986
FXUS21 KWNC 171817
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 17 2026

SYNOPSIS: Dynamical models favor a potentially major and long duration extreme
heat event over the southern tier of the contiguous U.S. (CONUS), with the
greatest risk focused over the Southern and Central Plains underneath a potent
mid-level high pressure center next week.  This mid-level high pressure center
may slightly shift westward with time, bringing increased risks for extreme
heat across the Southern High Plains, Rockies, and Desert Southwest, with the
potential for cooler conditions over the southeastern CONUS later in week-2.
Meanwhile, given antecedent dryness, the above-normal temperatures and
sub-normal rainfall expected for 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

High risk of extreme heat for parts of the Southern and Central Plains, and
adjacent areas of the Mississippi Valley, Tue-Wed, Aug 25-26.

Moderate risk of extreme heat for much of the Central and Southern Plains,
Rockies, Desert Southwest, parts of the Middle and Lower Mississippi Valley,
and adjacent parts of the Southeast, Tue-Fri, Aug 25-28.

Slight risk of extreme heat for portions of the Southwest, Great Basin,
Rockies, Central and Southern Plains, Middle and Lower Mississippi and
Tennessee Valleys, and Southeastern CONUS, Tue-Mon, Aug 25-31.

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 20 - MONDAY AUGUST 24:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR TUESDAY AUGUST 25 - MONDAY AUGUST 31: A continuation of amplified
subtropical ridging and accompanying extreme heat conditions throughout the
southern tier of the U.S. remains the primary hazard threat in the updated
outlook. While the ECMWF maintains a more amplified mean solution of this
mid-level ridge feature compared to the GEFS and GEFS AI early, there is good
model agreement in regards to the location of the ridge center, where tools
show elevated chances of 500-hPa heights exceeding 597 dm from the eastern Four
Corners to the Lower Mississippi Valley in week-2.  A pronounced ridge axis is
also featured in the ensembles extending from the ridge center northward into
Canada, supportive of above-normal temperatures and possible extreme heat
conditions emerging over portions of the Interior West, Plains, and Mississippi
Valley next week.  Downstream, both the GEFS and ECWMF ensembles also continue
to feature more amplified troughing over the eastern CONUS by early next week,
where a higher potential for cooler temperatures look to quell extreme heat
risks over the Southeast.  While much of the anomalous 500-hPa troughing is
favored to weaken later in week-2, dynamical models generally maintain mean
troughing over the eastern U.S. Paired with another mean trough that is also
favored to persist over the eastern Pacific, the predicted mid-level feature
begins to resemble more of an Omega block presentation, implying a stable
pattern with little respite to the predominant ridging and summertime warmth
for much of central North America later in August.



Based on the latest temperature and heat tools, a high risk of extreme heat
remains posted in proximity to the amplified ridge center, and is slightly
expanded in coverage across the Great Plains and now valid through Aug 26.
Within the highlighted high risk area, calibrated heat tools continue to
highlight elevated chances for heat index values exceeding the 90th percentile,
 Probabilistic Extreme Tools (PETs) show at least 40-60% chances for maximum
temperatures exceeding the 85th percentile, and the National Blend of Models
(NBM) indicate a number of locations with daytime temperatures approaching
daily records. In addition, there is good model support for daytime
temperatures exceeding 105 deg F, with heat index values above 110 deg F in the
high risk area. Surrounding the high risk area, a moderate risk of extreme heat
remains posted based on lesser, but increased chances in the temperature and
heat tools exceeding the upper climatological percentiles. The moderate risk
area is likewise expanded to include portions of the Southern and Central High
Plains where there is an uptick in heat signals in the PETs and raw tools
relative to yesterday.  Conversely, coverage has been removed over parts of the
Mississippi Valley and Southeast, where cooler temperatures are favored due to
the aforementioned troughing early in the period. The moderate risk of extreme
heat is also extended through day 11 (Aug 28) based on the ECMWF and GEFS
ensembles favoring a slight westward shift of the mean ridge center (595 and
594 dm heights, respectively) towards the middle of week-2. A broad slight risk
area of extreme heat is also posted for all of week-2, with much of its
coverage reshaped to include (exclude) portions of the High Plains and Interior
West (southeastern CONUS). Portions of the Northern Intermountain and Northern
Plains were considered for inclusion in the slight risk area based on stronger
support in the GEFS based heat tools, however due to decreasing sun angles over
the higher latitudes of the CONUS during late August, there is less confidence
for temperatures reaching hazardous thresholds.



No precipitation related hazards are issued in the updated outlook. Areas that
are expected to receive increased precipitation reside along and ahead of the
mean trough axes favored over the portions of the western and eastern CONUS.
While there is some support for a heavy precipitation risk early in week-2 over
parts of the Mid-Atlantic and Southeast in the GEFS, the ECMWF, Canadian and
GEFS AI favor drier conditions, precluding any heavy precipitation hazards at
this time. The region will continue to be monitored for heavy precipitation
risks in upcoming outlooks associated with any stalled frontal activity
favored.  Across the Desert Southwest, tools are rather muted in regards to any
influx of monsoonal moisture to support a heavy precipitation risk.  It is
worth noting that tools favor an uptick in tropical cyclone activity over the
eastern Pacific during the next few weeks, as any transient tropical
disturbances could trigger a Gulf of California surge event, and bring a heavy
precipitation risk during the period.  And despite the current lack of support
in the tools, isolated areas of heavy rain remain possible, and this region is
prone to flash flooding even with relatively low rainfall totals along dry
creek beds (arroyos) and in canyons. Caution should always be exercised in such
areas if thunderstorm activity is initiated nearby. Also, some convective
activity may result in little or no 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. Based on
the latest soil moisture conditions, the ROD area is expanded to include parts
of southeastern Texas and southern Louisiana to the Gulf coast in the updated
outlook. 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: Nick Novella

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