016
FXUS02 KWBC 231949
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
349 PM EDT Sat Aug 23 2025

Valid 12Z Tue Aug 26 2025 - 12Z Sat Aug 30 2025

...Waning heat in the Northwest; mild across much of the Plains,
Midwest & East...

...Heavy rainfall possible for parts of the South-Central Rockies
and Plains next week...


...Overview...

A stagnant/amplified pattern will be in place for much of
next week, with a ridge in the West supporting a heat wave
lingering over the interior Northwest into midweek and a cool
trough east of the High Plains with reinforcing shortwaves. The
main rainfall focus next week will be across the Great
Basin, Southern/Central Rockies and Plains, and into the Lower
Mississippi Valley with locally heavy rainfall possible.


...Guidance/Predictability Assessment...

Model guidance remains in generally good agreement with the
pattern described above. A reloading upper trough will be slow to
move eastward through the eastern third of the lower 48 next week.
00Z models varied in the exact timing of the trough shifting
eastward by late week, with the 00Z GFS faster and the 00Z CMC the
slowest even compared to ensemble members. Fortunately newer 12Z
models show better agreement. Ridging behind the trough will
gradually shift east as well. The details of northwest flow between
these features as well as details of frontal/outflow boundaries
will affect QPF but have low predictability out to this time range.


What is less certain on the relatively larger scale is potential
for shortwaves to come into the northwestern U.S. ahead of and with
an upper low in the eastern Pacific. Recent model guidance moves
the upper low more quickly toward the Pacific Northwest on
Thursday, but after that models show varying possibilities in its
evolution, including the upper low stalling, shortwave energy
moving eastward, and/or combining with another likely upper low
behind it. This leads to low confidence in elements like frontal
timing in the Northwest.

The WPC forecast blend was based on a general model compromise
during the early period. Gradually increased the proportion of
ensemble means as the period progressed to half by Day 6 and more
by Day 7 as individual model differences increased. This maintained
reasonable continuity.


...Weather/Hazards Highlights...

Multiple days of flash flooding concerns are expected for south-
central portions of the Rockies and Plains through much of next
week. Shortwave energy will round the western side of the trough
and provide ample lift from the jet stream in a broadly moist and
unstable environment (precipitable water values to 1.75"+ and
MUCAPE towards 1000 J/kg reaching the High Plains) in the vicinity
of a meandering front. A Slight Risk remains in place for Day
4/Tuesday for portions of southern Colorado and northern New
Mexico. The best chances for heavy rain look to redevelop north and
east by Day 5/Wednesday, where daytime convection and a nighttime
MCS may occur in overlapping areas. Models are generally agreeable
that this looks most likely in Kansas, so show a Slight Risk there
for the Day 5 ERO. Monsoonal moisture will be in place across much
of the West around the Four Corners ridge after a relatively dry
monsoon season in most places. The moisture combined with diurnal
heating will likely trigger storms capable of producing heavier
rainfall, which could cause localized flash flooding especially
over more sensitive areas (steep terrain, in/around burn scars, dry
washes/arroyos, urban areas). Broad Marginal Risks remain
highlighted for much of the Great Basin, Southwest, and
Rockies/Plains for both Tuesday and Wednesday for locally excessive
rainfall and isolated flash flooding. The Marginal Risks were
expanded north slightly from continuity to cover areas of the
northern Great Basin that are likely to see moisture (PW) anomalies
over the 95th percentile. Rain and thunderstorm chances should
slowly drift east later next week and focus in the South-Central
Plains and Mid-Lower Mississippi Valley.

Lake effect/enhanced showers and thunderstorms possible at times
through the rest of next week within the post-frontal cool air mass
near the Great Lakes. Down south, the front may stall over Florida
for some locally heavy rain there.

Behind these fronts, cooler than normal temperatures as well as
drier than normal dewpoints for August are likely. The South-
Central Plains should be particularly cool for highs (15-25F below
normal, only reaching the 60s and 70s) just behind the front and
where there is rain. Below average temperatures are likely through
most of the country from the Rockies eastward, with temperatures
more reminiscent of early fall than late summer. Record low maximum
and minimum temperatures are possible from the South-Central
Plains into the Midwest and Ohio Valley Tuesday and Wednesday, and
for widely scattered locations in the Southeast, southern
Appalachians, and Mid-Atlantic States on Thursday.

Meanwhile, the excessive heat prolongs over the Interior Northwest
though its footprint/magnitude shrinks each day. Major to locally
Extreme HeatRisk is forecast to continue into Tuesday for portions
of the Northwest, where high temperatures could exceed 100F. Some
moderation is anticipated Wednesday onward as an upper low and
resulting front approaches from the Pacific.


Tate/Roth


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 (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

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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