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Weather Whys

Freezing Rain In The US West

Question: Ice storms make headline news when they hit the eastern United States. Do they occur in the West as well?

Answer:

First, let me define some terms.

Freezing rain is rain that falls as liquid drops but freezes upon impact with horizontal or vertical surfaces. Freezing rain is characterized as either glaze or rime depending on the nature of the ice.

Ice storms are extended freezing rain events, lasting several hours to days at some location, with heavy ice accumulations. The US National Weather Service considers an ice storm with greater than 0.25 inches (0.63 cm) of ice accumulation a significant episode that would trigger an ice storm warning.

Short periods of freezing rain fall are often observed somewhere over the eastern United States during the passage of a winter storm system, but for the most part, such falls are light and brief and cause minor disruptions and damage, if any. Usually, the ice melts when warmer air invades the area with rain, or it is buried when colder air changes the precipitation to snow.

The conditions that produce such freezing rain and ice accumulations are most frequently found in the United States east of the Rockies and north of the Gulf Coast between September and April. About 99 percent of freezing rain observations are considered to be light falls of precipitation and most events last only a couple hours.

A recent study of freezing rain climatology across the US by Chris C. Robbins and John V. Cortinas Jr of the NOAA/ERL National Severe Storms Laboratory (A Climatology of Freezing Rain in the Contiguous United States: Preliminary Results, 1996) compiled freezing rain observation from US observing site from 1982-1990. The map below summarizes their findings for frequency of freezing rain observations.

Freezing Rain Events-US

Freezing Rain Events in the United States 1982-1990 (after Robbins and Cortinas, 1996)

Note the high frequency in the Great Lakes and Northeast Regions and the Mid-Atlantic States. This is the region for the classical ice storm events that I described in the article Ice Storms: Hazardous Beauty and characterized by the January 1998 Ice Storm.

Note too, the second maximum in the Northwest centered over the eastern Oregon/Washington and Idaho region. Robbins and Cortinas ascribe this peak to freezing rain events where storm systems entering the Pacific Northwest generate precipitation that falls as rain into cold air trapped in the inter-mountain valleys to produce freezing rain at the surface. (Similar conditions may account for some freezing rain events in the Eastern US mountain ranges as well.) At higher elevations, freezing rain may also occur when the rain falling from warm air aloft enters cold air lying near mountain summits. Such temperature stratifications are different in formation than those associated with the frontal zones of classical cyclonic storms.

Though severe ice storms are infrequent events at any given location, the impact of freezing rain across the United States is significant. According to statistics compiled by the National Weather Service from 1990 through 1998, ice storms were responsible for 69 fatalities, nearly 3000 injuries, and roughly $2 billion in damages.

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