189
FXUS07 KWBC 311906
PMD30D
Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT Sun Aug 31 2025


30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2025

The updated monthly outlook for September 2025 is based on dynamical model
ensembles and deterministic runs of the GFS and ECMWF models out to two weeks
in advance, the monthly CFS, the official CPC temperature and precipitation
outlooks for Weeks 3-4, Week-2, 6-10 days, WPCs Week-1 forecasts, September
climatology, recent observations, soil moisture anomalies, coastal sea-surface
temperatures (SSTs), and MJO lagged teleconnections. Tropical cyclone activity
in the Atlantic and east Pacific basins was also considered, as well as the
late season Southwest monsoon and potential moisture surges from the Gulf of
California.



The updated Temperature Outlook for September favors above-normal temperatures
over southern portions of Alaska, consistent with temperature outlooks at the
6-10 day, 8-14-day, and Weeks 3-4 time periods, and proximity to above-average
SSTs. This area of favored above-normal temperatures extends northward up the
West coast of Alaska and over much of the North Slope, and has not changed
since the August 21st outlook release. This is partly attributed to the
climatologically delayed formation of sea ice, and partly to a mixture of near-
and above-average coastal SSTs. Over the Lower 48 states, above-normal
temperatures are favored from the eastern Intermountain region and Rockies
eastward to the north-central states. Maximum forecast probabilities of 50-60%
are indicated over the northern and central High Plains, consistent with many
of the objective models and tools. Though the CFS is bullish on above-normal
temperatures over the Northeastern states, the constituent official forecasts
that make up the monthly period favor below-normal temperatures during the
first half of September switching to above-normal temperatures during the
second half of the month, with no clear mean temperature anomaly indicated.
Florida is another area where temperatures are uncertain, in this case, with
above-normal temperatures favored for the first half of September, changing to
Equal Chances (EC) of above-, near-, and below-normal temperatures for the
second half of the month. This essentially wipes out any preference favoring a
particular temperature category over Florida. Elsewhere, which includes a large
portion of the Contiguous U.S. (CONUS) and a substantial part of Alaska, EC is
favored.



The updated Precipitation Outlook for September features a tilt in the odds
towards above-normal precipitation from south-central and Southeast Alaska to
the Pacific Northwest. This is supported to various degrees by the 6-10 day,
8-14-day, and Weeks 3-4 Precipitation Outlooks. During Weeks 3-4, broad low
pressure over the northeastern Pacific favors widespread onshore flow across
these areas. Above-normal precipitation is also favored from central portions
of the Plains and Mississippi Valley southwestward across the Texas Panhandle
and much of New Mexico. This is supported by many of the model solutions,
especially over the central portions of the Plains and Mississippi Valley, tied
to expected frontal activity. The southwestward extension into New Mexico is
less likely (hence a minimal tilt of 33-40% toward above-normal precipitation).
This extension was included due to some of the model solutions and the
convergence of several possible factors, including back-door cold fronts and
associated upslope precipitation, late season monsoonal activity, and the
possibility of a gulf surge associated with any recurving east Pacific tropical
cyclones. The tropical cyclone possibility has some support from climatology
and lagged teleconnections from the MJO subseasonal signal, based on the main
enhanced convective envelope currently over Phase 3 (eastern Indian Ocean and
Maritime Continent). Though Arizona may experience above-normal precipitation
during the week-2 forecast period, it doesnt look like the expected total
monthly accumulation will be enough to warrant wetter-than-normal conditions
for September. Above-normal precipitation is also favored for the Gulf-,
Southeast-, and Mid-Atlantic coastal regions. This shows up in many of the
model solutions, and is consistent with what has been a persistent theme this
summer: stationary fronts and the resulting low-level convergence which focuses
the precipitation into elongated bands. A climatological factor which cannot be
ignored at this time of year is the peak month of the Atlantic hurricane season
in September. The long-track, Cape Verde-type storms generally begin to give
way to storms originating in the Gulf of America and Caribbean Sea during this
period. Elsewhere, there are no areas of favored below-normal precipitation in
the updated September Outlook over the CONUS or Alaska, due to many different
model scenarios. In addition, trying to pinpoint small-scale areas of slightly
better agreement where drier-than-normal conditions are possible is not
realistic for the monthly timescale. For the Hawaiian archipelago nearing the
end of its climatological dry season, below-normal precipitation amounts are
favored for September, along with drought expected to either persist or develop.



------------ Previous mid-month discussion is shown below ---------



El Nio Southern Oscillation (ENSO) neutral conditions are present with sea
surface temperatures (SSTs) near average across most of the equatorial Pacific
Ocean. The Madden-Julian Oscillation (MJO) is currently characterized by its
enhanced convective phase crossing the Indian Ocean, with the bulk of the
associated tropical rainfall located over the Maritime Continent region. The
latest MJO forecasts from dynamical models indicate considerable uncertainty in
the expected evolution of the MJO during the next several weeks.



