Where Freezing Rain Accumulations Are Most Common in the U.S.

Jonathan Belles
Published: January 15,2017
Freezing rain that results in the accumulation of ice on surfaces can be a major problem in certain areas of the country, while in other areas, it is nearly unheard of – even in places where cold weather is common.
Climatologically, there are several freezing rain corridors in the United States.
(MORE: Winter Storm Central)

The average number of days with freezing rain, based on 1948-2000 data. From Changnon and Karl, 2003.

Similar to severe weather season, the Gulf of Mexico can influence where freezing rain falls, providing much of the warm air and moisture needed for freezing rain to occur.
Cold air from Canada provides the necessary subfreezing air for ice accumulation, but if the cold air push is too strong and deep, precipitation will fall as sleet or snow.
Freezing rain occurs where the combination of these "Goldilocks" ingredients comes together.
It occurs least often in the Rockies, Southwest and near the Gulf Coast due to the lack of the right temperatures in the correct places. Generally, air in the entire depth of the atmosphere is too cold in the Rockies or too warm near the Gulf Coast to get freezing rain.
Freezing rain occurs when precipitation falls through a thick-enough warm (above freezing) layer in the mid-levels of the atmosphere, then drops through a shallow layer of below-freezing air near the ground, freezing on contact with any surfaces such as roads, cars, trees and power lines.

(MORE: Ice Storm Accumulations and How to Stay Safe)
This means that you need a good source of cold air near the surface to get rain to freeze when it hits the surface. There are a few ways this can happen.

1. Cold Air Damming

Since cold air is denser than warm air, it sinks. It takes some sort of forcing agent to lift cold air, or it will flow downhill.
Cold Air Damming (or CAD), sometimes referred to as "the wedge," occurs when cold air is pushed toward some sort of barrier that it cannot get over and has to go somewhere else. The best example of this is Appalachian CAD, which occurs when high pressure in the Northeast or mid-Atlantic states forces cold air down the east side of the Appalachian Mountains.
Typically, that colder air would flow westward on the southern side of a high-pressure system, but with the 5,000- to 6,500-foot peaks in the way, that cold air has nowhere to go. In this case, it plunges farther south into the Carolinas, Georgia and even sometimes Florida.

Setup for freezing rain and ice storms east of the Appalachian Mountains.

This becomes a source of cold air rather far south in the winter.
But the south is known for its warmth, right? That warm air lies just over the other side of the Appalachians, or a few thousand feet aloft. When it glides over that trapped cold air, you have the right setup for freezing rain.
Freezing rain in this setup will occur to the east of the Appalachians, the Green Mountains and White Mountains of New England and the Adirondacks and Catskills of New York as warm air and rain comes over the mountain tops and cold air is dammed up on the east side. Sometimes this warm air can come from storms off the Carolina coast as well.
Cold air damming can also occur in parts of Washington and Oregon in a very similar setup.

2. Large-Scale Movement of Warm Air Over Cold Air

Adapted from Rauber et al 2001.

Certain configurations of large-scale weather players are favorable for freezing rain. The placement of both high- and low-pressure systems are important because they dictate where air and moisture will go. One favorable configuration is shown above.
Favorable conditions are usually found on the northern or northeastern side of low-pressure systems where warm, moist air from the Gulf of Mexico is forced up and over cold surface air from Canada.
Sometimes, surface low pressure isn't even necessary. Simply a flow of moist, above-freezing air over fresh Arctic air is all that's needed.
(MORE: America's 10 Worst Ice Storms)

3. Cold Pools in Valleys

On calm, cool nights, freezing rain can form in valleys where freezing air settles. Any showers that pass over those pools of cold air will cause freezing rain in very small, isolated pockets as long as the clouds are comparatively warmer.
(MORE: Five Things to Know About Ice Storms)
Data for this freezing rain climatology was provided by the Midwestern Regional Climate Center, Rauber et al 2001, and Changnon and Karl, 2003.

MORE: January 18, 2015 Ice Storm