898
FXUS21 KWNC 181811
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 18 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
is favored to shift westward with time, bringing increased risks for extreme
heat across the Southern High Plains, Rockies, and Desert Southwest. Meanwhile,
due to antecedent dryness, above-normal temperatures and below-normal rainfall
forecast during next two weeks supports 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 Desert Southwest, Southern Rockies,
Southern and Central Plains, and adjacent areas of the Mississippi Valley,
Wed-Fri, Aug 26-28.

Moderate risk of extreme heat for much of the Desert Southwest, Rockies,
Central and Southern Plains, parts of the Middle and Lower Mississippi Valley,
Wed-Sun, Aug 26-30.

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, Wed-Tue, Aug 26-Sep 1.

Slight risk of heavy precipitation for the coastal Mid-Atlantic and Southeast,
Wed-Fri, Aug 26-28.

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

DETAILED SUMMARY

FOR FRIDAY AUGUST 21 - TUESDAY AUGUST 25:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR WEDNESDAY AUGUST 26 - TUESDAY SEPTEMBER 01: Since yesterday, the hazards
perspective remains on track as there continues to be elevated chances for
extreme heat conditions persisting into the week-2 period throughout the
southern tier of the CONUS. Dynamical models remain in good agreement and are
bullish with an amplified subtropical ridge center retrograding from the
Southern Plains into the eastern Four Corners region, where models maintain
some signal for 500-hPa heights exceeding 597 dm throughout much of the period.
One notable change since yesterday concerns the GEFS, which has been
consistently weaker with the subtropical ridging in relation to the ECMWF, now
favors a mean solution that is nearly equal in strength early in the period.
Given this, and the persistence of strong 500-hPa height signals in the
guidance, there is higher outlook confidence for a long duration and
potentially stronger heat event unfolding next week. In addition to the
subtropical ridging, a strong ridge axis is also featured in the ensembles
extending 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. Bookended by a pair of mean troughs
over the eastern Pacific and eastern CONUS, the 500-hPa mean height forecasts
reveal more of an Omega block presentation, implying the potential for stable
pattern with little respite to the predominant ridging and summertime warmth
for much of central North America later in August and possibly into early
September.



In proximity to the amplified subtropical ridge center, a high risk of extreme
heat remains posted and is slightly expanded to include additional portions of
the Southern Plains, and westward into New Mexico and southern Arizona, now
valid through day 10 (Aug 28). These adjustments are based on stronger and more
expansive temperature anomalies that are favored to persist into late next
week, as well as the persistence of elevated maximum temperature signals in the
Probabilistic Extreme Tools (PETs). Within the highlighted high risk area,
calibrated heat tools continue to highlight >60% chances for daytime
temperatures and/or heat index values exceeding the 90th percentile, where
there is good model support for daytime temperatures above 100 deg F (except
for higher elevations) with heat index values peaking above 110 deg F where
higher dewpoints are expected. Surrounding the high risk area, a moderate risk
of extreme heat remains posted (Aug 26-30) based on lesser, but increased
chances in the temperature and heat tools exceeding the upper climatological
percentiles. The moderate risk area is inclusive of locations in the National
Blend of Models which remain forecast to approach or break daytime highs and
nighttime lows, and where probabilistic guidance indicates elevated chances for
double digit daytime temperature departures at least through the middle of
week-2 before ensembles weaken the mean ridge center (below 594 dm). A broad
slight risk area of extreme heat remains posted for all of week-2 which
includes much of the southern tier of the CONUS, with its northern extent
capped to the Central Rockies and Central Plains. Despite the better support
for anomalously warm temperatures over the northern tier of the CONUS, there is
less confidence for temperatures reaching hazardous thresholds due to
decreasing sun angles over the higher latitudes during late August, which is
also supported in the heat index guidance depicting a limited potential for
values exceeding 100 deg F.



The extensions of both the high and moderate risks of extreme heat later in
August are also supported by feedback associated with rapidly drying soils and
drought being registered within the highlighted regions. The hot and dry
conditions favored are also expected to aggregate fuels and bring increased
chances for elevated fire weather conditions during the period. In addition, a
Rapid Onset Drought (ROD) risk also 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 expected during the next two weeks. Much of this ROD area
is within the moderate risk of extreme heat as increased evapotranspiration
rates look to accelerate surface moisture depletion.



For precipitation, the highest chances for above-normal precipitation reside
along and ahead of the mean trough axes favored over the portions of the
western and eastern CONUS. Relative to yesterday, daily ensembles and PETs show
some increased support for a heavy precipitation risk over coastal portions of
the Mid-Atlantic and the Southeast associated with stalled frontal activity.
Although raw tools show the highest amounts remaining offshore, both the GEFS
and ECMWF PETs are in agreement featuring at least 20% chances for 3-day
amounts exceeding the 85th percentile and 1 inch. Therefore, a slight risk of
heavy precipitation is issued from coastal South Carolina to the DelMarVa
region for Aug 26-28.



Across the Desert Southwest, the aforementioned subtropical ridge center is
forecast to be in a less favorable position for the advection of monsoonal
moisture. However, an uptick in tropical cyclone activity is expected over the
eastern Pacific in the coming weeks, where any transient tropical disturbances
nearing the Gulf of California could trigger a gulf surge event. Even with this
potential, precipitation tools remain rather muted at this time resulting in no
precipitation related hazards. 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.



In addition to the eastern Pacific, additional tropical cyclone formation is
also favored in the central Pacific during the next two weeks which is
highlighted in the National Hurricane Center (NHC) Tropical Weather Outlook
(TWO) for week-1 (https://www.nhc.noaa.gov/gtwo.php?basin=cpac&fdays=7), as
well as in the latest Global Tropics Hazards (GTH) Outlook for week-2
(https://cpc.ncep.noaa.gov/products/precip/CWlink/ghaz/index.php). Should any
development occur with a track in the vicinity of the Hawaiian, additional
rainfall is likely to exacerbate saturated ground conditions following the
passage of Hurricane Lala to the south of the Big Island this past week, as
well as possibly bring other adverse impacts to the state.

FORECASTER: Nick Novella

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