Forecast Updated Tue 9:02 AM — A quick update. From the latest GFS model v 16 (scheduled to replace the existing GFS model and become the operational GFS on March 16th) A picture’s worth a thousand words—
Latest GFS version 16 Snow water equivalence with superimposed PTYPE. Snow accumulation by 8 PM Thursday and predominant precipitation type by area. (The current, operation GFS version 15.3 has slightly higher snow totals. ) We know things will change and in recent storms, the GFS has over-predicted snow.
Snow starts between 6 and 9 AM Thursday.
Next update early evening Tuesday. Stay tuned.
Mon 09:59 PM Update — Tonight’s NAM has joined the rest of the models and now has snow at the start by 6-7 AM with 3-6 inches from the city north before the snow changes to sleet about 10AM – 1 PM.
NAM PTYPE Thursday 10AM (Click on image for a larger view.)
Expect further changes.
From this afternoon…
A complex winter storm system will approach on Thursday morning and develop a secondary coastal low later Thursday. As usual, there’s considerable spread regarding the precipitation type and amounts.
I’m going to start with my own mantra—
“Never ignore the NAM model”
While the current GFS, ICON and Canadian GEM have about 3-5 inches of snow, starting about 7-9 AM Thursday morning , the NAM has it starting as all sleet, little snow, and changing to sleet- freezing rain by about 1PM—
NAM model showing freezing rain and sleet to our north (magenta) by 1 PM. Notice the position of the 32º freezing line (white) just running through Philadelphia. The green-blue-yellow is ‘simulated radar’ rain or freezing rain. (Click on image for a larger view.)
The GFS has it starting as snow, but changing to sleet and freezing rain much later in the day Thursday. The GFS shows 5+ inches of snow.
If we follow the NAM mantra, the GFS is over-predicting the snow.
The Friday time frame is beyond the forecast range (84 hours) of the latest NAM, but the GFS shows a secondary coastal low development with continuation of precipitation through Thursday night. The GFS doesn’t currently show much snow at the back end of the storm.
Forecast Updated Sat 5:21 PM — The light freezing rain will continue through this evening with a heavier batch moving through about 10 PM. Precip tapers off towards morning Sunday.
For Sunday, just cloudy in the morning and the HRRR has some brightening skies in the afternoon.
Another batch of wintry precipitation midday Monday, likely in the freezing rain/sleet variety that will change to heavy rain by Monday night. Stay tuned for more updates.
Forecast Updated Fri 9:07 PM —
Tonight’s (00z) models are just becoming available. The HRRR is showing a light coating of snow during the late morning Saturday before a changeover to light sleet and freezing rain. The 00z Model Blend has this happening early to mid morning, while most models have some light snow/sleet/freezing rain between 11 AM and 2 PM.
The SREF shows all light sleet/freezing rain with no snow. A few weeks ago, the SREF was the only model to get the precip type (PTYPE) correct.
All models have only 0.05 to 0.10 inches water equivalent, so we’re talking about light precip. Nonetheless, if it’s freezing rain, it can be a travel issue, especially when it continues after sunset. (We’re getting to the point in the calendar year where higher sun angle and solar “insolation” through clouds can help melt precip on dark asphalt surfaces.)
Freezing precip ends around daybreak Sunday. Sunday still looks very cloudy and somewhat milder. High temp 37.8º ± 1.7º
Added Fri 10:35 PM : Tonight’s NAM and HIRESW-ARW have some breaks of sun midday Sunday.
We may luck out with the storm mentioned in my earlier post for Monday into Tuesday. Some models are showing it will be mostly rain here.
posted Friday afternoon…
Cold surface temperatures with warmer flow at 3000 feet will be a setup for freezing rain and sleet Saturday into Saturday night. Another light freezing rain/sleet/snow event Sunday night and a more significant storm for Monday night into Tuesday!
Low pressure moving off to the south and a front to our west will maintain sub-freezing surface conditions, but the wind flow above 3000 feet will keep temperatures too warm for snow on Saturday.
SREF Short Range Ensemble Forecast For Saturday 1 PM Low pressure moving off to the south and a front to our west will maintain sub-freezing surface conditions, but the wind flow above 3000 feet will keep temperatures too warm for snow on Saturday. (Click on image for a larger view.)
Saturday:
Cloudy early morning with darker, lower clouds by afternoon.
Very light precipitation is predicted by most models (a few hundredths of an inch of water), but it will fall as freezing rain and sleet for the immediate Philadelphia area.
The first period of precip expected late morning and early afternoon. Another period of light mostly freezing rain/sleet later afternoon and evening.
SREF Precipitation type 8 PM Saturday (Click on image for a larger view.)
Slippery conditions are ikely with ice accumulation Saturday evening and night.
High temperature Saturday 27.7º ± 2.2º Blue Bell.
Sunday:
Sunday starts with the same dark, low cloudiness, but the low level clouds will lift to mostly mid-level cloudiness by the afternoon. No sun expected. We should reach a high of 36º± 1.7º, which could be our high temperature for the week.
Forecast uncertainties: There’s a range of values for the quantity of precipitation falling (QPF) as a mix for Saturday into Saturday night. While most models have things on the very light side, freezing rain can be very problematic if large QPF values allow buildup on power lines and tree branches.
Additional light freezing rain Sunday night!
A significant storm is expected Monday night into Tuesday with more sleet, freezing rain, possibly changing to snow.
