147
FXUS21 KWNC 151819
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 15 2026

SYNOPSIS: A major, long duration period of extreme heat is expected to persist
into week-2 across the central CONUS and a large part of the Nation`s southern
tier. The highest chances for extreme heat cover the Central and Southern
Plains and Lower Mississippi Valley, but at least a slight risk covers most of
the southern tier of the Nation from the Desert Southwest to the South Atlantic
Coast. Anomalous mid-level high pressure forecast near the Four Corners is
forecast in concert with an incursion of Gulf moisture, producing the high heat
indices. This mid-level high pressure is favored to drift slowly westward or
southwestward while deamplifying, ending the extreme heat risk late week-2.
Meanwhile, given antecedent dryness, the above-normal temperatures and
subnormal rainfall expected over the next two weeks support a Rapid Onset
Drought (ROD) risk for portions of the Central and Southern Plains and Middle
and Lower Mississippi Valley.

HAZARDS

Moderate risk of extreme heat for much of the Central and Southern Plains, and
parts of the Lower Mississippi and adjacent Tennessee Valleys, Sun-Mon, Aug
23-24.

Slight risk of extreme heat for portions of the Southwest, Southern Rockies,
Central and Southern Plains, Lower Mississippi and Tennessee Valleys, and
Southeastern CONUS, Sun-Thu, Aug 23-27.

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

DETAILED SUMMARY

FOR TUESDAY AUGUST 18 - SATURDAY AUGUST 22:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR SUNDAY AUGUST 23 - SATURDAY AUGUST 29: Dynamical models remain in good
agreement regarding subtropical ridging across the south-central to
southwestern U.S. through most of week-2. The 0z European ensemble (ECENS),
GEFS, and Canadian ensemble (CMCE) means all start the period with a 596 dm
500-hPa height maximum centered near the south-central CONUS or eastern Four
Corners region. The ECENS and GEFS are in excellent agreement placing the
center of this feature over northern or eastern New Mexico while the CMCE mean
depicts this feature over the Red River (South) Valley, but the slightly
farther west placement is favored since it is the majority solution, and more
consistent with earlier runs. Meanwhile, ensemble means depict a somewhat
stronger and more persistent mid-level trough over the northeastern CONUS
relative to yesterday. The CMCE mean is strongest with this feature and the
ECENS mean the weakest, with the GEFS mean splitting the difference. The GEFS
mean, which approximately equals the average of all three ensemble means, is
favored with this mid-level trough.



Over the course of week-2, the 500-hPa height center remains roughly in place
while flattening and deamplifying. This process is expected to increase 500-hPa
heights in the Southeast for a day or two early in the period before the
mid-level trough to the north is reinforced, causing heights there to drop
again, and remain near normal for the last half of week-2. Confidence remains
high for a long duration heat event, with many tools indicating the potential
for oppressive conditions lingering into late August. But with a stronger, more
persistent mid-level trough forecast over the Northeast with an axis southward
along the coast, the greatest risk of extreme heat is focused a bit farther
west, over the Lower Mississippi Valley and Southern Plains early week-2. A
moderate risk of extreme heat is posted there for the first two days of the
period (Sun-Mon Aug 23-24). The broadening mid-level ridge, however, increases
the chances for at least a brief period of extreme heat across a broad area
stretching from the Desert Southwest to the lower South Atlantic Coast until
later week-2. By Fri-Sat Aug 28-29, the ensemble means all show the height
center of the mid-level ridge remaining over the eastern Four Corners Region or
sinking slightly southward into far northern Mexico while the greatest 500-hPa
heights drop to around 591 dm, which is expected to end the risk of extreme
heat.



Both the GEFS and ECENS Probabilistic Extremes Tools (PETs) depict at least a
40 percent chance of maximum temperatures exceeding the 85th climatological
percentile early week-2 over the Red River (South) Valley and Southern High
Plains. The GEFS PET is far more extensive with its depiction of elevated
chances for extreme heat, with odds topping 40 percent from the Central and
Southern Rockies through the Desert Southwest, Southern Plains, and Lower
Mississippi Valley. Given the strength, position, and size of the mid-level
ridge driving the hot weather, the higher likelihoods depicted in the GEFS PET
are preferred. After day 9 (Mon Aug 24), the ECENS PET shows the chances for
extreme heat dropping below 40 percent throughout the CONUS. This marks the end
of the moderate risk for extreme heat in the south-central CONUS, but higher
odds in the GEFS PET remain until later in week-2, and a slight risk for
extreme heat is maintained across the southern tier of the Nation until the
GEFS PET shows chances for extreme heat dropping below 40 percent. This is also
the point when the ensemble means all show the mid-level ridge beginning to
more quickly deamplify and weaken.



Increased humidity is expected to contribute to extreme heat index values from
the Southern Plains eastward, 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 in some
areas exceed 70 percent for maximum heat index values to exceed 105 deg F. The
likelihood of heat index values topping 110 deg F exceed 50 percent in South
Texas and the Lower Colorado Valley, and ranges from 20 to 40 percent across
Oklahoma, southernmost Kansas, Louisiana, Mississippi, Arkansas, and northern
Texas. In addition, the National Blend of Models (NBM) depicts nighttime lows
nearing or breaking daily records early week-2 from southern Florida to
southern California, resulting in little overnight relief from daytime 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 over the Four Corners, especially the higher elevations of Colorado.
However, the trend toward reduced signal strength has continued today, so no
hazards are posted in association with potentially heavy monsoon-related
convective activity; however, isolated areas of heavy rain remain possible, and
this region is prone to flash flooding even with relatively small precipitation
totals along dry creek beds (arroyos) and in canyons. Caution should always be
exercised in such areas should thunderstorm activity initiate nearby. Also, any
convective activity may result in relatively little rainfall while still being
capable of generating lightning, which poses a risk of wildfire initiation.



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
(near or under half of normal), with little rainfall expected during the next
two weeks. Furthermore, much of this ROD area is within the moderate risk of
extreme heat. This could potentially increase evapotranspiration rates and
accelerate surface moisture depletion.

FORECASTER: Rich Tinker

$$