Sunday 6PM Update:  This afternoon’s NAM and GFS model runs continue with a significant snowstorm for us starting after 1 AM Tuesday and continuing through at least late morning on Tuesday.  Total QPF is about 0.80 (NAM) and about 1.00 inch (GFS) for Philadelphia.   With a 12:1 ratio, we’re talking about 9-12 inches of snow as a current estimate.  It’s expected to be very windy as well.  Updates about 10:30 tonight.

---

Just a quick update:

The NAM has joined the GFS with a slightly more eastern track for the storm, putting us in the cold sector.   The storm looks like it will be a faster mover and not linger into Tuesday afternoon.

Current QPF values are about 0.75 inches water for the NAM and about 1.00 inches water for the GFS.  With a 12:1 ratio, we’re taking at least 8-10 inches for PHL, mostly occurring before daybreak and very early morning Tuesday.  Effects of the March sun through the clouds  won’t be a factor for much of the precipitation, but will allow for faster melting on roadways once the snow ends.    Temperatures remain below freezing.

The details really can’t be nailed down until Monday evening.  I’ll update later tonight. (The updates will be after 10:30 PM  and will only be for the NAM, with the GFS data coming out  an hour later.)

Friday 6 AM update:  Last night’s models bumped the QPF to about 0.35 inches water.  Snow accumulations more likely to be 2 inches around PHL, mostly on grassy surfaces, little remaining on roadways, mostly slush.

The latest NAM data has become available.  It’s QPF value has risen to 0.26 inches water for PHL.    Tonight’s GFS model data won’t be available until about 10:40 PM, so I’ll be basing this discussion on this afternoon’s GFS data.

There continues to be differences in the models-  The GFS starts the snow about 3 AM, while the NAM model doesn’t have the snow starting until about 7AM for the immediate PHL area.

The GFS has double the QPF of the NAM, or about 0.50 inches water.   That would make a big difference in the snowfall totals.

What’s interesting is that typically the NAM QPF is almost always higher than the GFS QPF.  But with this situation, it is reversed.    In past situations where the NAM QPF has been significantly lower than the GFS, the NAM has been correct.

Temperatures will be near or above freezing for most of the time. Ground temperatures are very warm.   Solar insolation through clouds in March has a significant negative effect on accumulations. As mentioned, predicting accumulations for daytime snows with low QPF values can be just an academic exercise in March.

So what does all this mean?  I’m going with the NAM model, although the differences between the models makes the confidence in this forecast lower than usual.

Here’s my best shot:  Snow starts just before 7 AM in PHL and the immediate suburbs, (earlier start further north from PHL)  and accumulates about 1-2 inches on grassy surfaces.  It will be a  wet snow.  Little lingering accumulation on roadways, especially as the morning progresses, although there will be periods of moderate snow between 7 and 10 AM which may be slushy at times.   Snow tapers late morning with lighter snow showers into early afternoon.

Areas north of PHL will have more accumulation, possibly 3-4 inches, mostly on grassy surfaces.

It will become windy and cold during the afternoon and wet areas will freeze during the evening.

As I’ve been saying for several days, the final forecast can’t be made until Wednesday night’s data comes in.  And latest NAM model data has just become available.

In conjunction with my previous post this evening regarding warm surface temperatures as an impediment to high snow accumulation, the latest NAM data suggests that much of the QPF will occur before surface temperatures in the immediate PHL area drop to freezing.

The NAM has 0.59 inches water falling before 7AM, but much of that will be rain,  snow mixed with rain,  and finally snow about 7AM.    The NAM has reduced the QPF to only 0.20 inches water after 7AM.

So I’m going to go against the TV forecasts and predict only 3-4 inches of heavy wet snow for the immediate PHL area, ending about 10 AM.

I might be wrong about this forecast, but you’re reading this blog to get the real scoop without the hype.  While everyone gets all excited about a big storm, I simply can’t ignore the warm surface temperatures with this one.

10:30 PM Update:  Latest GFS supports the forecast of only 3, maybe 4, inches of heavy snow accumulation in the immediate Philadelphia  area.  The GFS has only 0.18 inches QPF falling after 7AM.  It’s looking like most of the precipitation occurs before the cold air moves in.

I don’t think the snow will be very heavy after daybreak.

Wednesday 6PM Update: The afternoon model runs (1PM) usually show a reduction in QPF and that’s the case this afternoon.  Latest estimate- the transition to snow occurs about 4 AM and snow continues heavy until 10 AM, then light snow tapers off after noon.  Latest estimate for precipitation is 6-8 inches of snow in PHL.

Next Update about 9:15 PM

Wednesday 1 PM Update: Latest GFS has become available and it has  a significant amount of precipitation falling as snow between 4 AM and 10 AM on Thursday. Total QPF is over 1 inch water which could translate into 8-12 inches in the immediate PHL area, more north and west.  That means that both models have reverted back to yesterday morning model’s rather impressive snow potential.   All the factors cited of warm surface temperatures make exact accumulations difficult to predict, but this storm will likely be one for a snow blower, not a shovel.

The GFS and NAM are remarkably similar, which puts high confidence in this forecast. More interesting is that the GFS has higher QPF values than the NAM, something that is usually the other way around.

Will wait for the next model runs this evening, after 9:15 PM.

Quick Update- Latest NAM from this morning just in.  It has reverted back to yesterday’s scenario.   Precipitation continues into the morning hours due to greater intensification and it’s showing about 4 inches during the daytime hours alone.  Total accumulation for the immediate PHL area looks to be over 6 inches!  More north and west.

An interesting storm with many factors coming into play.  Won’t have a final handle until this evening’s data which becomes available after 9 PM