Prognostic Discussion for Long-Lead Seasonal Outlooks 
NWS Climate Prediction Center College Park MD 
830 AM EDT Thu Aug 15 2024 
 
SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS 
 
El Niño Southern Oscillation (ENSO) conditions are neutral with equatorial sea 
surface temperatures (SSTs) above average in the western Pacific, near average 
in the east-central Pacific, and below average in the eastern Pacific. La Niña 
is favored to develop during September-October-November (66% chance) and 
persist through the winter 2024-2025 (near 70% chance). 
 
The September-October-November (SON) 2024 temperature outlook favors 
above-normal temperatures across a majority of the contiguous U.S. with the 
largest probabilities (exceeding 60%) forecast for New England and parts of the 
Southwest. Increased below-normal temperature probabilities are forecast for 
southwestern Alaska, while above-normal temperatures are more likely across 
northern Alaska. 
 
The SON 2024 precipitation outlook depicts elevated probabilities for 
above-normal precipitation along the East Coast, parts of the Pacific 
Northwest, and western Alaska. Below-normal precipitation is favored for the 
Central to Southern Great Plains, Central Rockies, Southwest, southern 
California, and southern Alaska. 
 
Areas depicted in white and labeled “Equal-Chances” or “EC” are regions where 
climate signals  are weak, and so there are equal chances for either above-, 
near- or below-normal seasonal mean temperatures or total precipitation amounts. 
 
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 
 
Equatorial SSTs are above average in the western Pacific, near average in the 
east-central Pacific, and below average in the eastern Pacific. Weekly observed 
SST anomalies in the Niño 3.4 region are at 0.0 degrees Celsius. Subsurface 
temperature anomalies (averaged from 180-100W and 0-300 meters) are near -1.0 
degrees Celsius which is a decrease from early July. From July 14 to August 8, 
OLR anomalies were near average for much of the equatorial Pacific and Maritime 
Continent. Low-level (850-hPa) wind anomalies were easterly over the 
east-central and eastern equatorial Pacific. Upper-level (200-hPa) wind 
anomalies were highly variable east of the Date Line, with easterly anomalies 
from 160W to 140W and westerly anomalies around 100W. These atmospheric and 
oceanic conditions reflect a continuation of ENSO-neutral conditions. The 
Madden-Julian Oscillation (MJO) strengthened during early August with a more 
coherent wave-1 pattern developing over the global tropics. During the 
remainder of August, the MJO with anomalous upper-level divergence is forecast 
to propagate east from Africa to the Indian Ocean and Maritime Continent. This 
would likely provide a favorable large-scale environment for tropical cyclone 
development across the main development region of the Atlantic basin through 
the beginning of September. The MJO is also forecast to result in enhanced 
trade winds across the equatorial central Pacific which may contribute to 
negative SST anomalies becoming better established and a transition to La Niña. 
 
PROGNOSTIC DISCUSSION OF SST FORECASTS 
 
The CPC SST consolidation forecast for Niño 3.4 depicts decreasing negative 
anomalies during the fall with a -0.6 degree C anomaly persisting from 
October-November-December 2024 to January-February-March 2025. The CFSv2 has 
the most negative anomaly (below -1.0 degree C) among the model inputs to the 
NMME and C3S. The CPC ENSO outlook indicates a likely transition to La Niña 
during September-October-November with probabilities at or above 70 percent for 
La Niña conditions to persist during the late fall and early winter 2024-2025. 
By the spring 2025, ENSO-neutral conditions become more favored. 
 
PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS 
 
The temperature and precipitation outlooks for SON 2024 were based on dynamical 
model guidance such as the North American Multi-Model Ensemble (NMME) and 
Copernicus (C3S) along with statistical tools including the global SST-based 
Constructed Analog and ENSO-OCN. The Calibration, Bridging and Merging (CBaM) 
tool anchored to the NMME forecasts and "bridged" to the Niño3.4 index was also 
utilized. The objective, historical skill-weighted consolidation, that combines 
dynamical and statistical tools, was relied upon in many of the outlooks. La 
Niña composites were considered for the SON temperature and precipitation 
outlooks but were used more extensively from OND 2024 to JFM 2025 as its 
influence on the mid-latitude circulation pattern is expected to increase later 
this fall and into the winter. Long-term climate trends  were considered for all 
leads, but were relied upon most from the spring 2025 through SON 2025. 
 
