Given this week’s record blizzard in the Midwest—so far it’s the third largest snowfall in Chicago on record–I was curious about whether there is bias in predicting the amount of snowfall. In other words, do forecasts tend to overpredict the amount of snowfall? And if so, do we catch on and underprepare for snowstorms?
I found a paper by Bruce Rose from the Weather Channel (with Joseph Koval and Eric Floehr) that answers this question. They report that for a small set of snowstorms from 2007-08, there is an inverse relationship between the intensity of a snowstorm and the reported level of over-prediction, meaning that lighter storms are more exaggerated while heavier storms are less exaggerated. While they could not confirm the findings for a larger data set from the Weather Channel’s database, they write that there is a “marked tendency to over‐predict light events, but this trend is reversed with moderate and heavy snowfalls.” This seems to be consistent with what I have observed in Virginia, where there is much ado about virtually no snowfall.
As for whether people over-prepare or under-prepare for storms, I’ll have to answer that another day.
February 2nd, 2011 at 3:49 pm
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February 3rd, 2011 at 10:21 am
The Chicago Tribune reports that meteorologists accurately predicted the amount of snowfall–they predicted about 20 inches for a storm that officially brought 20.2 inches.
February 5th, 2011 at 10:54 am
The caption on the photo made my morning. As far as predictive bias and over-/under-preparing, the former is a statistical issue, the latter a behavioral one. I suspect the behavioral issue might be tied more to the excitement level of newscasters, reporters and government officials than to the numerical predictions. We got approximately a foot in the latest dump (plus, admittedly, some high winds and low temps). Heading into the storm, the talking heads were acting as if this were going to be a “severe” storm; but a foot in one dump is far from uncommon here in Michigan.
A conjecture: weak economy => tight budgets => fewer plows/less salt => longer delays clearing roads => more hype ahead of the storm (to discourage people from trying to drive the next day) => overpreparation by public (at least until we get wise to the new trend).