851
FXUS02 KWBC 151731
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
131 PM EDT Sat Aug 15 2026

Valid 12Z Tue Aug 18 2026 - 12Z Sat Aug 22 2026


...Long-duration heat wave continues across the South/Southeast...

...Midwest heavy rain/flooding threat renews Tuesday...


...Overview...

Southern tier upper ridging will be well established as the medium
range period begins Tuesday and continue to produce hazardous heat.
To the north of the ridge, relatively active flow will produce
rounds of rain and thunderstorms across the northern tier. Rainfall
amounts are forecast to peak in the Midwest Tuesday with a flash
flooding threat. Shortwaves should gradually deepen troughing
across the eastern U.S. into late week and slowly ease the
magnitude and scope of the heat in the Southeast, while maintaining
rounds of rain/thunderstorms there. However, the strong upper
ridge/high will keep a hold on the Four Corners states and High
Plains, growing north across the Intermountain West. Some monsoonal
moisture should stream north under the ridge for scattered convection.


...Guidance/Predictability Assessment...


The latest model guidance remains in good agreement on the large-
scale pattern, especially in regards to the persistence of a
southern U.S. ridge. In addition, the ECMWF, GFS, and CMC all
support a strong ridge shifting towards the Four Corners and much
of western U.S. late in the period, which also brings higher
confidence for excessive heat building across the southern states.
Model spread begins towards mid-period, especially in regards to
the timing, amplitude, and placement of the individual shortwaves
embedded within the northern stream, particularly across the
northern Rockies/Plains and Great Lakes. GFS seems to remain
slightly slower with the shortwave placements compared to ECMWF and
CMC. These differences will directly affect surface frontal
positions and precipitation placement/maxima. Towards later in the
period, a trough begins to push into the West Coast, but models
show some amplitude and timing differences as well. Therefore, the
forecast favored a multi-model deterministic blend through late
week, with increasing reliance on the ensemble means to reduce
sensitivity to the individual shortwave differences.


...Weather/Hazards Highlights...

A strong upper ridge remaining positioned over the southern U.S.
will maintain hot conditions across the southern Plains through the
Lower Mississippi Valley, Tennessee Valley, and Southeast.
Afternoon temperatures will commonly reach the upper 90s to lower
100s across the interior South, while high humidity and persistent
elevated dewpoints will produce oppressive heat indices of 105-115
degrees. Record-breaking warm overnight temperatures will further
limit recovery, particularly where several consecutive days of heat
occur. Widespread Major to Extreme HeatRisk conditions should
persist into much of next week, and several consecutive days of
high heat stress can lead to heat- related illnesses. There is some
gradual minor relief in sight for eastern areas late next
week/weekend due to the trough aloft and weak cold fronts, but the
south-central U.S. and back toward the Southwest will likely
continue to see dangerous heat. See WPC`s Key Messages for more
information.

Shortwave energy moving through the northern tier will carry
frontal systems that could focus moisture for rounds of showers and
thunderstorms next week. One strong shortwave and cold front are
forecast to bring increasing rain amounts to the Upper Midwest in
particular by Tuesday. A Slight Risk remains in place in the Day
4/Tuesday Excessive Rainfall Outlook for strong thunderstorms/MCS
activity that could cause flash flooding. Rain and thunderstorm
chances will push across the Great Lakes to Northeast back across
the Ohio Valley and central Plains by Wednesday with an isolated
flash flood threat. Farther south, a weak cold front should provide
a focus for scattered thunderstorms across the Southeast Tuesday-
Wednesday in the hot and unstable airmass. By late week, this
activity may all combine into chances for scattered showers/storms
across the Eastern Seaboard, and west across the Mid-South and
Mississippi Valley with the tail end of the front. Meanwhile, some
monsoonal moisture should bring scattered, diurnally driven
convection to the Southwest especially by Wednesday and beyond.
Moisture may continue spreading north across the Intermountain West
by Friday, allowing for rain and thunderstorm chances there.


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




$$