After the past two Decembers, where temperatures over most the western US were much warmer than average, December 2016 brought generally cool to cold conditions. This was especially evident across the PNW and into the northern Rockies and Great Plains, where up to now, the winter had been very warm (Figure 1). The temperature pattern over the west for December 2016 shows a largely near normal to warmer than normal central to southern California across the southern Great Basin and into the desert SW where temperatures were 0.5-4.0°F above average (Figure 1). Across the PNW and northern Rockies temperatures ranged from 2-6°F below normal. After a November that was wet along the coast and into the desert SW, but dry elsewhere, December flipped with a drier than normal coastal zone and wetter in the Great Basin (Figure 1). Nationwide the southern tier of the US and up along the eastern seaboard was near normal to substantially warmer than normal, while portions of the Midwest and Great Plains were generally cooler than normal (not shown). The main difference between November and December was a shift of the coldest polar air masses from Siberia across into northern Canada allowing cold air to fill down over the PNW and northern states during December. Precipitation amounts nationwide were mixed with the Great Basin, northern Plains, south Texas and the southeast seeing mostly greater than normal amounts. In addition to the west coast, drier than normal regions were seen in the southern Plains and into the northern Ohio River valley and across central Florida (not shown).

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