391
FXUS02 KWNH 180717
PREEPD

Preliminary Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
316 AM 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...

An upper high will be over the Four Corners states late week, with
ridging expanding north across the Intermountain West, gradually
shifting a bit east next week into the High Plains. This will
maintain hazardous heat across the Southwest to south-central
Plains and 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...

Model guidance shows good agreement regarding the anchoring upper
ridge/high through the period. Models also continue to show the
gradually deepening trough across the East. The details of
shortwaves within the trough will affect frontal positions and
sensible weather, though. 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. GFS runs including the 00Z
seem faster 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. Afternoon temperatures will reach the
90s and 100s in 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. Into the
weekend, there is some gradual relief in sight 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.


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



$$