Tag Archives: Snowstorm


The 1 AM model runs continue with the scenario that the heaviest precipitation is just getting started.  The NAM has an additional QPF  1.20 inches water from now through 7 PM this evening.  The GFS, has a somewhat less but similar QPF through 7 PM.

So the snow is just getting started; the storm is still near Virginia/Maryland right now —

Current RAP model showing position of low pressure
Current RAP model showing current position of low pressure at 8AM Wednesday

So the forecast remains on-track.  With heavier precipitation rates, temperatures should continue to drop to about 32 degrees.

[su_note note_color=”#ebf2d9″]Current forecast is 9-16 inches.  The NAM 10-1 snow map below is good estimate. [/su_note]

NAM Snow Predictions
NAM Snow Totals Prediction from the 06Z model run

Hey, I kept hearing on the radio yesterday that the we were going to get snow but the high temperature on Wednesday was going to be about 40!  Why do they say such things??

Who knows!

I’ll update with the latest model data after 9 AM today.



The tonight’s NAM model data is becoming available.  Here are the trends:

Higher QPF values- precipitation falling as water has increased significantly to 1.80 inches water, in some areas, over 2.0 inches water!

The predicted vertical thermal profile is much COLDER; indeed, critical “thickness levels” and temperatures now support snow into New Jersey where previously it was thought to be mostly rain or rain mix.

With these changes, there’s been a significant increase in predicted snow totals!  Here are the NAM 10:1 snow totals by area:

NAM Final predicted snow totals 10:1 ratio

There are other algorithms that try to take into account other factors such as melting.  One is called the Kuchera Snow Algorithm.

Kuchera algorithm
NAM Kuchera Algorithm Snow Accumulation

I don’t generally find the predefined snow algorithms that useful or accurate.  I believe the 10:1 simple algorithm will do best here and may understate snowfall at times. An average of the two might also prove correct for this storm.

The reason I’m presenting both predefined snow algorithms is to show that significant snowfall is now expected in Philadelphia, NJ as well as in PA.

[su_note note_color=”#ebf2d9″]So we’re looking at 9-17 inches of snow, based on your location, as shown on the above maps.[/su_note]

Timing:  Precipitation as rain is starting as scheduled (9PM) and will mix with and turn to snow by 1-2 AM in western sections and a bit later along the I-95 corridor.   Heaviest snow during the morning and afternoon hours on Wednesday, ending about 7PM or so.

Temperatures are expected to fall to about 32-33 degrees by 2-3 AM with dynamic cooling that will occur with heavier precipitation on Wednesday

Winds will be 15 to 20 mph with gusts near 25 during the day on Wednesday.

The GFS model data becomes available about 10:45 PM.  I expect the GFS QPF values to be less than the NAM values; it always is.  I still think we’re talking about a major snowstorm for Philadelphia and the immediate areas.  This is a high confidence forecast.

I’ll amend this post with the GFS data if it presents a problem with this forecast.

[su_note note_color=”#ebf2d9″]10:45 PM – Tonight’s GFS model data is similar to the NAM. QPF values 1.43  inches water with distribution of precipitation maximum centered around I95, similar to the NAM.  I believe the NAM 10:1 snowfall map above  is a good estimate. [/su_note]


At work…not much time, but a quick update for the snowstorm in Philly tomorrow.  The latest NAM has become available.  QPF values, somewhat less, still more than 1.09 inches water in PHL.  Band of heavier snow north and west from Valley Forge and Northern Montgomery county and north.

Temperatures at critical levels may allow some mixed precip in PHL during the morning, but a changeover back to all snow in the afternoon.  Timing still on track as earlier posts have mentioned. Still going with 12 inches in PHL, less along the adjacent counties SE of PHL, 18 + in western and northern suburbs.    (As was the case with the last storm, this is accumulation POTENTIAL.  March snowstorms have their own thermal issues from solar insolation effects, reducing totals.)

A busy day at work for me….won’t have time for elaborate posts or evalution of other models until this evening.   The all important data becomes available about 9:15 PM and the 3D temperature profiles about 9:45 PM.