MILD WEATHER PATTERN TO SWELL OVER MUCH OF LOWER 48 IN DECEMBER’S SECOND WEEK ~ The Washington Post

Temperatures could be 25 degrees above normal in the central U.S.

Temperature anomalies between Tuesday and Sunday across the Lower 48. (WeatherBell)
Temperature anomalies between Tuesday and Sunday across the Lower 48. (WeatherBell) 

By Matthew CappucciDec. 7, 2020 at 12:04 p.m. ESTAdd to list

It’s the second full week of meteorological winter, but temperatures more typical of mid-autumn or spring will spread across much of the Lower 48. The heat is slated to begin in the West, where records will topple amid critical fire-weather concerns, before expanding east and occupying most of the nation.

Highs some 15 to 20 degrees above average will pool along the West Coast before sloshing east, swelling over the Plains and being invigorated by high pressure dropping out of Canada on Tuesday. Thereafter, virtually the entire contiguous United States will be swallowed by springlike warmth.

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By late week, intermittent turrets of cold may oppose the balmy weather, but long-range forecasts favor near-to-above-normal temperatures for most of the country into the foreseeable future.

A toasty pattern

The evolving upper-air pattern over the Lower 48, as simulated by the American GFS model, favors atypical warmth. (WeatherBell)
The evolving upper-air pattern over the Lower 48, as simulated by the American GFS model, favors atypical warmth. (WeatherBell) 

Highs were already forecast to top record levels in California on Monday, contributing to serious fire-weather concerns unusually late in the year. San Francisco was expected to hit 70 degrees, which would set a record for the date. Sacramento should claim a record Dec. 7 high of 70, too, beating out its record of 67 degrees, established just last year. Average December temperatures are about two degrees higher in Sacramento nowadays than they were in the 1940s.

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The warmth will then blossom over the northern Intermountain West and Rockies, with strong high pressure building south out of Canada from Alberta and Saskatchewan. The toasty high-pressure system is so mild that columns of air beneath it have grown vertically by the size of 2½ football fields, since warm air expands. Weather balloons launched from Glasgow and Great Falls, Mont., as well as Bismarck, N.D., on Monday morning set atmospheric height records, with more likely in the days ahead.

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