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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
300 PM EST Sat Nov 30 2019


30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2019

The updated temperature and precipitation outlooks for December 2019 are based
on the WPC temperature and precipitation forecasts during the first week of
December, the CPC 8-14 day and Weeks 3-4 temperature and precipitation
outlooks, and the latest monthly forecasts from the CFS model. Although
equatorial sea surface temperatures (SSTs) are above average across much of the
Pacific Ocean, anomalous convection and winds continue to reflect ENSO neutral
conditions. The Madden-Julian Oscillation (MJO) constructively interfered with
the ongoing positive Indian Ocean Dipole (IOD) during late November. Dynamical
model forecasts are in reasonably good agreement that the MJO resumes its
eastward propagation across the Indian Ocean during the next two weeks. This
predicted MJO evolution would favor above normal temperatures across the
central and eastern U.S. during December.

A rapid pattern change is likely to occur over the middle to high latitudes of
the Northern Hemisphere at the beginning of December. Positive 500-hPa height
anomalies at the higher latitudes are forecast to be replaced by negative
500-hPa height anomalies. The Arctic Oscillation (AO) index is forecast to
become strongly positive at the beginning of the month. A positive AO index
typically corresponds to above normal temperatures across the north-central
U.S.  Many GFS ensemble members indicate that the AO index trends back to
neutral or even a negative phase by mid-December which slightly lowers forecast
confidence in the temperature outlook across the north-central U.S. Despite the
good agreement in recent model guidance, probabilities for above normal
temperatures were not increased too much across the north-central U.S. given
the poor consistency among daily CFS model runs during the final ten days of
November.  Multiple early season winter storms resulted in widespread, heavy
snow from the Sierra Nevada Mountains east to the Colorado Rockies during late
November. Anomalous snow cover and depth for this time of year may are likely
to result in a cooling effect on surface temperatures across the Great Basin,
at least through early December. Therefore, the updated temperature outlook
calls for equal chances of below, near, or above normal temperatures across the
Great Basin. The highest probabilities (above 60 percent) for above normal
temperatures are forecast across the central and southern Great Plains where
maximum temperatures are forecast to average 5 to 10 degrees F above normal
during the first week of December with above normal temperatures likely
persisting into the latter half of the month.

The updated precipitation outlook expanded the favored area of above normal
precipitation to include the entire West Coast. The greater chances for above
normal precipitation throughout the West are mostly due to a wet start to the
month along with an amplified 500-hPa trough extending from the Aleutians
southeastward to near the West Coast during the second week of December. The
highest probabilities (above 70 percent) for above normal precipitation are
forecast from San Francisco to the Sierra Nevada Mountain range where 7-day
precipitation amounts are forecast to be near the upper monthly tercile. Heavy
precipitation (2 to 7 inches, liquid equivalent) is likely across northern and
central California during the first few days of the month as a low pressure
system interacts with tropical moisture originating from Hawaii.

The revised precipitation outlook also increased the coverage for above normal
precipitation to include areas from the Midwest eastward to parts of the
Northeast. This expanded coverage is consistent with the upper-level trough
upstream, a northward shift in the mid-latitude storm track, and likelihood of
above normal temperatures for much of the central and eastern U.S. Also, heavy
precipitation at the beginning of December supports elevated odds for above
normal precipitation from the northern mid-Atlantic north to southern New
England. Based on a dry first week of the month coupled with a consistent dry
signal among recent CFS model runs, elevated odds for below normal
precipitation are forecast for parts of central and southern Texas along with
much of Florida.

Little to no change was needed for the updated Alaska temperature outlook with
the highest confidence remaining that above normal temperatures prevail along
most coastal areas of mainland Alaska and the Aleutians. Based on excellent
model agreement that an amplified trough becomes established over the North
Pacific with enhanced onshore flow, above normal precipitation is most likely
across the Kenai Peninsula.

----------- Previous message (from November 21) is shown below ------------

The December 2019 temperature outlook is based on Week-2 model guidance, Weeks
3 to 4 outlooks from the CFS and ECMWF models, recent daily CFS model output,
consideration of influences from the Madden-Julian Oscillation (MJO), and
long-term trends . Near to above-average sea surface temperatures were observed
across the east-central tropical Pacific Ocean during October and early
November. However, suppressed convection continued near the Date Line. The
oceanic and atmospheric observations reflect ENSO-neutral conditions which are
likely to persist through December.

The Madden-Julian Oscillation (MJO) strengthened in late October and its
enhanced phase quickly propagated east across the Western Hemisphere during
early to mid-November. The MJO is likely to constructively interfere with the
ongoing positive Indian Ocean Dipole (IOD) through the remainder of November
into the beginning of December. It is unclear how this constructive
interference between the MJO and IOD and the resultant enhanced convection
across the western Indian Ocean will influence the mid-latitude circulation
pattern. Lagged MJO composites are consistent with Week-2 model solutions that
feature upper-level ridging south of the Aleutians and a downstream trough over
the western U.S. The ECMWF ensemble mean maintains this anomalous ridge/trough
pattern through Week-3 (early December) and then indicates a retrogression of
the longwave pattern by mid-December. This evolving longwave pattern favors a
relatively cold start to the month across the northern Rockies and northern
Great Plains before a moderating trend or transition to above-normal
temperatures occurs by mid-December. Downstream of the upper-level trough over
the West, anomalous subtropical ridging is expected to result in a relatively
warm start to the month across the Southeast and Gulf Coast States. The highest
confidence (albeit limited at a half-month lead) in the temperature outlook
across the CONUS exists for these areas due to the likely warm start to the
month and good model consensus.  Forecast confidence decreases north of the
40th parallel across the central and eastern U.S. since the GFS and ECMWF
ensemble means continue to feature anomalous 500-hPa ridging over the Davis
Strait and Greenland persisting into the first week of December. Given the
persistence of this ridging at high latitudes, forecast confidence in the
temperature outlook is low across the Great Lakes and New England.

The precipitation outlook for December is based primarily on recent daily runs
of the CFS model and the calibrated North American Multi-Model Ensemble (NMME).
The area with enhanced odds for above normal precipitation across the Pacific
Northwest, interior West, and central to northern high Plains is supported in
part by the likelihood of an amplified upper-level trough forecast over the
Rockies or near the West Coast during early December. The slight tilt in the
odds for below normal precipitation forecast for south Texas is consistent with
the seasonal precipitation outlook. A large area with equal chances for below,
near, or above normal precipitation throughout much of the central and eastern
U.S. is necessary due to weak signals among precipitation tools along with low
predictability inherent with a monthly outlook at this time lead.  Revisions to
the December temperature and precipitation outlooks are likely with the
November 30 release.

The amplified upper-level ridge (trough) over the North Pacific (western North
America) is forecast to become established by the end of November and likely
continue into early December. This longwave pattern favors at least a slight
tilt in the odds for below-normal temperatures across eastern mainland Alaska.
The highest odds (above 60 percent) for above-normal temperatures are forecast
for areas of the state adjacent to the Bering and Chukchi Seas where sea
surface temperature anomalies are averaging as high as 2.5 degrees C above
normal. The favored area of above normal precipitation across Alaska is
consistent with the likelihood of enhanced onshore flow during early December
and supported by many of the precipitation tools.

FORECASTER: Brad Pugh

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.

The next monthly outlook...for Jan ... will be issued on Thu Dec 19 2019

These outlooks are based on departures from the 1981-2010 base period.
$$

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