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 Niño 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 Niña Watch is in effect, with La Niña 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 Niña 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 Niño indices did not change substantially during the month, with the 
latest weekly index values varying between +0.1°C (Niño-4) and -0.2°C 
(Niño-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 Niño-3.4 SST index predicts a weak and a short duration 
La Niña, as indicated by the Niño-3.4 index values less than -0.5°C. This 
month, the latest North American Multi-Model Ensemble (NMME) guidance was 
relied upon, which predicts La Niña 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 Niña 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 Niña that 
are higher than climatological probabilities through the upcoming winter. This 
anticipated La Niña signal played a role in the construction of these outlooks. 
However, the anticipated La Niña 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 month’s 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 Niña, 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 
Niña 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 Niña 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 Niña 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 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 Oct 17 2024 
 
 
1991-2020 base period means were implemented effective with the May 20, 2021 
forecast release. 
$$