746
FXUS02 KWBC 211934
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
334 PM 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...

For much of the extended period the flow aloft will be amplified,
with ridging present over the West and troughing over the East.
This pattern will continue to support a Heat Wave, shifting from
the Southwest to the the interior Northwest. Meanwhile to the
East, a series of shortwaves will reinforce the trough which will
bring substantial cooling for areas 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. Although there
is good agreement with the large scale pattern, differences with
various features among individual solutions remain in play that
will have direct implications on the sensible weather and QPF
amounts. One of which is when the trough over the East will shift
eastward near the end of the extended period. Another is the
stalled/stuck upper low off the Northwest Coast/Gulf of Alaska
region before it eventually begins to shift eastward.

As previously noted, the CMC favors strong ridging/more blocking
over western Canada late period and which keeps the low well out
over the northeast Pacific. This continues to be supported by the
ensembles as they depict the low drifting eastward with a position
just off the Northwest Coast by next Thursday. This track is also
supported by the CMC and ECMWF.

The preferred WPC blend began with a general model compromise for
the first half of the period, tipping heavier toward the ECMWF.
Late period increased ensemble mean contribution to 60 percent of
the blend to help mitigate the larger scale late period
differences. This maintained continuity as well as reducing the
noise for the latter periods.


...Weather/Hazards Highlights...

Showers and thunderstorms along and ahead of an advancing strong
cold front expected from the southern/central Appalachians into
the Northeast. The overall threat for flash flooding will likely be
limited due to recent dry conditions and progressive nature of the
frontal boundary; however, the potential will be greater for the
Mid-Atlantic and points south where the frontal progression will
likely be slower. Maintained the Marginal Risk that is in place for
much of the southern and central Appalachians on Sunday. As the
front exits the Northeast on Monday, locally heavy rainfall looks
more likely farther north. A Marginal Risk remains in place across
interior New England for Day 5. A significant drop in daily
temperatures expected in the wake of the front, leading to
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). Much of the Great Basin, Southwest, and
Rockies/Plains has highlighted Sunday and Monday as having an
elevated threat for locally excessive rainfall and isolated flash
flooding, therefore the broad Marginal Risk remains in place for
Sunday and Monday. An intensification of rainfall is expected
east of the Front Range and spread out into the Plains as shortwave
energy rounds the west side of the Eastern trough and interacts
with a front. The best signal focuses the heavy rainfall parts of
the Colorado Rockies and into southern Kansas/Oklahoma/Texas
panhandles. The inherited Slight Risk was expanded further across
southern Kansas and central/eastern Oklahoma. This activity should
continue into mid week as well with the front becoming stationary.

The excessive heat over the Southwest is expected to abate while
prolonging over the Interior Pacific Northwest. Major to localized
extreme HeatRisk likely across the Southwest and daytime
temperatures exceeding 110F in some locations into Sunday-- temperatures
should moderate beyond Sunday. Major to locally extreme HeatRisk
is forecast into next week for portions of the Northwest, with
possibly some moderation by next Thursday.


Campbell


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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