238
FXUS02 KWBC 241957
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
357 PM EDT Sun Aug 24 2025

Valid 12Z Wed Aug 27 2025 - 12Z Sun Aug 31 2025

...Heavy rainfall in store for parts of the southern and central
High Plains and the Middle and Lower Mississippi Valleys...


...Overview...
An amplified synoptic pattern continues through the medium range
period, featuring troughing along both coasts and varying degrees
of ridging in between. Monsoonal showers and storms across parts of
the Intermountain West will last through the end of the week
before drying out some over the weekend. A growing signal for heavy
rain remains in place from the central/southern High Plains to
parts of the Middle and Lower Mississippi Valleys for the latter
half of the week. Near to below normal temperatures will persist
across most of the Lower 48 to close out the month of August.


...Guidance/Predictability Assessment...
Model guidance remains in generally good agreement with its
depiction of the large scale pattern through the medium range
period. The prevailing 500mb flow is expected to remain amplified
and rather blocky with troughing flanking both sides of a
persistent ridge across the Central U.S. Uncertainty is mostly
found in the smaller scale details of shortwaves beyond Day 5, of
which there are many. These waves seem to shift both spatially and
temporally with each run, affecting the overall evolution of the
coastal troughing and their interactions with the Central U.S.
ridge late this week into the weekend.

Given the above, the WPC forecast was made using a blend of the
00Z and 06Z deterministic guidance through Day 4. For Day 5 and
beyond, the GFS nudged the trough across the Eastern U.S. too far
east compared to consensus (again due to timing and interactions of
shortwaves), so its contribution was removed. Similarly, the CMC
was too deep with troughing in the East, and too fast and close to
the coast in the Western U.S. by Day 7, so its contribution was
also removed. To build more stability and reduce the influence of
differences in shortwave timing and placement in the latter half of
the forecast period, a greater emphasis was placed on the EC and
GEFS ensemble means, which made up 40% of the blend by Day 6 and
60% by Day 7.


...Weather/Hazards Highlights...
Flash flooding concerns from the short range period spill over
into the medium range time frame for portions of the High Plains
and Mid/Lower Mississippi Valleys through much of the 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 of 1.75-2" and
MUCAPE towards 1000 J/kg) in the vicinity of a meandering front. A
Slight Risk remains in place for Day 4/Wednesday for Kansas and
vicinity which shifts east towards parts of the Mid/Lower
Mississippi Valleys on Day 5/Thursday with portions of CO and NM
also gaining a Slight Risk area due to expected rainfall and
anticipated soil saturation between now and then. Heavy rain
concerns lessen but aren`t zero for similar areas Friday heading
into the weekend.

Monsoonal moisture will remain in place across much of the
Intermountain West around the meandering ridge to the east of
approaching upper level troughing. 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 Wednesday and Thursday for the isolated
flash flood potential. The West should start begin to dry out
across portions of CA, OR, and NV on Thursday and be mostly dry by
Saturday outside of NM and southeast AZ.

Within the fall-like cold sector, overall pleasant weather is in
store across the Midwest, Ohio Valley, and the Mid-Atlantic states,
with plenty of sun, mild temperatures, and low humidity during the
day, and comfortably cool overnight temperatures at night. Lake
effect/enhanced showers, with thunderstorms possible at times, are
expected from the short range period into Friday within the post-
frontal cool air mass near the Great Lakes before relaxing by this
weekend. Down south, the front stalling over Florida may provide a
focus for locally heavy rain there, a risk that appears to
gradually increase Thursday through Saturday across the Southern
Peninsula.

In the vicinity of the Slight Risk ERO areas across the Plains and
Mid/Lower Mississippi Valleys, it should be especially mild/cool
for high temperatures (15-25F below normal, only reaching the 60s
and 70s) within the moderate to heavy rain shield poleward of the
front; record cool high temperatures are possible in some
locations. Below average temperatures are likely through most of
the country from the Rockies eastward. Record low maximum and
minimum temperatures are also possible in the Midwest and Ohio
Valley Wednesday, and for widely scattered locations in the
Southeast, southern Appalachians, and Mid-Atlantic states on
Thursday. The excessive heat footprint/magnitude should be fading
early on across the Northwest, as moderation of the heat wave in
the short range period is anticipated Wednesday onward as upper
lows/troughing and fronts approach from the Pacific.


Miller/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







$$