354
FXUS21 KWNC 211838
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 21 2026

SYNOPSIS: Models continue to favor a long-duration extreme heat event from the
southern tier of the contiguous U.S. (CONUS) northward into the Central Plains
and the Tennessee, Middle Mississippi, and Lower Ohio Valleys. The greatest
risks are focused from the Desert Southwest into the Great Plains, Lower
Mississippi Valley, and some adjacent areas under amplified and long-lived
mid-level high pressure. Some areas may experience heat more typical of
mid-summer into the first week of September. Meanwhile, unusually hot and dry
conditions over the past few weeks are expected to continue through much of
week-2, creating a Rapid Onset Drought (ROD) risk for portions of the Central
and Southern Plains, and the Middle and Lower Mississippi Valley.

HAZARDS

High risk of extreme heat for much of the Desert Southwest, Southern Rockies,
Southern and Central Plains, and Lower Mississippi Valley, Sat-Sun, Aug 29-30.

Moderate risk of extreme heat for the Desert Southwest, Southern Rockies,
Southern and Central Plains, Lower Mississippi Valley, and some adjacent areas,
Sat-Tue, Aug 29-Sep 1.

Slight risk of extreme heat for portions of the California Valleys, the
southern Great Basin, the Central and Southern Rockies, most of the Plains, the
Middle and Lower Mississippi Valley, the Tennessee and Lower Ohio Valleys, the
eastern Gulf Coast region, and the South Atlantic region, Sat-Fri, Aug 29-Sep 4.

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

DETAILED SUMMARY

FOR MONDAY AUGUST 24 - FRIDAY AUGUST 28:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR SATURDAY AUGUST 29 - FRIDAY SEPTEMBER 04: Dynamical models begin the period
with an amplified subtropical ridge and an accompanying extreme heat risk
mainly focused from the Lower Mississippi Valley through the Southern Plains,
the Four Corners region, and parts of the Desert Southwest. Today, the European
ensemble (ECENS) mean and derived tools have shifted toward yesterday`s GEFS
output, depicting a stronger and more persistent mid-level ridge and thus a
more robust extreme heat signal that only gradually deamplifies over the course
of week-2. The Canadian ensemble (CMCE) mean 500-hPa height field is
considerably more suppressed with the mid-level ridge and shows much less
support for extreme heat. However, with the ECENS guidance shifting toward the
more-consistent GEFS in featuring a stronger and more persistent subtropical
ridge (and thus more robust support for extreme heat), the CMCE is now an
outlier and its weaker, cooler solution is not favored.



The GEFS and ECENS today both forecast an amplified 500-hPa ridge centered near
the southern High Plains that erodes slowly over the course of week-2, but with
a significant axis extending northward through the central CONUS. Today, the
ECENS holds on to larger positive 500-hPa height departures later week-2 than
the GEFS, but both configurations support the continuation of a long-lived heat
event over a large part of the central and southern CONUS through at least the
first half of week-2, and perhaps well into early September in some areas. GEFS
and ECENS means both show 500-hPa heights peaking at 596-597 dm early in the
period over or near the southeastern Four Corners region. Both also show
standardized 500-hPa height anomalies topping out 2-3 standard deviations above
climatology early week-2, consistent with a long-duration, major heat event.



The mid-level ridge is bookended by a pair of mean troughs over the eastern
Pacific and near the eastern CONUS. These features will wax and wane with time,
and may drift a little over the course of the period, but by and large the
favored ECENS and GEFS means maintain the same trough-ridge-trough pattern
throughout week-2, with slow deamplification expected by later week-2.



Underneath the amplified ridge, a high risk of extreme heat remains posted
early week-2 in most areas from the Lower Mississippi Valley through the Desert
Southwest, excepting some of the higher elevations in the Rockies. The ECENS
Probability of Extremes Tools (PET) is considerably more robust with chances
for extreme heat than yesterday, depicting a better than 60 percent chance for
maximum temperatures among the highest 15 percent of past occurrences across
the lower Four Corners region at the start of week-2. The GEFS PET shows a
stronger signal for most of week-2, as it has much of this summer, depicting
better than 60 percent chances for daily highs among the highest 15 percent of
climatology from eastern Arizona through the Red River (South) Valley and
nearby areas of the south-central Great Plains and Lower Mississippi Valley.
The high risk area is extended one day from yesterday`s ending date (now
through Sun Aug 30), and extends eastward across a larger area than the GEFS
PET would strictly indicate due to the expectation that high dew points will
elevate heat index values considerably above actual temperatures from the Lower
Mississippi Valley eastward. The high risk area also covers a larger area in
the Desert Southwest, where the ECENS and GEFS PETs start the period showing at
least a 15 percent chance for temperatures to exceed 110 deg F. In fact, the
operational European model (ECMWF) forecasts temperatures near 120 deg F in the
Lower Colorado Valley near the Mexican Border. A moderate risk of extreme heat
is posted across a larger area surrounding the high risk area, and continues
for one additional day compared to yesterday (now through Tue Sep 1). These
changes are consistent with the changes observed in most guidance derived from
the GEFS and ECENS. Given the continuing model tendency to slow down
deamplification and maintain more ridging with time near the end of week-2, a
slight risk of extreme heat is extended for the duration of week-2 (through Fri
Sep 4) across a large part of the central and southern CONUS. There is some
support for a moderate risk of extreme heat east of the central Gulf Coast
region, but models have not been consistent with themselves nor with each other
in handling the mid-level trough stretching southward along the East Coast.
This introduces considerable uncertainty in this region, precluding a moderate
risk of extreme heat any farther east than the central Gulf Coast region.



The episode of extreme heat is being exacerbated by feedback processes
associated with rapidly drying soils and drought within many of the highlighted
regions. The hot and dry conditions favored are also expected to aggregate
drying fuels, increasing chances for elevated fire weather conditions during
the period. In addition, a Rapid Onset Drought (ROD) risk is designated across
portions of the Central and Southern Plains and Middle and Lower Mississippi
Valley. This area has registered 30- to 45-day precipitation deficits greater
than 2 inches below normal and/or under half of normal, with less than an inch
expected during the next two weeks. Much of this ROD area is within the
moderate and high risk areas of extreme heat, and the resulting increase in
evapotranspiration rates are likely to accelerate surface moisture depletion.



Tropical Depression 2C in the Central North Pacific and is expected to pass
south of Hawaii as a tropical storm within the next week. The National
Hurricane Center (NHC) is anticipating heavy rainfall in association with this
system early next week across portions of the state, likely exacerbating
saturated ground conditions associated with heavy precipitation from the
passage of Hurricane Lala last week. The NHC is also following an area of
disturbed weather farther east, assessing an 80 percent chance of it becoming
at least a tropical depression by later next week. There is a considerable
amount of ensemble member spread in the forecast track of this feature, but
there is the potential for this system to become the third tropical cyclone in
a 3- to 4-week period to bring locally heavy to excessive rain and other
impacts to the state.

FORECASTER: Rich Tinker

$$