422
FXUS02 KWNH 200646
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
246 AM EDT Wed Aug 20 2025

Valid 12Z Sat Aug 23 2025 - 12Z Wed Aug 27 2025

...Heat wave builds over the Southwest late this week and lingers
into the weekend...

...Overview...

An anomalously strong ridge of high pressure situated over the
Four Corners states as the period begins Saturday will continue the
threat for extreme heat across the Southwest. The ridge may weaken
some early next week moderating heat in the Southwest, but extreme
heat threats may continue into next week for the interior
Northwest. Downstream of the ridge, an amplifying trough across the
eastern half of the U.S. will build and generally hold through the
period with reinforcing shortwaves. A strong cold front will
accompany this trough, bringing a substantial cool down east of the
Continental Divide next week. Rainfall through the period will
mainly focus across the South and East, with surges of monsoonal
showers and storms over the Southwest/Rockies and eventually into
the central Plains.


...Guidance/Predictability Assessment...

Model guidance continues to advertise above average agreement and
predictability for most of the medium range period favoring an
increasingly amplified flow. There is good agreement on periodic
shortwaves reinforcing the Eastern U.S. trough but questions remain
on the details of which impact QPF, particularly for parts of the
Central Plains early next week. An upper low looks to get stuck off
the Northwest Coast/Gulf of Alaska region for several days, which
helps to further amplify troughing over the West with a Northwest
heat threat more likely. The CMC continues to be more aggressive
with shortwave energy into Western Canada with an overall weaker
ridge than is indicated by the consensus.

The WPC forecast started with a compromise of the deterministic
guidance, gradually increasing to include half of the ensemble
means and removing the 00Z CMC by the end of the forecast period.


...Weather/Hazards Highlights...

Heavy rainfall will focus along a lingering frontal boundary over
the southern states through the weekend with above normal PW values
and instability. Marginal risks remain in place on the Day
4/Saturday Excessive Rainfall Outlook focusing across the Southeast
and southern Appalachians. For Day 5, as the front sags southward
and weakens, convection may focus more across the northern Florida
peninsula ahead of the next stronger cold front. A marginal risk is
highlighted for this region on Sunday. To the north, showers and
storms will accompany the strong cold front through the East. The
front should be generally progressive, limiting the overall flash
flood threat, but opted for a marginal risk across parts of the
interior Northeast where there was the best overlap of instability
and anomalous moisture on Day 5/Sunday. 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 Marginal Risk ERO continues to be depicted for both
Days 4/5 valid for Saturday and Sunday. Shortwave energy rounding
the west side of the Eastern trough may also spread activity into
the central Rockies/Plains Sunday onward, with the marginal risk on
Sunday also extending into this region. Still some uncertainty,
but there is potential for heavier rainfall to monitor.

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 the weekend. Farther north into the interior West and
California valley, moderate to locally major HeatRisk is forecast.
Heat may abate across the Southwest late Sunday into Monday, but
ramp up across the Pacific Northwest into the middle of next week.

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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