NOAA December Outlook: Warmer and Drier Than Normal For Large Areas of the Country

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December temperature outlook. Credit: NOAA

The NOAA recently released its outlook for December. Not looking too good for snow lovers, except maybe the PNW.

Full discussion below:

The highest forecast confidence in the December temperature outlook exists across the Southwest and southern Great Plains where above temperatures are most likely, based on good agreement among inputs to the North American Multi-Model Ensemble (NMME) along with support from La Niña composites and decadal trends . Probabilities for above normal temperatures decrease farther to the north across the Great Basin and to the east along the Gulf Coast due to weaker signals among dynamical models. During late November, a large positive 500-hPa height anomaly center is likely to become centered over Greenland and the northern Atlantic as the North Atlantic Oscillation (NAO) becomes negative. The ECMWF ensemble mean depicts this anomalous ridging persisting into the beginning of December and a majority of GFS ensemble members feature a continuation of a negative NAO heading into December.  The week 3-4 model consensus for the 500-hPa height pattern, using the GEFS, CFS, ECMWF, Canadian, and JMA models initialized on Nov 15, depicts this negative phase of the NAO ending sometime during early December. Due to uncertainty on when the anomalous ridging downstream over the north Atlantic weakens and the associated negative NAO ends, a large area of equal chances (EC) of below, near, or above normal temperatures is necessary for the Ohio Valley, Great Lakes, Northeast, and Mid-Atlantic. Also, inputs to the NMME vary largely for these areas. The EC forecast for the northern Great Plains, northern Rockies, and Pacific Northwest is related to the lack of a signal in the calibrated NMME and conflicting signals between model output and wintertime La Niña composites.

Dynamical models and La Niña composites favor below (above) normal precipitation across the southern tier of the CONUS (Pacific Northwest and northern Rockies). The slightly elevated probabilities for above normal precipitation forecast for the Great Lakes is consistent with La Niña composites but also given the above normal Great Lakes temperatures, any cold air advection would likely lead to enhanced lake-effect precipitation. Although there is some support among dynamical models for a larger area of elevated probabilities of above normal precipitation across the north-central CONUS, large uncertainty at this time lead results in an EC forecast. The northern Great Plains and upper Mississippi Valley will be reevaluated for the updated monthly outlook (to be issued on Nov 30) when storm track predictability improves. Due to anomalous ridging predicted by the ECMWF and Canadian ensemble means heading into the beginning of December, the favored area of wetness for the Pacific Northwest is slightly reduced in coverage compared to what La Niña composites would typically support. This region, along with northern California, will be reassessed on Nov 30 when the longwave pattern over the northeast Pacific and western North America is better known.

Negative sea surface temperature anomalies and the calibrated NMME support increased probabilities of below normal temperatures across southern Mainland Alaska and the Aleutians. The favored area of dryness forecast for south-central Alaska is consistent with the predicted colder-than-normal temperatures. Elsewhere, across Alaska, precipitation tools feature little to no signal.

December precipitation outlook. Credit: NOAA

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