816
FXUS21 KWNC 131805
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 13 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
precipitation continues to be indicated across the Four Corners with elevated
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 and
the Middle and Lower Mississippi Valley, Fri, Aug 21.

Moderate risk of extreme heat for portions of the Central and Southern Plains,
Middle and Lower Mississippi and Tennessee Valleys, and Southeast, Fri-Sun, Aug
21-23.

Slight risk of extreme heat for portions of the Central and Southern Plains,
Middle and Lower Mississippi, Ohio, and Tennessee Valleys, Southeast, and
southern Mid-Atlantic, Fri-Thu, Aug 21-27.

Slight risk of extreme heat for portions of the Desert Southwest, Fri-Sun, Aug
21-23.

Slight risk of heavy precipitation for portions of the Four Corners, Northern
and Central Plains, and the Upper Mississippi Valley, Fri-Sun, Aug 21-23.

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

DETAILED SUMMARY

FOR SUNDAY AUGUST 16 - THURSDAY AUGUST 20:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR FRIDAY AUGUST 21 - THURSDAY AUGUST 27: 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 594-596dm 500-hPa height maximum over the
Southeast, Lower Mississippi Valley, and Southern Plains through the middle of
the period before slowly deamplifying or retrograding to the Southwest. 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.



Both the GEFS and ECMWF Probabilistic Extremes Tools (PETs) depict at least a
50 percent chance of maximum temperatures exceeding the 85th climatological
percentile at the start of week-2 centered over Arkansas, eastern Oklahoma and
Northern Louisiana, 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) depicts a number of locations with nighttime lows nearing and/or
breaking daily records, resulting in little overnight relief from the daytime
heat.



Therefore, a high risk of extreme heat is forecast for the Central and Southern
Plains and the Lower Mississippi Valley where heat index values may exceed 110
deg F, and is valid for Aug 21. The strongest signal continues to consolidate
over the Central and Southern Plains and the Middle and Lower Mississippi and
the high risk is correspondingly discontinued for the Southeast. The moderate
risk for extreme heat remains valid through Aug 23, and extends into the
Central Plains and Middle Mississippi Valley and through the Southern Atlantic
states where there remains elevated chances for heat index values to exceed 105
deg F. The ECENS based tools in particular support stronger extreme heat
chances for the Southeast relative to the GEFS. A slight risk, for all of
week-2, surrounds the moderate risk area.



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 of 3-day precipitation exceeding 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. 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 21-23. The southwest 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.



In the Southwest, there is a growing signal for extreme heat from the Week-1
period extending into the week-2 period. This is most evident in the
probabilistic HeatRisk tools from WPC which highlight 30-50% chances for major
HeatRisk over parts of the Desert Southwest. Ensemble guidance is finally
coming into better agreement regarding mid-level height anomalies building in
the Southwest and the West more generally. Therefore, a slight risk of extreme
heat is posted for the Desert Southwest, Aug 21-23.



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. The ROD shape was expanded today to include much of northern Louisiana
and the Mobile Bay area.

FORECASTER: Ryan Bolt

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