673
FXUS05 KWBC 211331
PMD90D
Prognostic Discussion for Long-Lead Seasonal Outlooks
NWS Climate Prediction Center College Park MD
830 AM EST Thu Nov 21 2024

SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS

El Nio Southern Oscillation (ENSO)-neutral conditions remain present, as
equatorial sea surface temperatures (SSTs) are near-to-below average in the
central and eastern Pacific Ocean. A La Nia Watch remains in effect, with La
Nia most likely to emerge in October-December 2024 (57% chance) and is
expected to persist through January-March 2025. Any La Nia event that develops

this autumn is favored, however, to be a weak short duration event.



The December-January-February (DJF) 2024-2025 temperature outlook favors
above-normal seasonal mean temperatures from the Southwest across the Southern
Plains to the Southeast, then northward from the Great Lakes to New England.
Below-normal seasonal mean temperatures are favored from the Pacific Northwest
to the Northern Great Plains.



The DJF 2024-2025 precipitation outlook depicts enhanced probabilities of
below-normal seasonal total precipitation amounts along most of the southern
tier of the contiguous U.S. (CONUS) and for parts of southeast Mainland Alaska
and Alaska Panhandle. The greatest odds of below-normal precipitation (greater
than 50 percent) are forecast for parts of the Southwest and Southern Plains.
Above-normal precipitation is favored for parts of the Pacific Northwest,
northern Rockies, central Great Lakes, and western and northern Alaska. The
strongest of those signals is over the northern Rockies.



Equal chances (EC) are forecast for areas where probabilities for each category
of seasonal mean temperatures and seasonal total precipitation amounts are
expected 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

ENSO-neutral continued through early November 2024, with near- to below-average
SSTs from about 170W to 100W along the equator and off equatorial SSTs just
above normal. The equatorial central Pacific SSTs, exhibited a mix of cooling
and warming during the last 4 weeks, with some portions of the Nino3.4 region,
so no strong movement toward La Nina was noted. The latest weekly Nio indices

ranged from +0.2C (Nio-4) to -0.3C (Nio-3.4), similar to values from
October. When removing the global tropical mean to construct a Nino3.4 value
relative to the global tropics, the relative value is -0.76C, so that may be
why we are seeing some signals consistent with La Nina but not robustly clear

indications, yet.



Equatorial ocean heat content for the area from 180 to 100W remained below
normal, near -0.5C. Below-average temperatures remain at depth in the
east-central and eastern Pacific Ocean, while above-average temperatures
prevail at depth and near the surface in the western Pacific. Above-average OLR
(suppressed convection and precipitation) was observed around the Date Line.

Below-average OLR (enhanced convection and precipitation) was evident over the
Philippines. Low-level (850-hPa) wind anomalies were mostly near average across
the equatorial Pacific Ocean, with a small region of westerly wind anomalies
over the eastern Pacific Ocean. Upper-level (200-hPa) wind anomalies were
mostly near average across the equatorial Pacific Ocean



Then MJO was active during October and moved across the central Pacific in late
October. The MJO is forecast to remain active for at least the rest of
November, potentially enhancing low-level easterly anomalies across the central
and eastern Pacific basins.



SSTs remain warmer than normal for most areas of the Atlantic and Indian
Oceans, as well as the mid-latitude regions of the Pacific. Below-normal ocean
surface temperatures are evident in parts of the Gulf of Alaska and the Bering
Sea, as well as the Chukchi Sea where there are some pockets of new ice at less
than 4-tenths coverage. Sea ice is largely in place for much of the Chukchi and
Beaufort seas (https://tgftp.nws.noaa.gov/data/raw/fz/fzak80.pafc.ice.afc.txt),
so any signal from a late onset of sea ice is not applicable during DJF.



Drought conditions that rapidly expanded across much of the CONUS during late
summer and early autumn have been ameliorated across much of the central Great
Plains while continuing to intensify across much of the Southeast, Central
Appalachians, and the Northeast.

PROGNOSTIC DISCUSSION OF SST FORECASTS

Taken collectively, statistical and dynamical model forecast guidance of the
Nio3.4 index favor the development of a weak and most likely short duration La
Nia event. Some statistical model forecasts do favor a continuation of
ENSO-neutral into and through winter 2024-2025. Dynamical model guidance

predictions tend to support weak La Nia conditions to develop, including the
majority of participant models from the NMME and C3S forecast suites. Most
recent observations and the forecast guidance noted above favor La Nia to
emerge during OND 2024 (57% chance) and it is expected to persist through JFM
2025. After JFM 2025, ENSO-neutral is the most likely category into the
northern hemisphere summer of 2025.



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

The development of potential La Nia conditions in the atmosphere-ocean system
in the Pacific contributed to the outlooks from DJF 2024-2025 through FMA 2025.
Although typical La Nia impacts are less likely to occur during a weak event
(currently favored), La Nia still tilts the odds in the outlook forecast
probabilities. Coastal SSTs and constructed analogue forecasts keyed to current
SSTs are utilized in preparation of the outlooks. Soil moisture is not largely
considered for the outlook during this time of year as correlations with soil
moisture are quite low for initial conditions during the cool season. Dynamical
model guidance from the NMME and C3S prediction suites (first 5 and 3 leads
respectively), statistical forecast tools, long-term temperature and
precipitation trends, and an objective, historical skill weighted combination
of much of the above guidance strongly contributed to the final outlooks.