The September 2025 Temperature and Precipitation Outlooks are based on a
combination of dynamical models, including CFSv2, the North American
Multi-Model Ensemble (NMME), the Copernicus Climate System (C3S), and to a
lesser extent statistical models/tools such as the Canonical Correlation
Analysis (CCA), Constructed Analog on Soil moisture (CAS), Optimal Climate
Normals (OCN), and a combined ENSO-OCN tool. The monthly outlooks are also
based on dynamical models and statistical tools for the Weeks 3-4 period which
cover the early and middle thirds of September. Uncertainty regarding the
evolution of the MJO precluded its use as a viable tool in the construction of
the September outlook. Impacts from the MJO will be re-evaluated at the end of
the month for the updated version of this Outlook. Local SSTs and antecedent
soil moisture anomalies also contributed to this outlook where applicable.



The September 2025 Temperature Outlook favors above-normal temperatures across
most of the Contiguous United States (CONUS). The regions of highest
probabilities (reaching 60 to 70%) are located over the upper Four
Corners/central Rockies area and over New England. For the former region over
the West which includes a broader 50-60% area encompassing much of the Interior
West, Rockies, and southern High Plains, the outlook is consistent with a
majority of the dynamical and statistical tools. A somewhat broader 50-60% area
is also favored over most of the Northeast, based on many of the dynamical
models/tools, historical trends (as indicated by the OCN tool), and to some
extent the proximity of relatively warm coastal waters of the western Atlantic.
Elsewhere, probabilities favoring above-normal temperatures are elevated
(50-60%) over most of the Florida Peninsula, based on various tools and a
strong trend signal. From the north-central CONUS to the Gulf and Southeast
coasts (excluding Florida), discrepancies between models and comparatively
weaker overall warm signals compared to other portions of the CONUS call for
modest probabilities tilting toward above-normal temperatures (33-40%). For the
coast of southern California, Equal Chances (EC) of above, near, and
below-normal temperatures are favored due to the proximity of near to
below-normal sea-surface temperatures (SSTs) near the coast. In Alaska,
above-normal temperature chances are elevated over the western and southern
Mainland, the Alaska Peninsula and eastern Aleutians, and Southeast Alaska.
This is largely attributed to the calibrated C3S tool, persistence of the area
of agreement between the ECMWF and GEFS Weeks 3-4 temperature tools, and
relatively warm coastal SSTs. The higher chances for above-normal temperatures
(50-60%) over northwestern portions of the Mainland are based on historical
trends that favor a delay in the onset of sea ice formation. For the remainder
of the Mainland and the western Aleutians, EC is favored.



The September 2025 Precipitation Outlook favors elevated odds of above-normal
precipitation from the eastern Gulf region (including Florida) across the
Southeast and Mid-Atlantic coast. Much of the dynamical and statistical
guidance warrants these elevated odds of above-normal precipitation, as does
the climatological peak of Atlantic tropical cyclone activity in September. In
addition, this enhanced wet signal is consistent with an increased likelihood
of stalled fronts.



For the Pacific Northwest, some (but not all) of the tools support increased
chances of above-normal precipitation, with September being the climatological
transition month from dry to wet season. From most of the Interior West,
Rockies, High Plains, extending eastward into the Ozarks odds favor
below-normal precipitation, with maximum probabilities of 50-60% over a sizable
multi-state area centered near the Great Salt Lake in northern Utah. To varying
degrees this is based on the NMME Consolidation, the CFSv2, and a few
constituent models of the C3S suite. The eastward extension of the relative
dryness across the south-central Plains and Ozarks area is based on recent soil
moisture anomalies. However, probabilities are modest given the uncertainty in
how much precipitation may fall over this region in September. Over far
southern portions of the Southwest a token EC is indicated due to the
possibility of a gulf surge or a recurving east Pacific hurricane bringing
precipitation into this area during the month of September. Well to the north,
above-normal precipitation is slightly favored over Southeast Alaska. Due to
conflicting indications among the dynamical and statistical models/tools, this
outlook for Alaska is based on persistence of the Weeks 3-4 precipitation
signals from the CFS, GEFS, and ECMWF models, the 30-day uncalibrated C3S, the
Meteo-France model, and the proximity of relatively warm SSTs. For the
remainder of Alaska, EC is favored for the precipitation outlook.

FORECASTER: Anthony Artusa

The climatic normals are based on conditions between 1991 and 2020, following
the World Meteorological Organization convention of using the most recent 3
complete decades as the climate 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.

The next monthly outlook...for Oct ... will be issued on Thu Sep 18 2025

These outlooks are based on departures from the 1991-2020 base period.
$$