Forecasters are underestimating the Philly snowfall

Forecasters are underestimating the Philly snowfall for the upcoming storm.  The latest GFS data just rolled in.   The latest GFS has a QPF of 2.01 inches water, remarkably close to the NAM data.    This translates easily into 25-30 inches of snow, using the built in GFS snow algorithm.  (In fact, the built in GFS algorithm has  32-36 inches.)

I’ve been saying we would get 20-30 inches of snow for two days now,  based heavily on a consistent NAM model.   With this morning’s GFS model  output being in the same ballpark, the likelihood of getting closer to 30 inches in Philly is looking very reasonable.    And if temps stay low, it could be more than 30 inches.

(The last time I listened to KYW radio at 11:30 AM, they were still going with 12-16 inches. ).

Updates later….


Philly Blizzard Update- 9:30 AM Fri

Latest NAM model has rolled out.  Extraordinary amounts of QPF ( 2.34  inches water)  falling as snow.  The NAM has been remarkably consistent with these high amounts and I’m thinking that at least 24 inches of snow and likely 25-30 inches will be final amounts.  

I know I’m well above the official forecast snow totals, but as I said on my earlier post today, the NWS tends to ignore the NAM model for snow amounts, preferring the various global models like the GFS and the ECMWF.    In the past decade, when they’ve underestimated snow totals, it’s seems like they have always  ignored the NAM model.  (Similarly, last year’s fizzled “blizzard”, where they were predicting a monster storm this same week in January,  was correctly modeled by the NAM and incorrectly by the GFS and European models. )

So despite the tendency for the NAM to run a bit high in QPF, I’m sticking with the extraordinary QPF of the NAM for this storm.

Other changes suggest a little heavier and early start for the snow tonight.  So while things were looking closer to a 10 PM start, the HRRR (based on the NAM model) shows snow starting in Philadephia as early as 7-8 PM.

Sustained winds, around noon tomorrow, may be as high as 32 mph with higher gusts.  We likely will be close or reach the true definition of blizzard conditions.

Philly Storm Update 7AM

Last night’s 1 AM models continue with the major snow storm  for Philly. Both models have  snow  starting earlier tonight (Fri), with snow starting as early as 7-8 PM, with  a few inches by midnight.

As is often the case, there are some differences between the NAM and the GFS models, with the NAM QPF  up to 2 inches of water and the GFS a bit lower than its previous run at 1.40 inches water.

In past storms, the average QPF between the NAM and GFS has worked.   That would still bring us into the 20-25 inches range for snow.

People ask me two things on the blog:  why don’t I quote the ECMWF more?  ….and why do I talk about the NAM model when no else does for winter storms?

Regarding the ECMWF (European Model), it is subscription only.   We’re talking big bucks for their data. They only provide certain data for free and QPF isn’t one of them.    If you read the NWS forecast discussions, the ECMWF hasn’t been all that great this season, so I’m not missing much.  (It was the ECMWF that gave us that “fizzled blizzard” last winter.)

As far the NAM, for some reason, maybe political, the mesoscale modeling branch of the NWS, which runs the NAM model,  isn’t taken  too seriously for winter season storms by the global forecast modeling branch of the NWS.

That said, after years of doing these forecasts,  I find the NAM can be remarkably good about rain/snow lines and even larger-than-predicted snowfalls.    Where the NAM goes wrong is overestimating  the QPF several days in advance.  But we’re getting into the timeframe where the NAM data is very useful.

Storm Update -Thurs 6 PM

Latest NAM and GFS data from the 1PM (18z) afternoon model run.  Both models now colder; little chance of changeover to sleet in PHL.  Both have the snow starting late evening on Friday in PHL and earlier south and west of the city.

The NAM cranks out 1.55 inches water (QPF) and the GFS cranks out 1.98 inches of water (QPF).    The snow algorithms suggest 20-30 inches of snow possible by late Saturday night.

The next model run is at 7PM (00z) and the data starts rolling out in raw form about 9:30 PM for the NAM and 10:45 PM GFS.

As we get closer to the actual ‘event’, confidence in the predicted QPF values increases.   But with both models having such high QPF values and similar thermal profiles,  the chance of a major snow storm is looking like a sure bet.

10 pm update:  tonight’s NAM has a QPF of over 1.80 inches water.  What’s new- starts between  7-10pm. Heaviest snow and highest winds Saturday morning. A lull late Saturday afternoon for the “dry slot” , then several more inches wrap-around Saturday night.  Snow totals 20-30 inches for PHL  by Sunday morning.

11pm update. Tonight’s GFS shows a 1.61 inches water. No dry slot and ending Saturday night. Taking average of NAM and GFS 20-24 inches snow total.

Weather… and other things "up in the air"