Wednesday, December 21, 2011

How Good, or Bad, Were the Blizzard Forecasts? Part 1.

One way meteorologists get better at what they do is by holding themselves accountable for their forecasts by validating them after the fact.

Here is the National Weather Service's total snowfall map as of midnight CST this morning.
click to enlarge

And, here is a detailed map of the High Plains where the heaviest snows fell.
The dark blue area in southeast Colorado is more than twenty inches.

This blog provided its first alert of a major winter storm in the Plains at 7:52pm Friday evening by linking to Mike Umschied's blog. I followed up with my own analysis of the potential storm at 9:18pm. It included this graphic for up to 11" that you can compare it to the NWS graphic (above) of actual snowfall.
This initial forecast, about 48 hours before the snow started falling in New Mexico, isn't too bad but is too far southeast. Because of the holiday travel period, I saw this posting mostly as a "heads up." My sense is it served its purpose.

The next update was 9:11am Saturday. Because of the uncertainty over the path of the storm, I presented probability maps so readers could gauge their risk as I thought it was too early to plot out an exact path.

I posted an update at 3:19pm Saturday, here is a reproduction of the most important part (click to enlarge):

I presented the northwesternmost and southeasternmost models and provided a list of roads (immediately above) with advice. This turned out to be exactly correct: All of the listed roads were closed.

Sunday evening, I posted the following:
As Interstate 70 was indeed closed between Salina and Colby (actually, WaKeeney to Colby), this was a very good forecast as was the forecast map. I was late catching on to the heaviest snow actually falling in southeast Colorado and the final map over forecast the amount of snow in parts of western Kansas by several inches.

The snow started in New Mexico shortly after the above "storm total" map was posted. So, from this point on, I was updating and nowcasting (short term forecasting) the storm. The above was the final map depicting the amount of snow. You can compare it to what actually fell. I believe they compare quite well but please form your own opinion.

In Part 2, I'll discuss the response to the forecasts.

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