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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 Long-Lead Seasonal Outlooks 
NWS Climate Prediction Center College Park MD
830 AM EDT Thu Oct 15 2020

SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS

La Nina conditions are present across the equatorial Pacific Ocean, as
represented in current oceanic and atmospheric observations. La Nina conditions
are likely to continue through the Northern Hemisphere winter and into Spring
2021.

The November-December-January (NDJ) 2020-2021 temperature outlook favors
above-normal seasonal mean temperatures for a majority of the CONUS and for
northern and western parts of Alaska. The greatest probabilities (larger than
60 percent) are forecast for parts of the Southwest. Below-normal temperatures
are most likely for areas of southeast Alaska, the Alaska Panhandle and parts
of the far Pacific Northwest.

The NDJ 2020-2021 precipitation outlook depicts enhanced odds for above-normal
seasonal total precipitation amounts for Alaska and parts of the Pacific
Northwest, northern Rockies and northern Great Plains. Below-normal
precipitation is most likely for much of California (slight tilt in the odds),
stretching eastward to include the Southwest, south-central Great Plains, lower
Mississippi Valley and Southeast.

Equal-chances (EC) are forecast for areas where probabilities for each category
of seasonal mean temperatures or seasonal total precipitation amounts are
favored 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:
http://www.cpc.ncep.noaa.gov/products/predictions/90day/tools/briefing

CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS

The coupled oceanic and atmospheric observations reflect La Nina conditions.
The latest monthly mean Sea Surface Temperatures (SSTs) through October 10,
2020 continue to indicate a strengthening and westward expansion of the Pacific
equatorial cold tongue with SST negative anomalies on or near the equator
stretching from 160E eastward to the South America coast. Some negative
departures reach as large as -1.5 degrees C in some areas. The latest Nino3.4
index value is -1.2 degrees C. Negative subsurface ocean temperature anomalies
continue to persist at a depth ranging from approximately 100-200 meters from
just west of the Date Line to 80W.

Suppressed convection continues over the west-central Pacific while enhanced
convection continues to increase in coverage in the Maritime Continent region.
Trade Winds have also been enhanced over the past 30 days across the central
Pacific Ocean.

PROGNOSTIC DISCUSSION OF SST FORECASTS

The CPC Nino3.4 SST consolidation forecast depicts negative anomalies
increasing in magnitude to near -1.5 degrees C by DJF 2020-2021, followed by a
gradual increase to near normal by AMJ 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 May 2021. Based on the
latest observations and model forecasts as of early October, the official
CPC/IRI ENSO outlook favors La Nina conditions persisting through the Northern
Hemisphere winter into Spring 2021 with La Nina conditions remaining the most
likely ENSO phase through FMA 2021.

PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS

Given the current ENSO conditions and predictions, the seasonal outlooks
through MAM 2021 utilized typical impacts during past observed La Nina events
as the guide for many areas of the forecast domain. This included standard
composite analysis, regressions anchored to forecasts of the Nino3.4 region and
via "bridging" techniques utilizing relationships between 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 also used heavily in the outlooks. Beyond AMJ 2021, the
consolidation of various statistical tools including long-term trends was the
primary basis for the outlooks from this point forward given little reliable
evidence of the influence from ENSO and 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 during 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 - NDJ 2020 TO NDJ 2021

TEMPERATURE

The November-December-January (NDJ) 2020-2021 outlook favors above-normal
seasonal mean temperatures for a majority of the CONUS based on La Nina base
state conditions along with consistent and considerable dynamical model
guidance support from the NMME and Copernicus program participant models and
their respective ensemble means. Highest odds are forecast for the Southwest
where long-term trends are strongly positive and dynamical model output
probabilities are the greatest. Less forecast confidence as depicted by lower
forecast probabilities for the Ohio Valley and Southeast are associated with
uncertainty 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 tilt in the odds for below-normal
temperatures for parts of the far Pacific Northwest consistent with La Nina
conditions especially later in the NDJ season. Lack of or below-normal coverage
in sea ice in waters in proximity to Alaska favor above-normal temperatures for
areas in northern and western Alaska.

Anticipated impacts associated with La Nina conditions, however, favor
below-normal temperatures for areas in southeast Alaska and the Alaska
Panhandle. Divergence in forecast tools and other factors and so high
uncertainty result in a forecast of Equal-Chances (EC) of below-, near-, or
above-normal seasonal mean temperatures for a region from the Pacific Northwest
to the western Great Lakes for NDJ 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 displayed in various statistical, dynamical and
hybrid (statistical-dynamical combined) guidance products. This typical
historical evolution is augmented by the latest dynamical model guidance and
considerations from the latest long-term trends and shows a gradual expansion
of favored 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. Later in Spring
2021, probabilities for above-normal temperatures across parts of the Southwest
and southern Great Plains are influenced by potential worsening or developing
low soil moisture conditions and its warming influence on surface temperatures
during the warm season after favored below-normal precipitation associated with
La Nina winters.

PRECIPITATION

Similar to the NDJ 2020-2021 temperature outlook, the NDJ 2020-2021 outlook for
seasonal total precipitation amounts are strongly influenced by typical impacts
associated with a La Nina base state during the winter and spring seasons. The
greatest confidence in the precipitation outlooks for the NDJ 2020-2021 through
FMA 2021 seasons are for elevated odds of below-normal precipitation first for
a region centered across Texas during NDJ 2020-2021 evolving to include the
Southwest, parts of the Gulf Coast and Southeast during DJF 2020-2021 through
FMA 2021.

Above-normal precipitation is most likely for the northern Rockies, parts of
the Pacific Northwest and northern Plains during the NDJ 2020-2021 season. This
favored region of above-normal precipitation expands eastward to include the
Great Lakes and Ohio Valley during the DJF 2020-2021 through MAM 2021 seasons.
Above-normal (below-normal) seasonal precipitation amounts are favored for
western Alaska (southeast Alaska and the Alaska Panhandle) during similar
seasons - consistent with typical La Nina impacts.

The precipitation outlooks in subsequent seasons are driven primarily by
long-term trends and areas labelled EC indicate a forecast of climatological
odds for either below-, near- or above-normal seasonal total precipitation
amounts.

FORECASTER: Jon Gottschalck

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
http://www.cpc.ncep.noaa.gov/products/predictions/long_range/tools.html
(Use Lower Cas e Letters)
Information on the formation of skill of the CAS forecasts may be found at:
http://www.cpc.ncep.noaa.gov/products/Soilmst_Monitoring/US/Outlook/outlook.shtm
l
(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 Nov 19 2020


1981-2010 base period means were implemented effective with the May 19, 2011
forecast release.
$$

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