123
FXUS02 KWNH 210655
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
254 AM EDT Thu Aug 21 2025

Valid 12Z Sun Aug 24 2025 - 12Z Thu Aug 28 2025

...Heat wave to wane over the Southwest by Sunday, but continue
into next week for the interior Northwest...

...Heavy rainfall possible for parts of the central Rockies and
Plains early next week...

...Overview...

The pattern over the CONUS will be rather amplified for much of
the medium range period with ridging over the West, and a trough
over the East. This will continue a Heat Wave which shifts focus
from the Southwest this weekend into the interior Northwest next
week. Meanwhile to the East, a series of shortwaves will reinforce
the trough which will bring a substantial cool down east of the
Continental Divide next week. Some rainfall will accompany the cold
front through the East on Sunday, but for the most part, the main
rainfall focus next week will be across the Great Basin, Central
Rockies, and into the central Plains with locally heavy rainfall
possible.


...Guidance/Predictability Assessment...

The latest model and ensemble guidance continues to show above
average agreement for much of the medium range period. Periodic
shortwaves will help to reinforce a deepening trough over the East,
and the interaction of these shortwaves with a front over the
south-central Plains will focus rainfall threats. Models show
agreement on the large scale set-up, but questions on the details
remain which would have implications for QPF across this region.
Later in the period, the trough may finally begin to shift off the
East Coast, but models disagree on the timing of that.

Out West, an upper low looks to get stuck off the Northwest
Coast/Gulf of Alaska region for several days before it eventually
begins to shift eastward. The CMC shows strong ridging/more
blocking over western Canada late period and which keeps the low
well out over the northeast Pacific. The ensembles seem to support
the low drifting eastward with a position just off the Northwest
Coast by next Thursday which is consistent with the GFS and ECMWF.

The WPC forecast for tonight used a general model compromise for
the first half of the period, with some weighting towards the
ECMWF. Late period, removed the CMC from the blend (due to its
issues in the Pacific) and increased ensemble mean contribution to
60 percent of the blend to help mitigate the larger scale late
period differences.


...Weather/Hazards Highlights...

A strong cold front moving into the East by Sunday will support
showers and thunderstorms along and ahead of the boundary from the
southern/central Appalachians into the Northeast. The front should
be progressive enough, with dry antecedent conditions, limiting the
overall flash flood threat. However, locally heavy rainfall looks
a bit more likely from the Mid- Atlantic southward as the front may
be slower to move out of that region so a marginal risk remains in
place for much of the southern and central Appalachians on Sunday
(trimmed out the northern part with WFO support). For Day 5/Monday,
as the front exits the Northeast, locally heavy rainfall looks
more likely farther north and so a marginal risk is in place across
interior New England on the ERO. In the wake of the front, a
significant cool down is expected, sending temperatures well below
climatology for late August.

Surges of moisture into the Southwest under the base of the Four
Corners ridge combined with diurnal heating will likely trigger
storms capable of producing heavier rainfall over more sensitive
areas (steep terrain, in/around burn scars, dry washes/arroyos,
urban areas). A broad Marginal Risk ERO continues to be depicted
for both Days 4 and 5 valid for Sunday and Monday from eastern
California into the central Plains. By Monday, with shortwave
energy rounding the west side of the Eastern trough and interacting
with a front, guidance continues to show increasing chances for a
more robust heavy rainfall signal especially for parts of the CO
Rockies and into southern KS/OK/TX panhandle. A slight risk is in
place on the Day 5/Monday ERO for this region. This activity should
continue into mid week as well with the front becoming stationary.

Heat threats will focus across the Western U.S. during the period,
with major to localized extreme HeatRisk likely across the
Southwest and daytime temperatures exceeding 110F in some locations
into Sunday. Temperatures should moderate after Sunday. Farther
north though, especially interior portions of the Northwest, major
to locally extreme HeatRisk is forecast into next week, with
possibly some moderation by next Thursday.


Santorelli


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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