Category Archives: Commentary


Today, with its morning model run, the National Center for Environmental Prediction (NCEP) will have upgraded the GFS model (version 14) to the new and improved FV3-GFS (GFS version 15.1.1). The FV3-GFS becomes the main global model of the National Weather Service today.

The FV3-GFS (Finite Volume Cubed Sphere) has been in development for well over 10 years and has been in testing for the past three. It uses a different three dimensional geometry to reduce errors inherent in all numeric weather models. It includes different modeling physics and new parameters.   Significant information on the FV3-GFS can be found here.  Background information is here.

The model data has been available on university web sites in a very limited fashion for awhile.  Over the past two weeks, the NCEP made the data available on the main weather model download server, “NOMADS”.

The FV3-GFS has been heavily evaluated and is considered “equal or better” than the current GFS.


My post from last year about Daylight Saving Time and the forecasts you see on the 10 and 11 PM TV news is useful information to check out.

The short version is that the latest runs of the GFS and other global models are not completed nor available in time for the late night TV weather segments when we switch to Daylight Saving Time on the east coast.

Indeed, the first “products” of the GFS model first become available about 11:32 EDT.

With the exception of the NAM, the forecasts you see on the 10PM or 11PM (east coast)  news/weather can only be based on older model runs from the short range models and [possibly] the newest NAM.   Indeed, some of the short range models at those times are, in turn,  “initialized” from the earlier afternoon NAM model, not necessarily the latest!


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Wow, the clouds never broke today and temperatures remained in the 40s!  Yesterday’s NAM had predicted temperatures remaining in the 40s and I ignored it; I thought it was a modeling error because it was so far off.  So one model was an outlier, yet it turned out to be correct.

from earlier this morning:

The rain will move out later this morning. Some of the models are forecasting a break in the clouds after noontime for a few hours, so we may get some sun.  If not, certainly brighter skies.

I know you may have heard about a high of 61 today.  Only the GFS has been predicting this high.  Most of the models have a high in the low to mid 50s.  I guess we’ll find out.

A cold front will move through between 4 and 6 PM. Widely scattered showers and some cloudiness moves through at that time. Temperatures drop into the low 30s by daybreak Monday.

It will become very windy this afternoon and even more so this evening.

Contrary to an often-heard incorrect explanation on TV forecasts, high winds are NOT due to a “wind tunnel effect”.

High winds and wind gusts are principally due to 1. Rapid changes in atmospheric pressure, specifically, the rate of change of pressure over time.  (dp/dt).    2. Thermodynamic effects that cause a mixing down of normally high winds aloft down to the surface.

If you’re looking for “Wind tunnel effects”, they can be felt on the corner of Walnut and 15th streets in Center City on windy days.

Every time I hear a TV person attribute windy conditions to a wind tunnel effect, I wince. 


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The computer modeling of this storm had many aspects go wrong…

  • The original track, consistently a near coastal storm for many days, became an inland storm.
  • The precipitation start time in Philadelphia, shown by most models to be 3-5 PM, didn’t start until 6-8 PM.
  • Thermal profiles, due to dynamic cooling and modeling error, allowed more snow than predicted before the changeover.
  • Temperatures that were expected to rise rapidly into the 40s shortly after midnight didn’t reach 40 until 6 AM today.
  • High temperature today of 49-50 (GFS) was never reached.
  • Winds that were supposed to pick up during the night and this morning, didn’t happen (yet.)
  • Winds are so light now that fog has developed in many areas, also unpredicted by the models.
  • Temperatures, predicted to rapidly drop to freezing by 1-2 PM, are now predicted to be in the 40s until noon!

With this storm, it’s not that one model did better than another. The GFS, NAM and NBM (the NBM is a composite of many models, including the European, Canadian, GFS, NAM, their statistical analogs, and the HRRR) all got it wrong.

Flash freeze?   I don’t think so.  “Flash Freeze” warnings became its own TV weather reality show when it was realized that this was going to be just a rain storm.  Originally based on model runs from early Thursday and predicated on a originally predicted temperature drop from about 49 to 32 in just a few hours, current short range models now show an unremarkable drop in temperatures during the afternoon today.  But the Flash Freeze likelihood has been dispelled by the model runs for two days.

The models are still showing high winds (likely) and a cold snap for later this afternoon and tonight.

Current temperatures are running warmer than even this morning’s models. It’s 43 northwest of the city at 11 AM, but the NAM run from this morning predicted 36.

The NBM is looking the best right now- temperature drops to 32 about 3-4 PM and 5 degrees by Monday morning.