Prognostic Discussion for Monthly Outlook 
NWS Climate Prediction Center College Park MD 
830 AM EST Thu Nov 19 2020 
 
 
30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2020 
 
The monthly temperature and precipitation outlooks for December 2020 are based 
on statistical and dynamical model guidance, the latest Week 3-4 CFS and ECMWF 
forecasts, La Nina composites, recent observations, coastal sea surface 
temperatures (SSTs), and consideration of the current and predicted states of 
the Madden Julian Oscillation (MJO). 
 
The latest ENSO Diagnostic Discussion (issued on November 12th) indicates a La 
Nina advisory remains in effect. La Nina is likely to continue through the 
Northern Hemisphere winter 2020-21 (~95% during January through March) and into 
spring 2021 (~65% chance during March through May). The cold event strengthened 
during October 2020, with below average near-equatorial SSTs extending from 
near the Date Line eastward to near the South American coast. A moderate to 
strong La Nina is predicted in December, with enhanced trade winds over most of 
the tropical Pacific, enhanced convection over Indonesia, suppressed convection 
near the Date Line, and a deep reservoir of relatively cold subsurface water in 
the central and eastern near-equatorial Pacific. 
 
The MJO crossed the Western Hemisphere during early November, favoring the 
development and rapid intensification of Hurricanes Eta and Iota in the 
Atlantic basin. At present, the enhanced convective envelope of the MJO is 
located over the western Indian Ocean. Widely divergent model forecasts 
indicate much uncertainty in the evolution of the MJO during the next two 
weeks, but current thinking favors a slow eastward progression and gradual 
decay of the subseasonal signal. Typically, the MJO crossing the Indian Ocean 
results in an anomalous wavetrain that is associated downstream with above 
normal temperatures focused over the Great Lakes region 3-4 weeks afterward, 
and a slight tilt toward below normal temperatures over the western CONUS. 
However, it remains to be seen if the tropical convection can force such a 
response, given the uncertainty regarding the future state of the MJO. 
 
The temperature outlook for December 2020 favors above normal mean temperatures 
across most of the Lower 48 states, with the greatest probabilities (50%+) over 
eastern and southern sections of the CONUS. This is supported by the Week 3-4 
CFS and ECMWF forecasts for the first half of December, the C3S model suite 
(Copernicus) and nearly all the constituent models that comprise the NMME, 
including the CFS, for the month of December. Above normal mean temperatures 
over approximately the southern half of the CONUS are broadly supported by an 
ENSO regression technique, where historical temperature anomaly data for the 
time of year is regressed onto the standardized Nino 3.4 index. Above normal 
temperatures are also favored over northern and western sections of Alaska, 
based on dynamical guidance, long-term trends  and the proximity of relatively 
warm coastal SSTs. The greatest odds for anomalous warmth (50%+) are predicted 
for the northwest corner of the state. For the southeastern Mainland of Alaska 
and the adjacent Panhandle region, below normal temperatures are slightly 
favored, consistent with some of the dynamical model guidance and historical La 
Nina composites. Below normal temperatures are also favored, for similar 
reasons, across the Pacific Northwest and the Northern Rockies. Elsewhere, 
Equal Chances (EC) of below, near, and above normal mean temperatures are 
predicted. 
 
The precipitation outlook for December 2020 favors above normal precipitation 
amounts over western Alaska, portions of the northwestern CONUS, and from the 
Ohio Valley northeastward across northern New England. These anomalies are 
commonly observed during cold season La Ninas, and are also well supported by 
the C3S Copernicus suite, and some of the dynamical models  that go into the 
NMME. These precipitation anomalies appear to be underdone in the calibrated 
version of the NMME (the PAC), with only the Northern Rockies wet signal 
getting past the skill mask (which incorporates historical skill information). 
Below normal precipitation amounts are favored over approximately the southern 
one-third to two-thirds of the CONUS, which is predicted by many of the 
dynamical models, though with variations on the overall spatial pattern. Below 
normal precipitation is also consistent with cold season La Ninas across much 
of the South, and is likely to exacerbate drought in the Southwest, especially 
after the paltry monsoon this past summer. Probabilities of below normal 
precipitation exceed 50% for portions of the Southwest, Southern Rockies, 
Southern Great Plains, and into Louisiana. Near the southern Alaska coast, from 
south-central Alaska to the Northern Panhandle region, drier-than-normal 
conditions are possible, though the tilt towards below normal precipitation is 
weak. This signal occurs frequently enough to show up in historical La Nina 
composites and has some support from the C3S and NMME dynamical model suites. 
The Alaska precipitation outlook for December is also consistent with the idea 
of persistent high pressure over the Gulf of Alaska, with onshore flow (wetter) 
in western Alaska, and offshore flow (drier) in southeastern Alaska. Elsewhere, 
Equal Chances (EC) of below, near, and above normal precipitation amounts are 
forecast. 
 
FORECASTER: Anthony Artusa 
 
The climatic normals are based on conditions between 1981 and 2010, following 
the World Meteorological Organization convention of using the most recent 3 
complete decades as the climate reference period.  The probability anomalies 
for temperature and precipitation based on these new normals better represent 
shorter term climatic anomalies than the forecasts based on older normals. 
 
An updated monthly outlook... for Dec will be issued on Mon November 30 2020 
 
These outlooks are based on departures from the 1981-2010 base period. 
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