125
FXUS21 KWNC 121816
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 12 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 support a
very strong heat event across the south-central and southeastern CONUS.
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 with enhanced
chances for heavy precipitation extended into parts of the Northern and Central
Plains and Upper Mississippi Valley.

HAZARDS

High risk of extreme heat for portions of the Central and Southern Plains,
Middle and Lower Mississippi Valley, and Southeast, Thu-Fri, Aug 20-21.

Moderate risk of extreme heat for portions of the Central and Southern Plains,
Middle and Lower Mississippi and Tennessee Valleys, and Southeast, Thu-Sun, Aug
20-23.

Slight risk of extreme heat for portions of the Central and Southern Plains,
Mississippi, Ohio, and Tennessee Valleys, Southeast, and southern Mid-Atlantic,
Thu-Wed, Aug 20-26.

Slight risk of heavy precipitation for portions of the Four Corners, Northern
and Central Plains, and the Upper Mississippi Valley, Thu-Sat, Aug 20-22.

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

DETAILED SUMMARY

FOR SATURDAY AUGUST 15 - WEDNESDAY AUGUST 19:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR THURSDAY AUGUST 20 - WEDNESDAY AUGUST 26: Dynamical model guidance remains
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 over the
Southeast, Lower Mississippi Valley, and Southern Plains through the middle of
the period before slowly deamplifying. As a result, confidence remains high for
a continuation of the long duration heat event through the middle of week-2. By
the end of week-2, the mid-level pattern becomes more uncertain with weaker
anomalies. This may just be a function of increased spread among the ensemble
guidance and continuation of the heat into week-3 remains possible.



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 through the middle of week-2. 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. In addition, the National Blend of Models (NBM)
also depicts a number of locations with nighttime lows nearing and/or breaking
daily records, resulting in little overnight relief from the daytime heat.



A high risk of extreme heat is forecast for the Central and Southern Plains,
Lower Mississippi Valley, and the Southeast, where heat index values may exceed
110 deg F, and is valid Aug 20-21. The strongest signal is reduced over the
southern Atlantic states and the high risk is correspondingly pulled back from
South Carolina and Georgia. The moderate risk for extreme heat is valid for an
additional day relative to prior forecasts, through Aug 23, and extends into
the Central Plains and Middle Mississippi Valley where there is increased
potential for heat index values to exceed 105 deg F. A slight risk, for all of
week-2, surrounds the moderate risk area and continues extends into the Upper
Mississippi Valley where some tools are suggesting at least a brief chance of
extreme heat.



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 early in week-2. Most of the guidance
suggests a reduction in the chances for heavy precipitation across the Four
Corners by the middle of week-2. However, some physical and machine learning
guidance favors elevated chances for heavy precipitation into portions of the
Plains and Mississippi Valley. Therefore, the slight risk is expanded towards
the Great Lakes region from the Southwest. Most tools suggest the heavy
precipitation risk times off by the middle of week-2. Therefore, a slight risk
of heavy precipitation is posted for portions of the Desert Southwest, Great
Basin, the Southern and Central Rockies, Central and Northern Plains, and Upper
Mississippi Valley, valid Aug 20-22. 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.



Toward the middle of week-2, troughing along the West Coast is favored to
retreat with rising heights across western Canada tied to the weakening of the
mid-level ridge over the southeastern U.S. At this time, tools do not favor
elevated extreme heat chances over the West but will be monitored.



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.

FORECASTER: Ryan Bolt

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