 
PROGNOSTIC DISCUSSION OF OUTLOOKS - SON 2024 TO SON 2025 
 
TEMPERATURE 
 
Above-normal temperatures are favored across a majority of the contiguous U.S. 
(CONUS) during SON 2024 which is supported by the NMME, C3S, consolidation 
tool, and decadal trends . Based on the dynamical models  and statistical tools, 
the largest probabilities (above 60 percent) for above-normal temperatures are 
forecast for New England and parts of the Southwest. Most guidance features a 
decreasing warm signal closer to the West Coast where equal chances (EC) of 
below, near, or above-normal temperatures are forecast. The previous seasonal 
outlook for SON featured EC across the Northern Great Plains and Upper 
Mississippi Valley. This was shifted to a slight lean for above-normal 
temperatures due to good agreement among the dynamical models  including 
relatively large probabilities (above 50 percent) in the CBAM + Trend along 
with the C3S. Although above-normal temperatures are the most likely outcome on 
the seasonal time scale for the eastern and central CONUS, northward tracking 
tropical cyclones could alter the mid-latitude circulation pattern and result 
in periods of cooler-than-normal temperatures for these areas this fall. 
Above-normal temperatures are favored along the North Slope of Alaska due to 
low sea ice and decadal trends . Below-normal temperatures are more likely for 
southwestern Alaska and the Aleutians due to below-normal SSTs along with 
consistency from the previous month. 
 
Minor changes were made to the previous outlooks for OND and NDJ. The slight 
lean towards below-normal temperatures in parts of the Pacific Northwest (OND 
and NDJ) and Northern Great Plains (NDJ) was adjusted to EC due to the lack of 
support from dynamical models  and any La Niña cooling influence may be delayed. 
Also, La Niña cooling is typically most pronounced, beginning in DJF. During 
OND and NDJ, the latest NMME supports an increase in above-normal temperature 
probabilities across the Southwest and Central to Southern Great Plains. The 
major climate driver during the winter and early spring 2024-2025 is expected 
to be La Niña and the outlooks from DJF 2024-2025 through FMA 2025 reflect this 
with above-normal temperatures favored for the Southern Great Plains, Lower 
Mississippi Valley, and Southeast. Elevated below-normal temperature 
probabilities are forecast for southeastern Alaska and also extend east from 
the Pacific Northwest to the Northern Great Plains and Upper Mississippi 
Valley. Later in the spring 2025 and continuing through the summer and fall 
2025, decadal trends  became the major factor in the temperature outlook. 
 
PRECIPITATION 
 
The SON precipitation outlook favors below-normal precipitation across the 
Central to Southern Great Plains, Southwest, and southern California based on 
the NMME, C3S, and consolidation tool. Given the excellent model agreement and 
consistency, below-normal precipitation probabilities exceed 50 percent for 
southern New Mexico and parts of western Texas. Based on the NMME and C3S, 
elevated below-normal precipitation probabilities extend east to inland areas 
of the Lower Mississippi Valley. EC were necessary closer to the Gulf Coast due 
to the potential for heavy precipitation associated with any landfalling 
tropical cyclones. The likelihood of an active Atlantic hurricane season was 
the primary factor in favoring above-normal precipitation along the East Coast. 
In addition, the NMME and C3S support above-normal precipitation for the 
Southeast and the consolidation tool features a wet signal for both the 
Southeast and New England. Above-normal precipitation is also favored for the 
Pacific Northwest and western Alaska based on the NMME and consistent with La 
Niña composites, while those tools support elevated below-normal precipitation 
probabilities for southern Alaska. Later in the fall and into the 2024-2025 
winter, the three-month precipitation outlooks are based largely on La Niña 
composites. The spatial coverage for below-normal precipitation probabilities 
exceeding 50 percent is largest during DJF and JFM across southeastern New 
Mexico, Texas, and parts of the Southeast where the dry signal is the strongest 
and occurs most frequently according to La Niña composites. Based on the same 
reasoning, above-normal precipitation probabilities are at their largest during 
DJF for the Pacific Northwest. Typically during La Niña, a tight gradient of 
dry to wet from the Gulf Coast to the Tennessee and Ohio Valleys becomes 
established during the winter and early spring. The consolidation tool and La 
Niña composites were relied upon for the Alaska precipitation outlook from OND 
2024 to FMA 2025. Similar to the temperature outlook at later lead times, the 
precipitation outlook during the late spring, summer, and fall 2025 was based 
largely on decadal trends . 
 
FORECASTER: Brad Pugh 
 
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 Sep 19 2024 
 
 
1991-2020 base period means were implemented effective with the May 20, 2021 
forecast release. 
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