Review of last week’s storm

[Correction: I was accidentally pulling data from snow-forecast.com that was in centimeters and not inches. Needless to say, this means there forecasts were actually the value below divided by 2.54. Or way wrong. They predicted zero out of 4 days correctly. Thanks to Joel Gratz for finding this error!]

There’s no snow in the forecast, so let’s look at the previous storm to compare and contrast forecasts.

Let’s start with Copper, because they received the highest 24 hour snow total. On the 25th, 16″ of snow fell on Copper. That’s a lot. Here’s a table that snows 24 hour snow forecasts along with the actual amount that fell. The Opensnow column is a range forecast in the format {low,high}.

DateNWSOpensnowSnowforecastForecast.ioSnow-forecastAvgActual
2014-11-226.49{4,8}81.8633.1*67
2014-11-237.09{3,6}145.2691.6*7.55
2014-11-245.28{4,8}106.5032.4*75
2014-11-255.9{2,6}65.585.1*616
Totals24.76{13,28}3819.21512.2*26.533

[* Snow-forecast values have been modified since originally published to their correct values. They were originally reported in centimeters. They are now in inches.]

Opensnow average forecast was 20.5″. As you can see, both the NWS and Opensnow under-predicted this storm by a large margin. But other sources did quite well. Snowforecast.com over-predicted snow amounts. Snow-forecast.com was the closest to actual and only missed the forecast by 2″ over 4 days. That’s pretty good. If we look at individual days, we can see that the most they were off was on the last day when Copper got 16″ of snow. They predicted 13″, which was the only forecast that day over 6″. So Snow-forecast was by far the winner with this storm. They accurately predicted 4 out of 4 days. [See correction at top of page!]

The NWS comes in with the second best overall forecast. They got 3 out of 4 forecasts: the first three days. They busted big on the last day, just like everyone else except Snow-forecast. My arbitrary cutoff for a “powder day” is 6″. They were the closest to predicting a powder day.

Coming in third is Opensnow who got 3 out of 4 forecasts as well. But on the critical fourth day, they predicted only 2-6″ of snow. So they busted this forecast.

Snowforecast.com actually didn’t do that well even though they predicted pretty close to the storm total. On two days, they predicted powder days when the actual snowfall was only 5″. And then on the actual powder day, they predicted only 6″. So they busted on 3 out of the 4 forecasts, but ended up getting lucky for the storm totals. Because they busted on three out of four days, Snowforecast.com comes in fourth for this storm.

I’m not sure why I track Forecast.io as their forecasts are always the worst.

I’m hoping that they help in creating a “best guess average”. That’s the “avg” column immediately to the left of the actual amount. Basically that value is a weighted average of all the other forecasts after accounting for any bias in the error distribution. For total amount, the “avg” came in second. It’s individual forecasts were a little off: 2-2.5″ on the second and third day. And on the fourth day, it barely predicted a powder day. I’m a little disappointed that it didn’t do better, but it’s just a simple algorithm, so I guess I can’t expect too much.

My name is Nathan Johnson and I have a Masters degree in Meteorology. I also snowboard and live in Boulder, Colorado.I have a strong desire to see precise, accurate snow forecasts. It is my hope that independent validation and verification leads to better forecasts.

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