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Official 90-day Outlooks are issued once each month near mid-month at 8:30am Eastern Time. Please consult the schedule of 30 & 90-day outlooks for exact release dates.

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HOME> Outlook Maps>Seasonal Forecast Discussion
 
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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