The featured chart is an attempt to explain why only a few inches of snow caused so much trouble for Des Moines on Thursday evening. The green line is the pavement temperature and blue line is the air temperature as observed by the Altoona RWIS site. The text abbreviations at the top are present weather reports from the Des Moines Airport sensor. (SN = snow, TSSN = thunder snow, BLSN = blowing snow, and the plus/minus indicate intensity with + being more intense than -) The colored blocks are the reported road conditions by the DOT for the location. The plot shows that prior to thundersnow report at 4 PM, temperatures were above freezing and the pavement temperatures were close to freezing for 30-45 minutes after the heavy snowfall started while air temperatures plummeted into the teens. So the situation was the initial intense snowfall melted, but soon after froze as air temperatures were way below freezing. The result was a layer of ice that formed on the roadways. The timing was terrible as well with this happening after 4 PM (the start of rush hour traffic). The plot does not show the intense wind speeds, which were creating ground blizzard conditions at the time.

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Tags:   rwis   2014