Monthly Archives: November 2019

Winter Storm Outlook

WEATHER UPDATE

[su_note note_color=”#defcdc”]Sunday 12 PM Update: This morning’s models have become available. The trends—

Less QPF is expected today, rain amounts reduced from 1 inch to about 0.6 inches water.

For Monday, temps in the upper atmosphere support snow fallling during the afternoon, but surface temperatures remain too warm until evening.

The upper atmosphere cools after 2 AM tonight and any precip will fall as rain >> snow by afternoon.  The lower atmosphere temps are slow to chill down, complicating accumulation forecasts.

The NAM has a coating up to 1 inch (with the lower range more likely) by late Monday evening.  The GFS has surface temperatures too warm for any accumulation in PHL and the immediate suburbs. 

For snow, I tend to go with the NAM. 
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[su_note note_color=”#defcdc”]Sun 7:30  AM Update:  The 1 AM run of the GFS and NAM has moved the start of the precip to about 9 AM.  The latest HRRR depicts the freezing rain  further north and west —

HRRR PTYPE forecast for 9 AM Sunday

As I said yesterday, with the precipitation onset being revised each model run suggests that the models are having difficulty with this storm’s forecast. 

That said, my rule of thumb — when the models are having trouble with the very short term forecast, it puts into question their forecast for the following day. 

The models are still predicting a mix of rain and snow during the day and evening Monday, significantly more snow to areas north of Doylestown.  Last night’s ECMWF is all on-board with that forecast.  Can we really trust it with Monday’s forecast?   Stay tuned.

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…from last night—

Earlier this week, I described this weekend’s weather as “interesting” and indeed it is a complex scenario involving a secondary coastal low pressure system and an upper atmospheric low that will interact in complex ways. See my previous discussions about the systems involved, as the general scenario appears to be playing out as predicted.  Here are the latest model trends —

Freezing rain and sleet begins 5-7AM from the city just north and west, and changes to all rain from 9 AM to noon. The city proper, south and east will have mostly rain. Areas north and west will have a prolonged period of sleet and freezing rain.  The graphic below is the latest NAM model forecast of PTYPE.

NAM forecast for 10 AM Sunday. Green rain, red freezing rain, magenta sleet. White line is surface freezing temp

The other models are in general agreement.

After the changeover, rain heavy at times is expected. QPF values about 1 inch water.

Sunday night, cold air filters in in the upper atmosphere.

For Monday —An upper low trailing the surface low will bring additional precipitation on Monday. A changeover to snow and snow showers is expected during the day, however for the immediate PHL area and adjacent suburbs, surface temperatures remain above freezing until late afternoon, so accumulation will be highly limited here. Currently, the models are showing a coating to an inch by the time it ends late Monday night, with considerably more in far northwest counties.

There remains uncertainty about this Monday’s snow. Stay tuned.

 

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

[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.

SREF model 7 AM Sunday Categorical PTYPE (precipitation type).  White line is freezing line at surface (actually measured 6 feet above ground).

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.