Yesterday at Copper

Copper got hammered yesterday! That was expected, but they got a lot more snow than predicted. The National Weather Service predicted that Copper would receive 7.28″ of snow and Opensnow predicted 4-8″ (with an average of 6″, obviously). Other resorts got hammered as well: Breck got 10″, Keystone 9″, and Crested Butte 11″. But those resorts aren’t open (yet!), so it doesn’t really matter.

But Copper is open. They don’t have any runs open yet that are worth driving out there for, but it’s only a matter of time. So let’s look at what went wrong. Joel writes:

The over production of snow along I-70 is more mysterious. I think it has to do with what I talked about this morning, the cold air from yesterday trapped near the surface and forcing the moisture streaming in from the west to rise and create snow. Sometimes all the factors just come together perfectly.

Since I’ve never been an operational meteorologist, I’m not going to second guess anyone. However, it’s clear that something was wrong with the models. He goes on to say:

I really really really dislike being wrong about a forecast. That said, I’d much rather have realistic expectations, have most mountains meet those expectation, and have a few areas come in much higher that forecast. That’s much better than shooting too high and being disappointed.

Here’s where I can offer input. Here Joel is essentially saying that he tries to under-predict snowfall. That’s what it means if you are right on 90% of the time, but that other 10% you under-forecast. That’s a bias in the system. As scientists, we should be trying to eliminate all biases. So what we should be doing is trying to forecast the exact amount every single time. Then the over-forecasts will be canceled out by the under-forecasts.

Let’s look at how he actual does: Copper Mountain Statistics.

As of today, the statistics table is as follows:

Days Under
Avg Under
Days Over
Avg Over
Open Snow163.31″25371.84″

The table says that Opensnow predicted 78 non-null days (starting on January 1st last season, plus a few this season). A null forecast is where the forecast was for 0″ of snow and there was exactly 0″ of snow. Non-snow days are fairly easy to predict, so we toss those out so we don’t skew the results. Of those 78 forecasts that either predicted snow or there was actual snowfall, Opensnow only under-predicted 16 times, or 21% of the time. Conversely, they over-predicted 47% of the time!

The National Weather Service is equally split amonst over- and under-predictions.

Further, when Opensnow under-predicts, the amount that they are off by is a lot larger than when they over-predict. So when they under-predict, they are under-predict by a huge amount! If you look at the NWS numbers, their average under-prediction is larger than their average over-prediction, but they are much closer. The Opensnow numbers are skewed.

I have code that makes histograms of the forecast error. I might break that out tomorrow to see if we can put some better numbers out there.

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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