599
FXUS05 KWBC 191231
PMD90D
Prognostic Discussion for Long-Lead Seasonal Outlooks
NWS Climate Prediction Center College Park MD
830 AM EDT Thu Sep 19 2024

SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS

El Nio Southern Oscillation (ENSO)-neutral conditions are present, as
equatorial sea surface temperatures (SSTs) are near-to-below average in the
central and eastern Pacific Ocean. During the last four weeks, negative SST
anomaly changes prevailed across the western and central equatorial Pacific
Ocean. As such, a La Nia Watch is in effect, with La Nia favored to emerge in
September-November (SON) (71% chance) and is expected to persist through
January-March (JFM) 2025. However, chances of a moderate to strong La Nia are
currently less than 50% through the Fall and Winter. ENSO-neutral conditions
are favored to re-emerge by the February-April (FMA) 2025 season.



The October-December (OND) 2024 temperature outlook favors above normal
temperatures for the southern two-thirds of the Contiguous United States
(CONUS), the eastern third of the CONUS, and northwestern Alaska. The largest
probabilities (greater than 60 percent) of above normal temperatures are
forecast across much of the Southwest and parts of the Rio Grande Valley and
southern High Plains. Conversely, a weak tilt toward below normal temperatures
is indicated for Southeast Alaska, parts of the southern Mainland, and parts of
the Alaska Peninsula. The OND 2024 precipitation outlook depicts enhanced
probabilities of below-normal precipitation amounts across much of the
south-central and southwestern CONUS as well as most of Southeast Alaska and
parts of the southern Mainland. The greatest chances (greater than 50 percent)
of below-normal precipitation are forecast for much of the Rio Grande Valley
and much of the Southern High Plains, where probabilities of below exceed 50
percent. Above-normal precipitation is more likely for the northwestern CONUS,
the Great Lakes and Northeast, and much of northwestern Mainland Alaska. Equal
chances (EC) are forecast for areas where probabilities for each category of
seasonal mean temperatures and seasonal accumulated 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 during August 2024, with near-average sea surface
temperatures (SSTs) observed across most of the equatorial Pacific Ocean. The
weekly Nio indices did not change substantially during the month, with the
latest weekly index values varying between +0.1C (Nio-4) and -0.2C
(Nio-3.4, 3, and 1+2). Below-average subsurface temperatures were also similar
to those in early August. Negative temperature anomalies continued to dominate
across most of the subsurface equatorial Pacific Ocean. Low-level wind
anomalies were easterly over most of the equatorial Pacific, and upper-level
wind anomalies were easterly over the east-central Pacific. Convection was
slightly enhanced over parts of Indonesia and was near average near the Date
Line. Both the Southern Oscillation index and the equatorial Southern
Oscillation indices were positive. Collectively, the coupled ocean-atmosphere
system reflected ENSO-neutral.



Since late August, the Real-time Multivariate Madden Julian Oscillation (RMM)
index observations show the Madden Julian Oscillation (MJO) signal has
struggled to fully propagate out of the Maritime Continent. The signal has
regained amplitude in the past week, but still has shown little sign of
eastward propagation. Dynamical models have come into better agreement favoring
an eastward propagating Western Pacific MJO event during the next few weeks,
with several ensemble members maintaining the signal with a moderate to high
amplitude as it enters the Western Hemisphere. Upper-level velocity potential
forecasts from the models remain somewhat mixed in regards to the evolution and
strength of the MJO, with the European Centre for Medium-Range Weather
Forecasts (ECMWF) favoring more robust activity heading into October. Despite
some uncertainties, conditions are expected to become more conducive for
tropical cyclone development over the tropical Americas.

PROGNOSTIC DISCUSSION OF SST FORECASTS

The most recent International Research Institute (IRI) for Climate and Society
plume forecast of the Nio-3.4 SST index predicts a weak and a short duration
La Nia, as indicated by the Nio-3.4 index values less than -0.5C. This
month, the latest North American Multi-Model Ensemble (NMME) guidance was
relied upon, which predicts La Nia to emerge in the next couple of months and
continue through the Northern Hemisphere winter. The continuation of negative
subsurface temperatures and enhanced low-level easterly wind anomalies supports
the formation of a weak La Nina. In summary, La Nia is favored to emerge in
SON (71% chance) and is expected to persist through JFM 2025.

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

Dynamical model forecasts from the NMME, the Coupled Forecast System Model
Version 2 (CFSv2) , the Copernicus (C3S) multi-model ensemble system were used
extensively for the first six leads when they are available, as was the
objective, historical skill weighted consolidation and Calibration, Bridging,
and Merging (CBaM) guidance, that combines both dynamical and statistical
forecast information.



Additionally, the official ENSO forecast depicts probabilities of La Nia that
are higher than climatological probabilities through the upcoming winter. This
anticipated La Nia signal played a role in the construction of these outlooks.
However, the anticipated La Nia is likely to be weak, which may temper some of
its impacts. At later leads, decadal trends in temperature and precipitation
were increasingly relied upon in creating the seasonal outlooks.

