Making Sense of Weird Weather

La Nina may get the attention, but if forecasts of unusually wild weather this spring come true, lesser-known forces like "zonal jet streams" and "Bermuda highs" will be responsible.

UW-Madison weather expert Thomas Achtor said the lesser-known phenomena triggered this unseasonably mild winter, including February temperatures that were way above normal. And they may also bring strong storms this spring, Achtor says.

Achtor, a senior research program manager with the Space Science and Engineering Center, said La Nina years typically bring colder-than-normal temperatures to central and eastern North America. La Ninas are defined by a sharp cooling of water in the central equatorial Pacific -- the opposite of El Nino conditions the year before.

Indeed, some ferocious cold of minus-70 degrees or more has struck Alaska and Canada, but bitter cold never plunged southward. Achtor said the cold air was bottled up because the jet stream has been in a strong east-to-west holding pattern, forging straight east through the Pacific Ocean and North America with almost no fluctuation.

A stronger than normal "Bermuda high," or an anticyclone that funnels warm, moist air inland from the Atlantic Ocean, is also keeping things interesting, Achtor said. Combined with the zonal jet stream, this could pump unusually high amounts of moisture into already volatile conditions. "The strength of these two systems means we will likely see a very active storm season with more precipitation that normal," Achtor said.

Achtor and others at SSEC track global weather patterns through the Cooperative Institute for Meteorological Satellite Studies. Achtor can be reached at (608) 263-4206; or: [email protected].

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