739
FXUS02 KWBC 171749
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
149 PM EDT Mon Aug 17 2026

Valid 12Z Thu Aug 20 2026 - 12Z Mon Aug 24 2026


...Long-duration heat wave continues across the southern tier...


...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.


...Guidance/Predictability Assessment...

Guidance continues to show relatively good agreement on the large
scale synoptic pattern through the period, with a strong upper high
situated over the Four Corners and the ridge extending northward
through the Intermountain West into the central High Plains.
Meanwhile, broad troughing is expected to deepen gradually across
eastern U.S. These features are well supported both in the
deterministic guidance as well as the ensemble means, bringing
higher confidence of the anomalous heat developing over the
southern tier. The greatest uncertainty remains with the evolution
of the shortwaves and their interaction with the developing eastern
trough, especially towards the weekend. This will likely impact
confidence with the development of convective activity along the
Eastern Seaboard. Another emerging source of uncertainty is the
progression of the eastern Pacific closed low/trough. Most guidance
support the arrival of a mid-level energy ejecting into the Pacific
Northwest by the weekend, but the differences with timing and
amplitude persist. Therefore, to account for some of the
differences, the forecast was based on a multi-model blend through
the first portion of the period, while incorporating more of the
ensemble means towards middle to latter portions of the period.


...Weather/Hazards Highlights...

Stronger upper ridging will maintain hot conditions across the
southern U.S. into late week. Afternoon temperatures will commonly
reach 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. Widespread Major to Extreme
HeatRisk conditions should persist, and several consecutive days
of high heat stress can lead to heat-related illnesses. There is
some gradual relief in sight for eastern areas late week/weekend,
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. See WPC`s Key Messages for more information.

Shortwave energy moving through the Great Lakes to northeastern
U.S. 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
stretching from portions of the Northeast/Mid-Atlantic into the
Ohio Valley and Mid-Mississippi Valley on Thursday. The
thunderstorm focus should be from the Mid-Atlantic to Southeast by
Friday as part of the front sinks southward, and then there is some
uncertainty with how much rain remains onshore in the Eastern
Seaboard vs. offshore over the weekend. Some isolated flash
flooding and strong to severe thunderstorms are possible. Farther
north, a shortwave is likely to push a round of rain/thunderstorms
across the Upper Midwest Thursday and Great Lakes Friday, reaching
the Northeast over the weekend.

Monsoonal moisture is forecast to bring scattered, diurnally
driven convection to the Southwest to southern/central High Plains
into Thursday. By Friday and into the weekend, moisture should
continue spreading north across the Intermountain West, allowing
for rain and thunderstorm chances there as well. 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.


Oudit/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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