super bee cam

Copper Mountain Super Bee cam on November 3, 2014.

Yesterday saw a lot of snow for many of the resorts. Unfortunately, it wasn’t well forecast. Another bummer is that snocountry only reports data for open resorts. That mean we only have measured totals from Arapahoe basin, Copper Mountain, Keystone, and Loveland.

Let’s start with the good news. Both the NWS and Opensnow predicted there would be 5″ of snow at Abasin. 6″ were recorded. That’s about as good as can be expected given certain limitations. For instance, the NWS forecast is a point forecast for a certain grid box in their model. The model doesn’t know were the recording station is, and it probably doesn’t care.

Opensnow predicts using a range for the amount of snowfall. Since the NWS uses a single value (and other sources too) , I need to convert their range into a most likely value. I use the mean. Mathematically, this doesn’t make sense. There are a lot more low snowfall days than high snowfall days, so assuming a normal distribution (which is implicitly what I do when I take the mean) is wrong. However, I think it’s good enough for our purposes here. So fore a forecast of 4-6″ of snow, I use a value of 5″ as a best guess.

So both the NWS and Opensnow accurately predicted the amount of snow at Abasin. Let’s move a little ways away to Loveland. NWS predicted 6.1″ while Opensnow predicted 5″. The amount recorded at snocountry was 8″. Obviously NWS did a little better than Opensnow. by 1.1″.

When we head over to Keystone, things get much worse. Both NWS and Opensnow busted the forecast. NWS predicted 1″ and Opensnow 5″. They recorded 9″!

Copper saw NWS predict 2″ and Opensnow 3″ with a snow total of 8″.

I’m in the process of developing a Bust Index. Basically, I want an index where we can compare forecasts from different forecasters at the same resort as well as compare forecasts at different resorts. At the moment it’s still very crude and I don’t want to use it publicly yet, but it does seem to bring something to the

table. Let’s look at some specific numbers for NWS forecasts at Loveland over the last 3 days.

DatePredictedActualBust IndexBust or Not
2014-11-126.180.31Not busted

Looking at this table, it’s clear that the higher the Bust Index, the larger the difference between the forecast and the actual snowfall. A snowfall of 3.82″ would be good skiing or boarding, but wouldn’t be a great powder day. But 7″ of snow would likely be a good powder day (depending on what time the snow actually fell). Missing a 4″ day is vastly different than missing an 8″ day. The Bust Index on the 11th is too high though. If the weather prediction is 1″ of snow and it snows 4″, so what? A 4″ day isn’t going to be spectacular unless there have been 4 or 5 of them in a row.

I’d like to hear your thoughts. What makes a busted snow forecast?

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.

Posted in snowstats

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