329
FXUS02 KWBC 202000
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
400 PM 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 show solid agreement and
predictability for most of the medium range period, favoring an
increasingly amplified flow pattern. Several major shortwaves and
vorticity maximums will help to carve out and reinforce a deepening
trough across the eastern U.S. through the weekend and into next
week, flanked upstream and downstream by stout ridging.
Additionally, an upper low looks to get stuck off the Northwest
Coast/Gulf of Alaska region for several days. Lingering ridging
across the Pacific Northwest and lower heights off the coast of
California and Baja will likely lead to a weak Rex block. Guidance
has had a solid handle on this general pattern for the last couple
of days, with recent runs of the CMC and its excessively
progressive evolution of the Northwest Coast upper low now more in
line with consensus.

Despite an overall good handle on the pattern, the details of the
smaller scale waves still need some time to be resolved in the
coming days, especially in the longer time ranges. Thus, the WPC
forecast started with a compromise of the 00Z and 06Z deterministic
guidance, gradually increasing to include half of the ensemble
means 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 pushing through the
East. The marginal risk across parts of the interior Northeast
remains in place where there is the best overlap of instability,
anomalous moisture and dynamics. Farther south, the flash flood
threat becomes more uncertain given that the driving shortwave and
best dynamics lift out and to the east of Hudson Bay. This could
cause the front to weaken and slow down some as it sags into the
Mid-Atlantic the southern Appalachians, especially when it begins
interacting with terrain during the day on Sunday. With plenty of
instability and moisture to work with ahead of the front, there`s
at least some potential for storms to produce heavy downpours that
could lead to isolated flooding concerns. As a result, opted to
extend the marginal risk southward to include parts of the Mid-
Atlantic and southern Appalachians to account for this scenario. 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.


Miller/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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