598
FXUS02 KWNH 160741
PREEPD

Preliminary Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
339 AM EDT Sun Aug 16 2026

Valid 12Z Wed Aug 19 2026 - 12Z Sun Aug 23 2026


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


...Overview...

A southern tier upper ridge at the start of the period Wednesday
will gradually focus across the Southwest/southern Plains northward
into the Intermountain West as troughing slowly deepens in the
East. This leads to hazardous heat continuing across the south-
central U.S. and increasing in the Southwest, but lessening in the
Southeast. Rounds of shortwaves and fronts to the north and east of
the ridge will carry rain and thunderstorm chances to portions of
the central U.S. to Eastern Seaboard. Meanwhile some monsoonal
moisture should stream north under the ridge for scattered
convection across the West.


...Guidance/Predictability Assessment...

The latest model guidance remains in very good agreement with the
overall pattern mid to late week. Models show the upper high
building north over the Four Corners states and northern Rockies
Thursday-Friday in response to being sandwiched between eastern
Pacific and eastern U.S. troughing. The ridge/high should gradually
move east a bit with its axis over the High Plains by next Sunday.
Details of the eastern and western upper troughs are somewhat more
questionable, especially by next weekend. There is a complex
pattern setting up in the Pacific, with energy both in the
midlatitudes and in the northeast Pacific that contribute to the
pattern over the Northwest. This lowers confidence in how the
troughing evolves and moves eastward in any particular solution.
Then in the East, there are also multiple shortwaves at play, which
especially by Sunday lead to model differences. The 00Z GFS
indicates the flow could be split, showing a southern stream
shortwave while its core upper low is farther north of consensus
well north into Canada. Other models are more phased, like the 00Z
ECMWF. The 00Z CMC is a bit strong with its upper low over the
Great Lakes Sunday, but it does have some support from the EC-AIFS
(though the AIFS is farther east). These details will require
additional time to determine.

The WPC forecast was based on a blend of deterministic/AI model
guidance through the early to middle medium range period. By next
weekend, gradually increased the proportion of ensemble means to
around half given the increasing spread within the troughs in
particular.


...Weather/Hazards Highlights...

A strong upper ridge remaining positioned over the southern U.S.
will maintain hot conditions there. 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 northern tier midweek and
eventually deepening troughing in the East will carry frontal
systems that should focus moisture for rounds of showers and
thunderstorms. Convection is likely across portions of the Great
Lakes, Ohio Valley, Mid-Mississippi Valley, and central Plains near
a frontal boundary on Wednesday. Farther south, a weak cold front
should provide a focus for scattered thunderstorms across the
Southeast in the hot and unstable airmass. By Thursday, combining
frontal boundaries will push rain chances across the Eastern
Seaboard and back into the Mid-South and Mississippi Valley with
the tail end of the front. Scattered thunderstorms are likely to
continue into Friday and Saturday across the East. Some isolated
flash flooding and strong to severe thunderstorms are possible each
day.

Monsoonal moisture is forecast to bring scattered, diurnally
driven convection to the Southwest to southern/central High Plains
on Wednesday and Thursday, with localized flash flooding possible.
Into late week and next weekend, moisture should continue spreading
north across the Intermountain West, allowing for rain and
thunderstorm chances there as well. Elsewhere, a shortwave is
likely to push a round of rain across the Upper Midwest Thursday
and Great Lakes Friday.


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


$$