Thursday, January 24, 2013

Rare Freezing Rain Event in Salt Lake City

Residents of Salt lake City woke up to some unusual weather conditions today. A light freezing rain that began before dawn was coating exposed surfaces with ice. The temperature when the precipitation started was only 20°F, and temperatures remained below freezing all day. There was only .09 inches of rain, but temperatures well below freezing it takes very little rain to create an icy mess.

Radar image from Salt Lake City at 5:00 MST January 24.

The glaze of ice turned roads, interstates, and parking lots into skating rinks. At 9:00 a.m. MST all runways of the Salt Lake City International Airport were closed to departures, and it was past mid afternoon before two runways could be opened. It was the first time in 24 years the airport was closed due to ice.Since 1940 there have been only nine other freezing rain events recorded at the airport. The 0.09 inches of freezing rain measured today was the greatest since December 31, 1983. There were dozens of weather-related accidents, and dispatchers reported more than 240 reports of other accidents and slide-offs. 

Freezing rain fell as far north as Boise, Idaho. The Idaho Transportation Department closed a 70-mile stretch of Interstate 84 between Boise and Glenns Ferry this morning, and schools closed because of the weather conditions.

Schematic of freezing rain formation.
Credit: University of Illinois Department of Atmospheric Sciences.
Freezing rain occurs when rain becomes supercooled and freezes on impact with objects on the surface. Typically snow falls though an elevated warm layer where the snow melts and becomes rain. The rain drops then fall through shallow layer of cold air near the surface, where they become supercooled. When the rain hits objects below freezing (the ground, trees, power lines, etc.) it immediately freezes.

The atmospheric sounding at Salt Lake City this morning exhibited this typical profile in fairly dramatic fashion.

Southwest flow ahead of a system approaching from the Pacific Northwest brought warm and moist air into the region at mid-levels while very cold air hugged the surface in the valleys. The cold air layer near the surface was only about 900 feet deep, while the warm layer immediately above was approximately 3700 feet deep. This set up the unusual freezing rain event in northern Utah this morning.

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