981
FXUS02 KWBC 170752
PMDEPD

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

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. One question is whether a rainfall axis
sets up onshore for the Mid-Atlantic and Carolinas or offshore for
the end of the week. 00Z models (EC, GFS, and CMC) generally
trended toward more QPF offshore by Saturday compared to previous
runs, looking more like the AIFS. Then, after some hints of it in
the 12/18Z model cycle, recent 00Z AI and dynamical models have
trended toward a closed upper low developing over the Great Lakes
region by Sunday. The EC/CMC and AIFS/AIGFS are showing a slow
track east of the upper low by next Monday, while the GFS is on the
faster side at shifting it east as a more open trough.

Meanwhile in the West, uncertainty in the details remains
prevalent. Midlatitude energy comes into the Northwest likely
Saturday but with models varying in speed of the shortwaves moving
east, and then additional northern stream shortwaves could come in
and affect the pattern in the northwestern/northern tier into early
next week. No real outliers were seen, but the variable models
lower confidence in any particular solution.

Thus, the WPC forecast used a multi-model 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
shortwave details.


...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.


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



$$