350
FXUS02 KWBC 182001
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
401 PM EDT Tue Aug 18 2026

Valid 12Z Fri Aug 21 2026 - 12Z Tue Aug 25 2026


...Hazardous heat will lessen in the Southeast but will affect the
south-central U.S. and Southwest...


...Overview...

During the extended period an upper high over the Four Corners
region will slowly expand northward into the Intermountain West
while shifting eastward into the High Plains. This will continue to
support widespread heat/hazardous heat from the Southwest to the
Plains/Lower Mississippi Valley. Monsoonal moisture is forecast to
stream north under the ridge for scattered convection across the
West. Meanwhile, troughing is set to slowly deepen in the East,
lessening heat concerns across the Southeast while rain and
thunderstorms are likely across the Eastern Seaboard through the
weekend.


...Guidance/Predictability Assessment...

The latest suite of guidance show good agreement with the
anchoring upper ridge/high through the period and in addition to
the gradual deepening trough across the East. There will be
multipleimpulses/shortwave traveling through the trough that
will affect frontal positions and sensible weather, and the
specifics are still a bit uncertain given model variability. This
is initially evident with some differences in where a rainfall axis
sets up in the Mid- Atlantic and Carolinas Friday and Saturday,
including questions about if it will be on or offshore Saturday.
Then, models are relatively more agreeable now that a closed low
should develop within the trough by Sunday-Monday in the Great
Lakes and northward. The 00Z GFS now indicates a closed low as
well, though its older runs showed an open and more progressive
trough. Meanwhile the recent 00Z EC/CMC maintain their closed low
but are farther north and more progressive than their previous
runs, so closer to a compromise with the GFS. However, the AIGFS
and AIFS are still on the slower and farther south side, lingering
across the eastern Great Lakes/Northeast into Monday, so the EC/CMC
faster trend is uncertain.

Elsewhere, AI and dynamical model guidance agree on a shortwave
moving from the eastern Pacific into the Northwest Saturday, but
timing differences remain a challenge. The GFS continues to be more
progressive than consensus as the shortwave moves quickly across
southern Canada Sunday- Monday and toward the Upper Midwest
Tuesday. A trough emerging from the northeast Pacific and
potentially reaching the Pacific Northwest by Tuesday also has some
timing spread.

Thus, the WPC forecast used a multi-model (dynamical and AI) blend
for the early to mid part of the period, and used some ensemble
means in increasing proportions for the later period as spread
increased with the details of the pattern.


...Weather/Hazards Highlights...

Hot conditions are forecast all across the southern tier into
Friday under upper ridging. Daily maximum temperatures expected too
climb into the 90s and 100s across the South, while high humidity
and persistent elevated dewpoints produce oppressive heat indices
of 105-115 degrees. The Desert Southwest can expect highs in the
low 110s. Record-breaking warm overnight temperatures across the
southern tier will further limit recovery, particularly where
several consecutive days of heat occur. Warmer than average
temperatures including record warm lows are forecast north through
the Intermountain West under the ridge Friday-Saturday. Some
gradual relief may be expected for eastern areas, as the trough
aloft and weak cold fronts lessen the magnitude and scope of the
hazardous heat in the Southeast. However, the heat is forecast to
continue across the south-central U.S. and back toward the
Southwest. Widespread Major to Extreme HeatRisk conditions should
persist in the southern Plains and Lower Mississippi Valley.
Several consecutive days of high heat stress can lead to heat-
related illnesses.

Shortwave energy moving through the Great Lakes to Northeast
trough that gradually deepens in the East will carry frontal
systems that should focus moisture for rounds of showers and
thunderstorms. Convection is likely near a frontal boundary across
the Mid-Atlantic and Southeast on Friday, with isolated flash
flooding possible. Farther north, a shortwave is forecast to push a
round of rain/thunderstorms across the Upper Midwest and Great
Lakes Friday. These systems should interact/combine and lead to
rain chances across the Eastern Seaboard through the weekend, and
across the Appalachians and Great Lakes region on the backside of
the low pressure/frontal system as it slowly moves northeast.

Monsoonal moisture is forecast to bring scattered, diurnally
driven showers and thunderstorms to the Southwest and Intermountain
West through the period. Localized flash flooding is possible
especially across sensitive areas like burn scars, favored terrain
like slot canyons, and urban areas. A round of rain is also
possible with a shortwave in the Pacific Northwest and into the
northern Rockies, likely peaking Saturday. Areas of rain and storms
are also possible in the central U.S. as shortwaves round the
northeast side of the ridge and dive southeast. Placement of these
shortwaves and thus the rain chances are uncertain, but generally
the northern/central Plains and the Mississippi Valley could be
impacted.


Campbell/Tate


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key Messages
are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw





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