617
FXUS21 KWNC 101822
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 10 2026

SYNOPSIS: A major, long duration period of extreme heat continues to be
predicted across portions of the south-central contiguous U.S. (CONUS) and
Southeast persisting into week-2 tied to anomalous mid-level high pressure
combined with high humidity levels. Model guidance has continued to trend
stronger with this event supporting an expansion of the high risk area into
more of the Central Plains, Middle Mississippi Valley, and Southeast compared
to yesterday. Antecedent dryness, combined with little precipitation favored
over the next two weeks supports a Rapid Onset Drought (ROD) risk for portions
of the Central and Southern Plains and Middle and Lower Mississippi Valley.
Enhanced monsoonal activity continues to be indicated across the Four Corners,
although with less certainty with downstream progression across the Midwest as
some of the stronger signals have now shifted into week-1.

HAZARDS

High risk of extreme heat for portions of the Central and Southern Plains,
Middle and Lower Mississippi Valley, and Southeast, Tue-Fri, Aug 18-21.

Moderate risk of extreme heat for portions of the Central and Southern Plains,
Middle and Lower Mississippi and Tennessee Valleys, Southeast, and southern
Mid-Atlantic, Tue-Sat, Aug 18-22.

Slight risk of extreme heat for portions of the Central and Southern Plains,
Middle and Lower Mississippi and Tennessee Valleys, Southeast, and southern
Mid-Atlantic, Tue-Mon, Aug 18-24.

Slight risk of heavy precipitation for portions of the Four Corners, Tue-Sat,
Aug 18-22.

Rapid Onset Drought (ROD) risk for portions of the Central and Southern Plains
and the Middle and Lower Mississippi Valley.

DETAILED SUMMARY

FOR THURSDAY AUGUST 13 - MONDAY AUGUST 17:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR TUESDAY AUGUST 18 - MONDAY AUGUST 24: Dynamical model guidance is in good
agreement regarding subtropical ridging persisting across the south-central and
southeastern U.S. into week-2. The 0z ECMWF, GEFS, and Canadian ensembles
depict a 594-596 dm 500-hPa height maximum slowly retrograding from the
Southeast to the Southern Plains throughout the period, with its amplitude
steady or even increasing slightly. As a result, confidence remains high for a
long duration heat event, with many tools indicating the potential for
oppressive conditions lingering well into late August. Both the GEFS and ECMWF
Probabilistic Extremes Tools (PETs) depict at least a 60 percent chance of
maximum temperatures exceeding the 85th climatological percentile at the start
of week-2 centered across the Central Gulf Coast states, and remaining above 40
percent for most of the period. The ECMWF PET is generally more east-focused
with a greater extension of higher probabilities into the Southeast, with the
GEFS favoring higher odds over the Southern Plains and Lower Mississippi Valley
tied to quicker ridge retrogression. A large contribution from increased
humidity is expected, with a strong signal in the calibrated skill-weighted
GEFS-ECMWF heat risk guidance, particularly across the Central and Southern
Plains and adjacent Mississippi Valley where probabilities exceed 60 percent
for maximum heat index values to exceed the 95th climatological percentile. The
high risk of extreme heat is extended into the Central Plains, Middle
Mississippi Valley, and the Southeast, where heat index values may exceed 110
deg F, and is valid Aug 18-21. However, the retrogression of the ridge may
allow some of the highest heat to shift out of the Southeast by the middle of
week-2 consistent with the evolution in the PETs. In addition, the National
Blend of Models (NBM) also depicts a number of locations with nighttime lows
nearing and/or breaking daily records, where little overnight relief to daytime
heat is expected. The moderate risk for extreme heat is now extended through
Aug 22 and is also extended slightly northward where there is increased
potential for heat index values to exceed 105 deg F. A slight risk surrounds
the moderate risk area and continues for all of week-2. The GEFS is more robust
in depicting increasing heat expanding into the Northern Plains later in
week-2. Since the ECMWF and Canadian ensembles keep the heat across the Central
Plains and south, no northward expansion is indicated in today`s outlook but
trends will continue to be monitored.



Troughing initially near the West Coast of the CONUS favors an influx of
moisture across much of the western U.S., and enhanced Southwest Monsoon
activity continuing over the Four Corners where the GEFS and ECMWF PETs depict
at least a 20 percent chance 3-day precipitation exceeds the 85th
climatological percentile and a half-inch justifying a slight risk of heavy
precipitation, Aug 18-22. This region is prone to flash flooding even with
relatively small precipitation accumulations especially along dry creek beds
(arroyos) and in canyons. Caution should always be exercised in such areas
should thunderstorm activity initiate nearby. Some of this moisture is likely
to propagate overtop the ridge axis over the south-central and southeastern
CONUS and lead to enhanced frontal or Mesoscale Convective Systems (MCSs)
across the Midwest, Great Lakes, and possibly into the Northeast or
Mid-Atlantic. However, the PETs are not indicating any enhanced signal for
hazardous precipitation, and uncalibrated guidance, particularly from the 0z
ECMWF, indicates a stronger chance of a shortwave moving through late in
week-1. While it is not out of the question for additional activity during
week-2, confidence is no longer high enough to support a slight risk of heavy
precipitation over these areas.



Toward the middle of week-2, troughing along the West Coast is favored to
retreat with rising heights across the West tied to expansion of the ridging
over the south-central CONUS. This evolution would decrease the moisture
advection into the West, and also promote an increase in above-normal
temperatures and possibly extreme heat across portions of the Interior West
where the GEFS PET depicts increasing coverage of probabilities greater than 20
percent for maximum temperatures to exceed the 85th climatological percentile.
However, due to lower probabilities in the ECMWF PET, no related extreme heat
hazards are posted across the West at this time.



A Rapid Onset Drought (ROD) risk remains designated across portions of the
Central and Southern Plains and Middle and Lower Mississippi Valley. This ROD
area continues to register 30-day precipitation deficits greater than 2 inches,
with little rainfall forecast during the next two weeks. Much of this ROD area
is also included within the moderate risk of extreme heat as the potential for
increased evapotranspiration rates also looks to reinforce this risk later in
August.



Across the eastern Pacific, the National Hurricane Center (NHC) is monitoring
three areas located near 140 deg W, 130 deg W, and 115 deg W, with 90 percent,
60 percent, and 10 percent chances of tropical cyclone (TC) development,
respectively, during the next seven days (as of 11am PDT). For the western
watch area near 140 deg W, both dynamical model and Google AI guidance are
indicating a clustering of tracks near or just to the south of the Hawaiian
Islands, although potential impacts are shifting more into the late week-1
timeframe. However, the GEFS and ECMWF PETs continue to depict at least a 20
percent chance 3-day precipitation totals exceed the 85th climatological
percentile early in week-2 across most of the state, and interests across
Hawaii are advised to monitor the latest forecasts from NHC
(https://www.nhc.noaa.gov/).

FORECASTER: Thomas Collow

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