PROGNOSTIC DISCUSSION OF OUTLOOKS - OND 2024 TO OND 2025

TEMPERATURE

Above-normal temperatures are favored throughout a majority of the southern and
eastern CONUS and northwestern Alaska during OND. Conversely, below normal
temperatures are more likely for much of southern Alaska. EC of below, near, or
above normal temperatures are forecast for the northwestern and north-central
CONUS. This EC area is due to weak or conflicting signals among temperature
tools. Probabilities of above-normal temperatures are reduced slightly relative
to last month across parts of the north-central CONUS to the Southeast due to a
trend toward a colder solution in the temperature consolidation coupled with a
low confidence EC forecast for the month of October across many of these areas.
Above normal temperatures remain likely (greater than 60 percent) across the
Southwest, due to good agreement among both dynamical and statistical guidance.
The area of favored above-normal temperatures are expanded to include much of
California due to a stronger signal in this months consolidation. Guidance is
similar across much of Alaska relative to last month. Increased probabilities
of below-normal temperatures are indicated for southern parts of the state,
aided in part due to an anticipated cooler than normal October and the
potential impacts of La Nia, especially later in the season. Above-normal
temperatures remain favored for northwestern Alaska due to recent trends and
observed below normal sea ice extent.



From November-December-January (NDJ) through JFM, impacts from the predicted La
Nia continue as above normal temperatures are favored across the Southern
CONUS and the Eastern Seaboard while enhanced below normal temperature
probabilities persist across southeastern Alaska and expand northwestward
toward the central Mainland and southeastward to the northwestern and
north-central CONUS by December-January-February (DJF). By FMA, the potential
impacts of La Nia begin to wane as ENSO-neutral conditions become increasingly
likely. Below-normal temperatures favored for much of Alaska disappear by
April-May-June (AMJ) and from all of the CONUS by May-June-July (MJJ).
Thereafter, the forecast pattern increasingly reflects trends with above-normal
temperatures generally favored across most of the southern and eastern CONUS
next spring, expanding to most of the West Coast by the Summer and to the
remainder of the CONUS next fall. Across Alaska, a quick transition toward
milder than normal conditions is predicted with above-normal temperatures
favored for almost the entire state by late Spring and through the Summer and
much of next Fall.

PRECIPITATION

Model and statistical guidance remains generally consistent from previous
months depicting elevated probabilities of below normal precipitation for much
of the southern CONUS, and increased chances of above normal precipitation for
the Pacific Northwest, Great Lakes, and parts of the Northeast during OND.
However, despite dynamical models favoring below-normal precipitation in the
Southeast, EC is indicated due to the potential for tropical activity early in
the season and increased probabilities for a wet October. Precipitation
consolidation guidance has trended wetter across the northern tier of the
CONUS, resulting in an expansion of elevated above-normal precipitation
probabilities across parts of the northern High Plains and Great Lakes relative
to last month. However, due to a lack of dynamical model support, EC remains
forecast across most of the north-central CONUS. A slight tilt toward
below-normal precipitation is indicated for parts of southeastern Alaska, based
primarily on dynamical model guidance from the CFSv2 and a dry signal derived
from natural analogs to the current ENSO state. Conversely, above-normal
precipitation is favored for northern and western Alaska due to trends with
support from C3S and CBaM output.  EC is indicated for most of the rest of the
forecast domain as signals for the various dynamical and statistical tools are
too weak or conflicting to issue a forecast with a sufficient degree of
confidence.



As we progress further into late fall and through the winter, dynamical and
statistical guidance persist with a dry signal across the southern CONUS and
southeastern Alaska, consistent with a La Nia signature. An expansion of
enhanced above normal precipitation probabilities is noted across the northern
CONUS, peaking in coverage during the winter months. A wet signal is also
favored to persist across northwestern Alaska and expand southward from the
Great Lakes across much of the east-central CONUS later in the winter through
the Spring, consistent with dynamical model guidance and trends. Thereafter,
trends become the dominant factor with increased chances of above normal
precipitation generally indicated across parts of the Northeast and parts of
the Mid-Atlantic in the Summer and shifting southward along the Mid-Atlantic
Coast by early Fall. Conversely, trends favor a dry pattern for much of the
Southwest during early spring shifting northward to the Northern Rockies by
late Spring, with increased chances of dryness for parts of the Northern and
Central Plains next summer as well. As dryness potentially expands into the
northern and central Plains next summer, a corresponding weak tilt toward above
normal precipitation makes a brief appearance for parts of the Southwest
Monsoon Region during June-July-August (JJA) and July-August-September (JAS).
Thereafter, the area of enhanced probabilities of below-normal precipitation
begins to recede over the west-central CONUS, based primarily on combined
statistical guidance from trends and ENSO. By next Fall, a small area of
favored above-normal precipitation appears in the northwestern CONUS due to
combined statistical guidance from trends and ENSO. Above-normal precipitation
is also favored for much of northern Alaska late Spring and early Summer and
shifting to southeastern Alaska by the Fall, consistent with trends.

FORECASTER: Scott Handel

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 Oct 17 2024


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