Winter Storm Outlook

WINTER STORM OUTLOOK

[su_box title=”Winter Storm Outlook Update Tues 9 PM” box_color=”#defcdc” title_color=”#000000″]Reviewing today’s models, there isn’t much change from this morning’s update. The small disturbance that will accompany a warm front will bring some light snow Thursday night. Current QPF is 0.12 inches water, or 1-2 inches from midnight Thursday to daybreak Friday.

Most models have the large storm for late Saturday into Sunday tracking far enough inland that the precipitation will be mostly rain for PHL and immediate suburbs; the changeover to snow looks to be late enough to limit snow accumulations significantly. Stay tuned. [/su_box]

[su_box title=”Winter Storm Outlook Update Tues 8 AM” box_color=”#defcdc” title_color=”#000000″]The small disturbance that will accompany a warm front will bring some light snow Thursday night.

The large storm for late Saturday into Sunday is now forecast to track far enough inland that the precipitation will be mostly rain for PHL and immediate suburbs; the changeover to snow on Sunday may be late enough to limit snow accumulations significantly. This scenario has been one that has alternated with a more coastal track.

Stay tuned. [/su_box]

[su_box title=”One more thing… ” box_color=”#defcdc” title_color=”#000000″]While focusing on the weekend, I neglected to mention a warm front type snow that is forecast to occur this Thursday night into early Friday morning. There’s some question whether it will be snow or a mix, but tonight’s NAM has it as snow. An active week coming up. Stay tuned. [/su_box]

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The first truly impressive storm of this winter season is forecast to affect our area in the late Saturday through Sunday timeframe.

This storm has been predicted by the long range models for well over two weeks. All models are currently on-board for some sort of winter precipitation, very windy conditions, followed by deep cold.

As always, there are differences among the models regarding the speed of the initial impulse, degree of intensification and the precipitation type. The GFS, the FV3-GFS and the ECMWF (European) are the most impressive.

GFS-FV3 forecast
GFS-FV3 forecast for Sunday at 1 PM

As of today, it appears that precipitation will start as rain Saturday night, then change to a mix of sleet and rain and then a final changeover to all snow from west to east during the day Sunday.

As we know, the models didn’t accurately depict the the past weekend precipitation, so it’s a shot in the dark to predict snow totals 6 days in advance.

That said, a 6-10 inch snowfall (based on the latest ECMWF) with high winds is looking possible at this time, north and west of Philadelphia. Philadelphia may stay a mix for a longer period. This storm looks to be impressive, with high winds. Stay tuned.

Philadelphia Area Weather Forecasts

SNOW UPDATE

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.

[su_note note_color=”#d9f2da”]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.  [/su_note]

Weather… and Other Things 'Up in the Air'