654
FXUS05 KWBC 171231
PMD90D
Prognostic Discussion for Long-Lead Seasonal Outlooks
NWS Climate Prediction Center College Park MD
830 AM EDT Thu Oct 17 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 favored to emerge during September-November (SON) 2024 (60% chance) and is
forecast to persist through January-March (JFM) 2025. Any La Nia event that
develops this autumn is favored, however, to be a weak short duration event.



The November-January (NDJ) 2024-2025 temperature outlook favors above-normal
seasonal mean temperatures for most of the Contiguous United States (CONUS)
from the Southwest eastward to include most of the central U.S. and all of the
eastern CONUS. The largest probabilities (greater than 60 percent) of above
normal temperatures are forecast for parts of the Southwest. There also is an
enhanced likelihood of above-normal temperatures for northern Alaska.
Below-normal temperatures are most likely for much of southern Alaska.



The NDJ 2024-2025 precipitation outlook depicts enhanced probabilities of
below-normal seasonal total precipitation amounts along most of the southern
tier of the 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. Slight
tilts toward above-normal precipitation is depicted for parts of the Pacific
Northwest, northern Rockies, central Great Lakes, and western and northern
Alaska.



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 October 2024, with near-average sea
surface temperatures (SSTs) observed across the equatorial Pacific Ocean as a
whole. The equatorial central Pacific SSTs, however, have cooled recently and
negative anomalies of greater than -0.5 degrees C for the equatorial region
from 165 W to 125 W are evident. In fact, the latest weekly Nio-3.4 index
value is now -0.5 degrees C. Below-average subsurface ocean temperatures remain
in place with cooler than normal conditions from approximately 160 W to 90 W to
a depth of about 150 m. Low-level wind anomalies are easterly over the
east-central equatorial Pacific, and upper-level wind anomalies are westerly
over the eastern Pacific. Convection has been near average over Indonesia and
slightly suppressed over the Date Line.



Then MJO has been active during September into October and aided a trade wind
surge across the central Pacific in early September. The MJO is forecast to
remain active for at least the rest of October and another trade wind surge
supporting cooling of SSTs in the central Pacific is possible in late October.



SSTs at higher latitudes remain warmer than normal for most areas of the
mid-Pacific, Atlantic, and Indian Oceans. Enhanced upwelling along the U.S.
West Coast has allowed negative SST anomalies to develop in parts of this
region and below-normal ocean surface temperatures are evident in parts of the
Gulf of Alaska and the Bering Sea.



Drought conditions have rapidly expanded across much of the CONUS during late
summer and early autumn especially for areas in the central U.S.

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 SON (60% chance) and is expected to persist through JFM 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 NDJ 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
soil moisture anomalies are utilized in preparation of the outlooks at early
leads. 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 - NDJ 2024 TO NDJ 2025

TEMPERATURE

The NDJ 2024-2025 temperature outlook favors above-normal seasonal mean
temperatures for much of the contiguous U.S. (CONUS) to include the Southwest,
central Rockies, the central and Southern Plains, Southeast, Ohio Valley,
mid-Atlantic and Northeast. Highest odds for warmer than normal conditions is
forecast for the Southwest and southern Plains. The temperature outlook for
this season is primarily based on dynamical model guidance from the NMME and
C3S prediction suites and long term positive temperature trends across much of
the country.



Expectations for the development of La Nia conditions contributed to the
temperature outlooks and their evolution from DJF 2024-2025 through FMA 2025.
Even though the La Nia event is favored to be a weak and shorter duration
event at this time, typical winter season impacts influence the outlook during
these seasons with below-normal temperatures most likely from the Pacific
Northwest eastward to the northern Plains beginning in DJF 2024-2025 through
FMA 2025 and potentially lingering for parts of the Pacific Northwest and
northern Rockies in MAM 2025. Enhanced odds for above-normal temperatures
forecast in NDJ 2024-2025 gradually decrease in coverage and magnitude across
the CONUS through FMA 2025 for areas of the western CONUS, Midwest, Great
Lakes, Ohio Valley and Northeast. Some statistical model guidance including the
ENSO-OCN forecast tool and composites taken over the 1991-2023 period for
combined weak La Nia and ENSO-neutral winter seasons support this outlook
evolution.



Negative temperature trends for the north-central U.S. - especially centered
near the month of February - played a considerable role in the outlooks. The
official outlooks deviate significantly in some areas in the north-central
CONUS from the majority of the NMME and C3S dynamical model guidance. Given
weak ENSO forcing this winter, variability within the winter seasons is most
likely to be above-average and greater than last winter. The variance from key
subseasonal modes of variability from both the Tropics (e.g., MJO, etc.) and
higher latitudes/stratosphere (i.e., AO/NAO, etc.) is likely to dominate the
eventual seasonal mean temperature anomalies rather than more persistent
periods during the winter. Positive temperature trends in the dynamical model
guidance may be overemphasized in the model predictions in these regions
resulting in warmer solutions overall for the winter seasons from DJF through
FMA 2025 as the observed temperature trends are either neutral or negative in
most areas.



For Alaska, over these seasons, dynamical model guidance is in overwhelming
agreement between models and consistent with typical La Nia related impacts.
Below-normal temperatures are most likely for south-central and Southeast
mainland Alaska as well as for the Alaska Panhandle through MAM 2025 with the
maximum forecast coverage during JFM 2025. Above-normal temperatures are
favored for northwest Alaska during this period with the greatest odds during
NDJ 2024-2025 due to more open water (below-normal sea ice extent) and
anticipated warmer than normal SSTs.



Progressing through the spring and summer months of 2025, the outlooks
primarily rely on the forecast consolidation of statistical model guidance and
long-term temperature trends with high uncertainty in the future evolution of
ENSO. Forecast coverage for above-normal temperatures is greatest during the
late spring and summer seasons of MJJ and JJA 2025.

PRECIPITATION

The NDJ 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. 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.



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: Jon Gottschalck

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 Nov 21 2024


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