The featured chart presents a time series of observed visibility and wind gust from the Des Moines Airport for five recent blizzard events. These events are when Polk County was under a Blizzard Warning. The definition of a blizzard is somewhat vague in that visibility should be frequently reduced to a quarter mile or less and wind gusts be frequently in excess of 35 mph for at least a three hour period. The highlighted sections in the chart denote the periods at quarter mile visibility and 35+ mph winds. For last week's blizzard (top chart), the visibility was only briefly at a quarter mile. Automated weather sensors do not do a good job reporting these low visibilities during snow and blowing snow conditions, so blizzards tend to be verified by people like police and snow plow drivers who are out in the storm and can attest to the frequent poor visibilities and strong winds over a three hour period.

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Tags:   blizzard