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Prognostic Discussion for Long-Lead Seasonal Outlooks
NWS Climate Prediction Center College Park MD
830 AM EST Thu Nov 19 2020
SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS
La Nina conditions are present across the equatorial Pacific Ocean, as
indicated by current oceanic and atmospheric observations. La Nina conditions
are likely to continue through the Northern Hemisphere winter and into Spring
The December-January-February (DJF) 2020-2021 temperature outlook favors
above-normal seasonal mean temperatures for a majority of the CONUS and for
northwestern areas of the Alaska mainland and Aleutian Islands. The greatest
probabilities (larger than 60 percent) are forecast for parts of the Southwest
and the Rio Grande Valley. Below-normal temperatures are most likely for
southeastern Alaska, parts of the Pacific Northwest, the far Northern Rockies,
and the Northern Plains.
The DJF 2020-2021 precipitation outlook depicts enhanced probabilities for
above-normal seasonal total precipitation amounts for northwestern areas of
Alaska, as well as across the northern tier of the contiguous U.S., including
parts of the Pacific Northwest, the Northern Rockies, the northern Great
Plains, the Ohio Valley, and the Great Lakes Region. Below-normal precipitation
is most likely for Central and Southern California, stretching eastward to
include the Southwest, the southern and central Great Plains, the Lower
Mississippi Valley, and the Southeast.
Equal-chances (EC) are forecast for areas where probabilities for each category
of seasonal mean temperatures or seasonal total precipitation amounts are
predicted to be similar to climatological probabilities.
BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS
Note: For Graphical Displays of the Forecast Tools Discussed Below See:
CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS
Tropical oceanic and atmospheric observations reflect ongoing La Nina
conditions. The latest monthly mean Sea Surface Temperatures (SSTs) through
November 11, 2020 continue to indicate a Pacific equatorial cold tongue with
negative SST anomalies on or near the equator stretching from west of the Date
Line to the coast of South America. Negative anomalies are as large as -2.0
degrees C in some areas. The latest weekly Nino3.4 index value is -1.0 degrees
C. Negative subsurface ocean temperature anomalies persist to depths of
approximately 100-200 meters from just west of the Date Line to 80W.
Suppressed convection continues over part of the western and central Pacific
while enhanced convection continues over parts of the Maritime Continent. Trade
Winds are enhanced over the past 30 days over parts of the western and
east-central Pacific Ocean.
PROGNOSTIC DISCUSSION OF SST FORECASTS
The CPC Nino3.4 SST consolidation forecast depicts median negative anomalies
increasing in magnitude to near -1.5 degrees C by DJF 2020-2021, followed by a
gradual warming to about -0.5 degrees C around MAM 2021. The North American
Multi-Model Ensemble (NMME) ensemble mean forecast for the Nino3.4 SST anomaly
has a similar evolution during the next several months through MJJ 2021. Based
on the latest observations and model forecasts as of early November, the
official CPC/IRI ENSO outlook indicates high confidence in the persistence of
La Nina conditions through the Northern Hemisphere winter (probabilities
exceeding 90 percent through JFM 2021) with La Nina conditions now likely to
extend through MAM 2021 (probabilities exceeding 60 percent).
PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS
With an increasing likelihood of the persistence of La Nina conditions, the
seasonal outlooks utilized typical impacts during past observed La Nina events
as guidance for many areas of the forecast domain through at least MAM 2021.
This guidance included regressions of temperature and precipitation relative to
the CPC consolidation forecasts of the Nino3.4 region and via "bridging"
techniques utilizing statistical relationships between dynamical model
forecasts of the Nino3.4 index and observed temperature and precipitation.
Dynamical model guidance from the North American Multi-Model Ensemble (NMME)
and the model suite from the Copernicus program are a significant component of
guidance for the temperature and precipitation outlooks through MJJ 2021.
Beyond MJJ 2021, the consolidation of various statistical tools, including
decadal trends , was the primary basis for the outlooks, with little remaining
influence from ENSO or other reliable large-scale climate variability for these
forecast leads. Also, based on current and potential drought conditions in many
areas across the southern tier of the CONUS, low soil moisture conditions
influenced the temperature outlooks for the spring and early summer 2021. At
later leads, decadal trends in temperature and precipitation were the primary
tool used in creating the seasonal outlooks.
PROGNOSTIC DISCUSSION OF OUTLOOKS - DJF 2020 TO DJF 2021
The December-January-February (DJF) 2020-2021 outlook favors above-normal
seasonal mean temperatures for a majority of the CONUS based on the impacts of
a La Nina climate state along with consistent calibrated dynamical model
guidance from the NMME models and the consolidated multi-model ensemble mean.
