768
FXUS21 KWNC 071914
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT September 07 2025

SYNOPSIS: The favored development of strong mid-level high pressure across
central North America next week is anticipated to result in a pattern change
from week-1, with widespread above-normal temperatures and possible extreme
heat conditions emerging from the central contiguous U.S. (CONUS) to the
Appalachians during week-2. The abnormally warm temperatures, combined with
antecedent dryness and below normal precipitation favored during the next two
weeks support a Rapid Onset Drought risk over parts of Mississippi and Alabama.
Lingering mid-level low pressure forecast over the western third of the CONUS
early in the period favors increased chances for episodes of high winds over
the West Coast and parts of the Interior West. A stalled frontal boundary
remains forecast across the southeastern U.S. bringing an increased likelihood
for enhanced precipitation and high winds. Surface low pressure is forecast to
bring increased chances of elevated wind speeds to southern and southeastern
coastal Alaska.

HAZARDS

Slight risk of extreme heat across portions of the eastern Great Plains, Middle
and Lower Mississippi, Tennessee, and Ohio Valleys, and southern Great Lakes,
Mon-Thu, Sep 15-18.

Slight risk of high winds over the West Coast and portions of the Interior
West, Mon-Wed, Sep 15-17.

Slight risk of heavy precipitation across the Florida Peninsula and coastal
Southeast, Mon-Fri, Sep 15-19.

Slight risk of high winds across the central and eastern Gulf Coast, and the
lower Eastern Seaboard, Mon-Fri, Sep 15-19.

Slight risk of high winds across coastal portions of southern and southeastern
Alaska, Mon-Fri, Sep 15-19.

Rapid Onset Drought risk across portions of the Lower Mississippi and Tennessee
Valleys.

DETAILED SUMMARY

FOR WEDNESDAY SEPTEMBER 10 - SUNDAY SEPTEMBER 14:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR MONDAY SEPTEMBER 15 - SUNDAY SEPTEMBER 21: By the outset of the period,
there is good agreement between the 0z GEFS and ECWMF ensemble mean 500-hPa
height forecasts, featuring a strongly amplified ridge over central Canada,
with a pair of mean troughs, one situated over the Gulf of Alaska and the other
oriented along the eastern Seaboard of the U.S. Consistent with previous
guidance, the ECMWF continues to be most bullish with the Canadian ridge,
favoring a mean height anomaly center in excess of 240 dam along and to the
west of Hudson Bay late in week-1 and persisting into early week-2. In response
to this predicted mid-level pattern, above-normal temperatures are
predominantly favored for much of the central CONUS, with end of summer heat
conditions possible for many parts of the Midwest. With this blocking ridge
expected to have some staying power in the longer leads, guidance continues to
favor a deamplification of the Gulf of Alaska trough with above-normal heights
building back over western North America. Though, the GEFS and ECMWF differ
somewhat on this evolution, as the latter model shows hints of continued
shortwave troughing which could bring additional shots of enhanced onshore flow
and unsettled weather into the northwestern CONUS.



Based on the latest raw and calibrated temperature tools, a slight risk of
extreme heat remains posted across parts of the eastern Plains, Mississippi,
Tennessee and Ohio Valleys, and southern Great Lakes, valid through Sep 18.
Within the highlighted region, Probabilistic Extremes Tools (PETs) indicate at
least a 20% chance for maximum daytime temperatures exceeding the 90th
climatological percentile, with increased chances for temperatures exceeding 90
(95) degrees F over the Middle Mississippi and Ohio Valleys (Lower Mississippi
and Tennessee Valleys). Although these PET heat signals extend into the Upper
Midwest closer to the Canadian ridge, daytime temperatures are not expected to
reach hazardous thresholds due to lowering sun angles at these higher latitudes
with the autumn equinox soon approaching. Relative to yesterday, the GEFS PET
has become more robust with the heat signals (upwards of 40% chances for
exceeding the 90th percentile) supportive of a higher risk designation, however
the uncalibrated GEFS and ECMWF PET are comparatively milder with the heat
potential. In addition to the above-normal temperatures, the combination of
favored below-normal precipitation over this part of the country supports a
Rapid Onset Drought (ROD) risk, and remains posted mainly over portions of
Mississippi and Alabama. Within this area, many local areas are registering
less than half their normal rainfall accumulation over the past 60 days.



