Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
830 AM EST Thu Nov 21 2019


30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2019

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 probabilities (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.

An updated monthly outlook... for Dec will be issued on Sat November 30 2019

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