Freezing Rain Climatology
One of several winter weather-related hazards we experience in the Midwest is freezing rain. The accumulation of ice results in extremely slick roads and walkways, causes trees branches to snap, and coats power lines. The latter two impacts can combine to cause power outages, especially if an ice storm is associated with strong winds.
from Cortinas et al, Weather and Forecasting (WF), 4/2004
The Midwest is one of four areas in the United States where ice storms are common; the Pacific Northwest, the Northeast, and the Mid-Atlantic states are the other three. The image above depicts these areas based on the number of hourly freezing rain observations in the period from 1976 to 1990.
Freezing rain can occur anytime between November and April, with the average number of hours with freezing rain per month peaking in January and February (see graph above). Note that Burlington averages the most hours with freezing rain followed by Moline then Dubuque. This reflects our temperature climatology where Dubuque averages the coldest and thus has more snow than Moline and Burlington rather than freezing rain.
The map above depicts the average number of days per year with freezing rain. Most locations average around 4 days per year, but this can vary from 0 days with freezing rain in some years to around 8-10 days on the higher side. The highest number of days of freezing rain found during the period of study (1948 to ~2000, depending on the location) was 9 at Dubuque in 2000, 14 at Moline in 1949 and 15 at Burlington in 1951.
The National Weather Service issues a Winter Weather Advisory when Freezing Rain or Freezing Drizzle is expected to occur and possibly cover surfaces resulting in hazardous travel conditions. When the accumulation of ice is expected to reach 1/4 inch or more, an Ice Storm Warning is issued.