Category Archives: Winter Weather


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Tuesday Forecast

Update Mon 02/27 @ 8:15 PM — The heavier rain will be ending before daybreak. Tuesday morning will have very low clouds, areas of fog and some widely scattered sprinkles as a weak trough remains over central PA and Maryland.

The low clouds gradually lift during the afternoon, but it may be mid (to late) afternoon before things break up for some sun.

Mon 8:15PM Forecast Review — It’s interesting that the weather outside is unfolding closer to yesterday’s models than this morning’s models.

Examples : The rain started around 4-5 PM as forecast yesterday but today’s models had forecast a later start time.

Example: The transition to sleet and snow is further south, almost as predicted by yesterday’s models, but today’s models had all rain in the city and just outside.

If we backtrack to yesterday’s forecast, it will changeover to rain by later this evening.

Updated Monday Forecast

Update Mon 02/27 @ 9:11 AM — Current models have decreased the extent and duration of the sleet this evening.

Clouds increase by 1 PM and lower and thicken during the afternoon.

Rain starts between 6 and 7 PM in the city, but areas near Yardley/Trenton, it may start as light snow.

It’s looking like it stays rain in the city and immediate suburbs, but northwest, it transitions to a mix of sleet and rain around 9 PM. Here’s where the mix occurs according the the HRDPS

Canadian HRDPS precip TYPE forecast for 9 PM tonight. Much less sleet than forecast last night. Click on image for a larger view.)

It then moves back to all rain except areas near Quakertown and Allentown and north where a mix lingers longer before changing to all rain after midnight. Temps remain above freezing here.

Update Mon 02/27 @ 10:20 AM It looks like much of Tuesday through early afternoon will be very cloudy with sprinkles in the morning.

Previously Posted Sun 9:10 PM —

Monday Evening Sleet Rain mix

The rapidly changing weather pattern will produce yet another change Monday evening.

Low pressure will bring clouds Monday afternoon, then precipitation Monday late afternoon, starting as early as 4 PM but possibly as late as 6 PM.

Due to the initial rain falling through very dry air, the onset of the rain is expected to result in evaporative cooling.

This evaporation cooling will drop the air temperature to the “wet bulb temperature”, which will be below freezing in such a dry column of air.

So precipitation may start as rain, but this dynamic evaporative cooling process will cause the precipitation to change to sleet as the rain falls and freezes falling through this colder lower layer of air.

Tonight’s 00z NBM model Precipitation type at 9 PM (Click on image for a larger view.)

By 11 PM to midnight, Philadelphia and the immediate surrounding areas should change back to all rain.

Here’s the NBM by 3 AM —

NBM forecast for 3 AM. Tuesday. Only areas north of Allentown will have snow, but that will also mix with rain alter. (Click on image for a larger view.)

Some accumulation of sleet possible in the immediate PHL area during the evening but it should all melt with the considerable additional rain expected, so no accumulation around here by morning.

These dynamic temperature situations sometimes causes surprises, but there appears to be good model agreement with the current forecast.


Update Sat @ 11:47 AM — The snow should be ending in the immediate PHL area by 1 PM, later east in NJ.

Here are estimated snow totals based on MRMS data (11:1 snow water ratio) as of 11 AM—

MRMS derived estimated snow totals 11:1 snow:water ratio as of 11 AM (Click on image for a larger view.)

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Update Fri @ 11:02 AM — The latest GFS just became available. As is almost always the case, its totals are somewhat less than the NAM—

GFS 12z snow totals White contours are 1 inch increments. (Click on image for a larger view.)

I’m providing this morning’s NAM forecast again for easy comparison—

NAM 12z (Click on image for a larger view.)

My rule of thumb is to average the GFS and NAM for a best forecast.

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Things seem to be coming together, based on the latest Water Vapor image and superimposed NAM/GFS potential vorticity (see explanation below)—

  • Last night's (Thurs) Water Vapor with Potential Vorticity

Update Thu @ 11:16 PM — Quick update. Latest GFS has increased snow totals almost as much as NAM.

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Previously Posted Thu 5:32 PM —

The storm late Friday through Saturday time frame has entered into the range of the high resolution models.

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With the high resolution models coming into the mix, here’s the forecast:


An upper air disturbance and cold front will move through. We’ll have light snow break out northwest of the city as early as late morning Friday. Light snow/flurries on and off, mostly north and west of the city. Accumulation a dusting to 1/2 inch. High 34º sd 1.7º NBM model Blue Bell.

Snow starts Friday evening.

Most of our snow with this storm will occur Friday night into early Saturday morning.


Cloudy, cold and WINDY. Snow tapering and ending in the morning but lingering at the NJ shore. High 23.2 sd 2.9º NBM model Blue Bell


Sunny and cold. Windy. High 26.7 sd 2.2º

Earlier Storm Coverage:

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Mon 08:32 PM Forecast Review —We had more snow than forecast.  The models (with the exception of the Canadian GEM model) underestimated the QPF and didn’t fully take into account the cold upper level atmospheric temperatures, which resulted in more snow and less rain. The NAM erroneously hastened the transition to rain, despite having the QPF somewhat in-range.   

The Canadian “Regional GEM” model is one model I always look at, but it it isn’t necessarily a model that is more often accurate.  I’ve seen it over-estimate snow totals many times.   Last night’s GEM had these higher snow totals, but as I said, “Not sure what to make of it” 

Here are the NWS official snow totals:Here’s the graphic posted yesterday afternoon from the GEM model—

CMC GEM Model Snow totals 


Here’s the GEM model that I looked at 11 PM last night but I didn’t know what to do with—

Last night’s Canadian GEM snow totals.  (The GEM shows snowfall as snowfall rate in Kg2/m2)  These numbers are correct.  The 4-5″ mentioned in last night’s  “update” was a quick, mental approximate conversion to inches of snow.)





Update— Tonight’s Canadian model continues to forecast 4-5 inches of snow in central parts of Montgomery county.  Not sure what to make of it. 

Tonight’s latest models (HRRR, RAP, NAM)  shows the following changes— snow changeover a bit delayed northwest of the city, although snow totals are pretty much the same.  (It might be snowing an hour longer, but it will be increasingly difficult for accumulating snow.)

Dark, paved roadway surfaces will likely see less accumulation than grassy surfaces.

Snow starts about 9-11 AM from west to east and changes to rain from southeast to northwest by 2 PM.    Precip ends 3-4 PM.

The differences with this storm and recent previous storms  were outlined in an earlier post:

  •  The models have been consistent with total water falling (QPF) will only be 0.3-0.4″ water equivalent or less in the immediate PHL area, in  contrast to much greater amounts in previous storms.
  • The thermal structure of this storm is vastly different.   This storm will have southerly winds at the surface and surface temperatures will be at or above freezing for most of the area once things get going.   (Previous storms had northeasterly or easterly winds at the surface.)
  • In contrast, upper air temperatures will be colder northwest of the city, supporting snow formation but the surface temperatures will go against accumulation.
  • We’re into the final part of February.  The solar angle is higher and “insolation” effects through clouds are more likely to interfere with and cause melting on dark asphalt road surfaces when the precip occurs during the daytime hours.

Things can be summarized with this graphic slide show:

  • RAP model 10 AM Precipitation moves in
  • NAM model 1 PM Changeover northwest of the city. (note 32º surface temp white line)
  • RAP model Snow totals 4 PM


RAP model snowfall on grassy surfaces.  (Click on image for a larger view.)