly after noon today for Philadelphia but continues in southern Delaware and Chester counties and southern NJ.
So… this was a boundary condition forecast which was poorly handled by the NAM model. The NAM has done well in these situations in the past, and it was prudent to report its forecast as a high probability.
The NAM forecast was predicated on a secondary low intensification at the coastline that will take place, but slightly further south.
Looking back, the GFS did better with the timing (starting earlier Saturday evening) and with the lower QPF. I can think of many past storms where the NAM did better than the GFS.
The NBM did well with the QPF, but poorly with the onset of the snow. It had the snow starting 10-midnight which is what I used. It also had forecast the secondary low intensification for Sunday night, which appears to be the same error made by the NAM.
Low pressure will approach on Saturday and pass to our south Saturday night. Philadelphia will be on the northern fringe of the snow shield.
Saturday will have increasing cloudiness. It will be cold, but not as windy as the past two days. High temperature 32-33
Light snow develops late evening, probably after 10 pm. Light snow continues through the night and tapers early Sunday morning.
Sunday will cloudy. There may be some clearing mid to late afternoon. High 33.
Tonight’s NAM has a QPF of 0.16 inches water. Tonight’s GFS has reduced QPF to 0.07 inches water. One inch of snow is the current call, less north of the city, more south. The trend is towards less snow.
Low pressure will move up from the south and intensify somewhat as a coastal low on Saturday. As mentioned in my post earlier this week, this would ordinarily be a snowstorm here, but the lack of cold air will make this a rainstorm.
Saturday will be rainy through most of the day, tapering late afternoon and ending early evening. High temperatures 46-48. It will be somewhat windy.
Things dry out for Sunday.
Sunday will be mostly sunny, windy and cooler. High temperatures 44-46. (Average highs now are 39.)
Forecast Update Saturday 11 PM
Sunday forecast looks to be cloudy in the morning with sun breaking out in the afternoon. Temperatures chill down for Monday (Highs in the 30s) with a brief warm up Tuesday (High near 50); Temperatures chill down again Wednesday night into the weekend.
The medium range models are showing some winter-like cold air moving in Wednesday night into Thursday and lasting into the weekend. This dip in the jet flow will primarily affect the eastern and northeastern sections of the US.
After another warmup, the overall weather pattern is expected to gradually change into a much colder pattern during the third and fourth week of January as a mass of colder air moves down from Canada into the central US.
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.