Highest probabilities are forecast for the Southwest and Rio Grande Valley
where decadal trends are strongly positive and dynamical model probabilities
are the greatest. There is less forecast confidence and lower probabilities of
above-normal temperatures for the Ohio Valley and Mid-Atlantic, due to
uncertainties in the phase of the Arctic Oscillation / North Atlantic
Oscillation and other mid-latitude variability, especially during the months of
December 2020 and January 2021. There is a slight increase in the probabilities
of below-normal temperatures for parts of the Pacific Northwest, far Northern
Rockies, and the Northern Plains, consistent with impacts from La Nina
conditions, especially later in the winter season. Above normal sea surface
temperatures and decadal temperature trends favor above-normal temperatures for
areas in northern and western Alaska.
Expected impacts associated with La Nina conditions, however, favor
below-normal temperatures for southeastern Alaska. Model forecasts predict a
transition from likely above normal temperatures to a greater probability of
below normal temperatures and, as a result, there is high uncertainty in the
DJF temperature forecasts for parts of the western Midwest and Great Lakes
regions. A forecast of Equal-Chances (EC) of below-, near-, or above-normal
seasonal mean temperatures for this region is indicated for DJF 2020-2021.
The evolution of the temperature outlooks from DJF 2020-2021 through MAM 2021
are consistent with typical impacts associated with La Nina conditions spanning
the winter and spring seasons indicated by statistical, dynamical and hybrid
(combined statistical and dynamical) model guidance products. This typical
historical evolution is augmented by the latest dynamical model guidance and
consideration of decadal trends . Outlooks show a gradual expansion of likely
below-normal temperatures for areas along the northern tier of the CONUS,
primarily for parts of the Northern Plains and western Great Lakes as well as
portions of the Pacific Northwest and southern Alaska, through early spring
2021. Later in Spring 2021, probabilities for above-normal temperatures across
parts of the Southwest and southern Great Plains are enhanced by potential
worsening or developing low soil moisture conditions and its warming influence
on surface temperatures during the warm season, following likely below-normal
precipitation associated with La Nina.
Similar to the DJF 2020-2021 temperature outlook, the DJF 2020-2021 outlook for
seasonal total precipitation amounts are strongly influenced by typical impacts
associated with a predicted La Nina climate state during the winter and spring
seasons. The greatest confidence in the precipitation outlooks for the DJF
2020-2021 and JFM 2021 seasons are for elevated probabilities, greater than 60
percent, for below-normal precipitation for a region extending from
southeastern Arizona to western Texas. Probabilities exceeding 50 percent for
below-normal precipitation during DJF 2020-2021 extend from Southern California
across parts of the Southwest, into the Southern Plains, along the Gulf Coast,
and across the Florida Peninsula. From JFM through MAM 2021, the area of
probable below normal precipitation evolves to extend further north into the
Great Plains, while decreasing in extent over the Southeast, such that
eventually only a small area of likely below normal precipitation remains over
the Southwest in AMJ 2021.
Above-normal precipitation is most likely for parts of the Pacific Northwest
extending eastward across the northern Rockies and northern Plains to include
the Great Lakes, the Ohio Valley, and parts of the Northeast region during the
DJF 2020-2021 through FMA 2021 seasons. This favored region of above-normal
precipitation continues over a smaller area of the Pacific Northwest into MAM
2021. Above-normal precipitation is favored for western Alaska during similar
seasons while below-normal precipitation is favored over the southeastern
Alaska Mainland and the Alaska Panhandle, consistent with typical La Nina
The precipitation outlooks in subsequent seasons are driven primarily by
decadal trends , predicting below normal precipitation for parts of the
northwestern CONUS during the warm seasons, MJJ through JAS 2021, and above
normal precipitation for parts of the central and eastern CONUS from AMJ 2021
through ASO 2021, continuing for parts of the Northeast for SON and OND 2021.
Areas labelled EC indicate a forecast of climatological probabilities for
below-, near- or above-normal seasonal total precipitation amounts.
FORECASTER: Dan Collins
The Climatic normals are based on conditions between 1981 and 2010, following
the World Meterological Organization convention of using the most recent 3
complete decades as the climatic 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.
For a description of of the standard forecast tools - their skill- and the
forecast format please see our web page at
(Use Lower Cas e Letters)
Information on the formation of skill of the CAS forecasts may be found at:
(use lowercase letters)
Notes - These climate outlooks are intended for use prior to the start of their
valid period. Within any given valid period observations and short and medium
range forecasts should be consulted.
This set of outlooks will be superseded by the issuance of the new set next
month on Dec 17 2020
1981-2010 base period means were implemented effective with the May 19, 2011