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.

So we’re looking at 9-17 inches of snow, based on your location, as shown on the above maps.

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.

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. 


I tore myself away at work to look at the 1PM NAM model data that just became available.  QPF values are back to 1.30 inches and higher north and west.  Thermal profiles are COLDER!   It looks like mostly snow even in Philadelphia!  It does not look like rain will mix in after it starts.

Sorry for the vacillation on forecast amounts but I think we’re back to 8-10 inches in Philadelphia and 10-18 north and west of the city!

Weather forecasting is a tough business; glad I do other things when I’m not forecasting.  🙂

I’ll update with a final forecast about 9:45 PM or so.


I’ve had some unexpected free time this morning and was able to look at the GFS and NAM models.

QPF values for PHL have reduced to 0.85 GFS and 1.00 NAM.  Higher QPF amounts west and north of Philadelphia.   Thermal profiles are a bit warm around daybreak around the city, allowing a mix in with rain, reducing snow totals in the city and south/east.

Wet snow tonight may mix with rain around daybreak.  Snow mixed with rain in the city until late morning, when it all changes to snow.  In the city and areas near I-95,  mix with rain  will reduce accusations to about 4-6 inches, mostly accumulating late morning and in the afternoon.

Just outside of the city where it should stay as snow (Valley Forge, Main Line and north and west)  10-16 inches.

The real data you can hang your hat on will be available about 9:45 PM. I’ll update a little afterwards


I took a quick look at the thermal profiles from this morning’s model run…   A bit warm in the atmosphere around daybreak, (rain mixes in) before the storm begins to deepen.   This will reduce accumulations in Philadelphia area by several inches.   8 inches looking like a good guess, still the higher amounts, 12+  north and west of the city.

We’ll need to wait until this evening for the real final totals- about 9:45 PM