Tied to shortwave troughing that is favored to encroach the western CONUS early
in week-2,  both the GEFS and ECMWF PETs indicate increased chances for winds
speeds exceeding the 85th percentile, and a slight risk of high winds remains
posted for the West Coast, and into portions of the Interior West. This slight
wind risk is extended through Sep 17 in the updated outlook before positive
mid-level height departures generally begin to overspread the western CONUS by
the middle of week-2. Any additional episodes of high winds could prolong the
risk for wildfire development due to very low soil moisture values being
registered for many parts of the West. Associated with more onshore flow
favored in the ECMWF height forecasts, this is likely contributing to elevated
(>40%) precipitation signals in the ECMWF PET for 3-day amounts exceeding the
85th percentile across many parts of the northwestern CONUS. By comparison, the
GEFS PET remains much more muted with this potential, and given that these
signals in percentile space are more the result of low precipitation
climatologies in the northwestern CONUS, with actual amounts favored to remain
below hazardous thresholds, no corresponding precipitation hazards are issued.
If anything, any rainfall above climatology would provide needed relief to many
areas in the northwestern CONUS that are experiencing moderate to extreme
drought conditions based on the latest U.S. drought monitor.



Over the southeastern CONUS, WPC days 3-7 surface prognosis maps depict a
stalled frontal boundary located over the Gulf of America and into the western
Atlantic, which looks to be the focus for enhanced and possibly heavy
precipitation during week-1. This surface feature is expected to persist well
into week-2, with the ECMWF (GEFS) PETs continuing to favor a more southward
(northward) solution of the heavy precipitation potential early in week-2. By
the middle of week-2, the uncalibrated and calibrated ECMWF tools begin to
shift the precipitation risk back into Florida, becoming better aligned with
the GEFS PET with 3-day amounts exceeding the 85th percentile and 1 inch.
Therefore, a slight risk of heavy precipitation remains posted mainly over the
Florida peninsula, and now valid through Sep 19. Due to continuous enhanced
precipitation over the state during the next two weeks, any heavy rainfall may
trigger isolated flash flooding during the week-2 period. The wind PETs also
continued to support a broader slight risk area of high winds from the central
Gulf to the lower Mid-Atlantic. Consistent with previous thinking, surface low
formation along the frontal boundary cannot be ruled out, which if realized,
could bring a renewed threat of rainfall and high winds later in week-2.



Since yesterday, a tropical disturbance (Invest 91L) located over the Main
Development Region (MDR) has dissipated and the National Hurricane Center is
indicating no other areas in the tropical Atlantic for potential development as
of 1:00 PM EDT. In the eastern Pacific, the ECMWF probabilistic tropical
cyclogenesis tool continues to favor additional tropical cyclone development to
the south and west of Mexico, which could possibly trigger a Gulf of California
surge event into the Desert Southwest in week-2. The ECWMF favors an
enhancement of mid-tropospheric moisture into the lower Four Corners, along
with increased chances (20-30%) for 3-day precipitation amounts exceeding the
85th percentile through the middle of week-2. However, the GEFS remains largely
unsupportive of a heavy precipitation risk, resulting in no precipitation
related hazards, though this potential will continue to be monitored in
upcoming outlooks.



The mean troughing favored over the Gulf of Alaska, and associated mean surface
low pressure is expected to bring enhanced precipitation with potentially
hazardous high winds to parts of south-central and southeastern coastal Alaska
during week-2. Consistent with previous guidance, the ECMWF PET depicts
elevated chances (40%-60%) of wind speeds exceeding the 85th climatological
percentile and 40-mph across southern and southeastern coastal Alaska, whereas
the GEFS PET remains more limited with this potential.  While a moderate risk
was considered for the outlook, only a slight risk of high winds remains
designed for this part of Alaska for Sep 15-19 due to weaker mean surface
pressure favored and troughing aloft compared to yesterday. A slight risk of
heavy precipitation was also considered for inclusion based on increased
signals in the PETs, however there is less confidence that actual totals will
exceed hazardous thresholds and this potential will be reassessed in upcoming
outlooks.

FORECASTER: Nick Novella

$$