PROGNOSTIC DISCUSSION OF OUTLOOKS - DJF 2024 TO DJF 2025

TEMPERATURE

The December-January-February (DJF) 2024-2025 temperature outlook favors
above-normal seasonal mean temperatures from the Southwest across the Southern
Plains to the Southeast, then northward from the Great Lakes to New England.
Below-normal seasonal mean temperatures are favored from the Pacific Northwest
to the Northern Great Plains. The DJF outlook has slightly lower odds for
below-normal temperatures across North Dakota and Minnesota than the DJF
outlook released in October due to the continued delay in La Nina and slightly
increased odds for a weak event. Additionally, the NMME suite was judged to be
too warm over the Northern Great Plains despite a shift toward cooler outcomes
in the model suite, many of the models were still showing above-normal

temperatures. Based on a weak La Nina and models overdoing trends, observed
trends become more of the signal. Furthermore, higher frequency patterns (AO,
MJO, and stratospheric variability) that result in increased uncertainty can
also play a larger role. Those modes are largely not predictable on seasonal
timescales, though La Nina events and the westerly phase of the QBO can result
in less polar vortex splits or shifts.



Similar patterns (above-average temperatures favored largely across the
southern tier and below-normal temperatures across the northern tier) are
present in the outlooks through Feb-Mar-Apr 2025, with below-normal
temperatures favored from the Northwest to the Northern Great Plains. A band of
equal chances from central California to Wyoming, and across much of the Great
Plains where higher frequency modes can often have their largest impacts.



Across Alaska, the DJF outlook for temperature favors above-normal temperatures
across northern Mainland Alaska while favoring below-normal temperatures across
southern Mainland Alaska and the Alaska Panhandle. The northern signal is
related to trends and outlooks from the NMME and C3S, while the southern signal
is more from La Nina, statistical tools, and the NMME model suite. Those two

areas remain in the outlooks, with some minor modifications, through
Mar-Apr-May, where the signal for below-normal temperatures recedes as trends
become the dominant predictor.



During late Northern Hemisphere spring and early summer of 2025, trends become
the dominant climate forcing. Areas in the Great Plains have the lowest odds
being the areas with the weakest trends and higher variance.  Coming out of a
La Nina winter, dry signals across the southern tier can reinforce some of the
trends in temperatures, especially across the western CONUS, so the odds of
above-normal temperatures are relatively the highest in the Rockies and
Southwest through the summer and autumn.

PRECIPITATION

The DJF 2024-2025 precipitation outlook favors above-normal seasonal total
precipitation amounts for portions of the Pacific Northwest, northern Rockies,
Michigan, and northern and western Alaska. Drier-than-normal conditions are
most likely for the southern tier of the U.S. with the highest odds forecast
for the Southwest and Southern Plains. For Southeast Mainland Alaska and the

northern Alaska Panhandle, a slight tilt toward drier than normal conditions is
forecast through MAM 2025.



As noted above, the favored development of La Nia conditions approaching the
winter and consistent dynamical model forecast guidance is the basis for the
outlook. The odds for above-normal precipitation are increased during DJF
2024-2025 relative to the outlook for DJF from October 2024 as the tools are

showing more certainty. The most uncertain area in the precipitation outlook
for DJF 2024-2025 is across the northern Great Plains, as signals are weak and
mixed (NMME favors above-normal precipitation, where as calibrated versions
remove the signal, and trends vacillate season to season). Moving through the
FMA 2025 season, the enhanced likelihood of above-normal precipitation in the

Great Lakes and Ohio Valley regions expands in coverage and probabilities
increase maximizing in the southern Great Lakes and northern Ohio Valley area
in JFM 2025. Positive precipitation trends also contribute strongly to these
outlooks for some areas in the Midwest and Great Lakes. The precipitation

outlook is highly uncertain east of the Appalachians in the mid-Atlantic and
Northeast, as the storm track during La Nina years is typically more inland,
but a weak La Nina will likely let other, shorter-lived climate forcings play
larger roles in the overall storm track.



Favored below-normal precipitation across the south through FMA 2025 remains
generally consistent with the forecast coverage and odds greatest for JFM 2025.
The majority of the dynamical model guidance from the NMME and C3S model suites
highlight substantially elevated odds for below-normal precipitation for the
Far West (i.e., California/Nevada) for the winter seasons through FMA 2025.

Given the anticipated generally weak ENSO forcing, this seems overdone,
especially since some of the dynamical model guidance can sometimes
overemphasize ENSO impacts during weaker events.



Given the lack of a clear, reliable ENSO signal after this winter/early spring,
longer lead outlooks are primarily based on long term precipitation trends and
depict low forecast coverage. The longer lead precipitation outlooks during the
warm season generally have low forecast skill.

FORECASTER: Matthew Rosencrans

The Climatic normals are based on conditions between 1991 and 2020, 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 Case 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 Dec 19 2024


1991-2020 base period means were implemented effective with the May 20, 2021
forecast release.
$$