This movie illustrates a few cold air outbreaks (CAOs) that hit the continental U.S. during a sample month (December), as simulated by the NCAR Community Climate System Model version 3 (CCSM3).
The first cold surge strikes the western U.S. during days 8-10. Steered by the northeasterly wind flow aloft and the very high air pressure at the surface, it produces a mass of sub-zero air that pushes down to the Southern Plains. The second CAO struck the central U.S. in days 15-17, as a –40°F air mass in northern Canada migrated southward. This Arctic air mass was associated with extremely high air pressure (1060 hPa) and was driven equator-ward by northerly winds at the surface and aloft. The third CAO formed from –50°F air that developed over northern Canada around day 22 and clipped the northeastern U.S. around the 26th-27th. This one was also steered by northerly winds, but it didn't have such an impressive high pressure cell.
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