First Alert Weather In-Depth: Ice storm history on this date

Great Ice Storm of 1991

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ROCHESTER, N.Y. – Do you remember this storm? It happened 33 years ago on Monday. It was the great ice storm of March 1991 and it was devastating for the Rochester area.

Statistics on this particular storm system were amazing. First, we had moderate freezing rain that lasted for more than 17 hours. That freezing rain produced a glaze or an ice accumulation that was two inches thick. That huge amount of added weight damaged or destroyed at least 10,000 trees – just in the city. That does not include the additional tree damage in the outlying areas. Thousands of electric customers were without power for days and weeks. Unfortunately, in the most extreme cases, some were without power for almost three weeks.

The meteorological reasoning for an ice storm of this magnitude was due to a process called “overrunning”.  Warm, moist air rises over a wedge of cold air that is below freezing at the surface of the Earth. This ice formation is highly dependent on the precise thickness of the cold air. If this cold air is very shallow, this creates a setup for super cooled water droplets that only freeze on contact. Unfortunately, that contact is usually made on trees and powerlines. It is estimated that a powerline with a half inch of glaze can add 1000 pounds of weight to the line. 

This ice may have been pretty to look in the days following the storm, but it also meant days and weeks of cleanup. Statistically this was considered 100 year storm for Rochester. In other words, a storm of this magnitude could happen once every 100 years.