[su_note note_color=”#defcdc”]Sat 5PM Update: Current Trends: Precipitation starts later, about 7:00 AM Sunday.  Several hours of light sleet and freezing rain possible before changing to rain  late Sunday morning.  Far north and west will have extended sleet/freezing rain.

There have been ongoing changes with the onset of the precipitation, suggesting the models are having trouble with this complex surface and upper air system.

For Monday, still a coating of snow for PHL and immediate counties north and west on Monday.  Areas north of Trenton and Doylestown will have more— potentially a few inches of snow on Monday.  I’ll update later this evening. [/su_note]

…from Sat morning:

[su_note note_color=”#defcdc”]Sat AM  Update: After reviewing last night’s model runs, here are the current trends— For Sunday morning, all models show a period of sleet and freezing rain until about 10 AM Sunday morning,  longer duration far northwest.  Below, the SREF (Short Range Ensemble Forecast Model) shows sleet/freezing rain in magenta, snow in purple, rain in green.

The rest of Sunday, we’ll have rain, heavy at times.

Still uncertainty about Monday, especially Monday evening. The SREF, NAM and ECMWF (European) have light snow with a coating of accumulation possible, more north of Allentown. The GFS and the CMC (Canadian) have snow showers, very light and scattered, no accumulation in our area.  I’ll continue to update again this weekend.

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…from Fri evening—

The latest models have come in.  Based on the latest NAM, NBM and WRF,  there are some changes in the forecast.  The background for the weather this weekend is discussed in previous posts.

Saturday will be dry.  Some sun early with high, thin cloudiness increasing throughout the day. Lower level clouds move in later in the afternoon  than previously forecast. Very light winds. High 44.

About 3-5 AM Sunday morning, precipitation starts, likely as sleet, freezing rain and wet snow mixed in.  After daybreak Sunday, sleet continues and mixes and changes to rain during the morning.  Some freezing rain possible north and west early.  Driving may be affected early. Tonight’s WRF models have freezing rain/sleet until mid morning.

Rain, heavy at times Sunday. High 45.

A secondary low may linger and intensify off the coast on Monday, with cold air changing precipitation to wet snow on Monday and Monday evening. Still unclear if there will be any accumulation, but a coating to 1 inch possible by Tuesday morning.

[su_note note_color=”#defcdc”]Fri Late Afternoon Update: The forecast for the weekend remains on track.  I’m updating because the Monday forecast period now falls into the range of the NAM model.

For Monday: The latest NAM model supports the scenario discussed as a possibility over recent days— a secondary coastal surface low is held back or regenerated along the coast due to the upper atmosphere low pressure system to the west playing catch-up with the surface system.

Once they come together and become “vertically stacked”, the coastal system will further intensify.  This setup allows the collision of cold air and wrap-around moisture to create the chance of snow Monday afternoon and evening over our area.  About an inch  is a possibility by daybreak Tuesday morning, although warm surface temperatures may interfere with accumulation.   I think there’s still uncertainty about the amount of snow on Monday and I expect better clarification over the weekend.  Stay tuned.

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…from Friday morning—

[su_note note_color=”#defcdc”]Fri AM Update: There’s still uncertainty with the forecast for Monday.  As mentioned over past days, the strong upper atmosphere low remains west of the surface low on Monday.  Precipitation lingers while the “critical thickness” levels indicating lower level/upper level temperatures supporting snow  moves to our south and east, allowing a change from rain to snow.

So the possibility of light snow or snow showers on Monday still a possibility.  A range of warmer surface temperatures may keep us from getting much, if any, accumulation.   Below is the SREF (Short Range Ensemble) “statistical mean” forecast.  Stay tuned.

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….from Thursday night:

Happy Thanksgiving!

A wet and windy Sunday….

The trends mentioned in the previous posts continue.  A deep low pressure system will spawn a secondary coastal low late Sunday as the upper atmosphere low pressure system hangs back.

Saturday will be dry.  Sunny in the morning with increasing cloudiness expected by early afternoon.   High 48.  Precipitation, originally forecast to start Saturday afternoon is further delayed until the hours before daybreak Sunday.

For Sunday, precipitation will be rain, heavy at times.  QPF values around 1 inch of rain.  There is a possibility the the precip starts as wet snow before daybreak but warm temps will change everything to rain.  High 43.  It will be windy.

The deep low pressure will intensify off the coast. Colder air will be brought in before daybreak on Monday.  There is the chance of change back to wet snow at that time. No accumulation is forecast by the models.

Monday will be cloudy, windy and colder. High 40. The upper atmosphere low and the coastal low will combine to our east.  The models are forecasting snow showers during the day Monday, but surface temperatures will be above freezing.  Little or no accumulation is forecast by the models.

I think there’s some uncertainty in the model forecast for Monday. Some of the snow showers could be more than forecast, but I don’t like to second guess the models.   Monday is just beyond the forecast window for the shorter range, higher resolution models.   Stay tuned.