843
FXUS02 KWBC 300757
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
357 AM EDT Sat Aug 30 2025

Valid 12Z Tue Sep 02 2025 - 12Z Sat Sep 06 2025


...Overview...

Mean upper troughing will be in place over the eastern half of the
lower 48 next week, with reinforcing energy arriving around
midweek. The atypically deep troughing developing and a cold front
will promote well below average temperatures across the Plains,
Midwest, and Great Lakes/Ohio Valley. A round or two of rain and
thunderstorms are possible with shortwaves and along and ahead of
the front. Out West, mean upper ridging will be in place, leading
to above average temperatures especially for the Northwest. Some
monsoonal moisture sneaking in could bring showers to parts of the
Great Basin and Southwest and reaching the southern Plains at
times.


...Guidance/Predictability Assessment...

Recent model guidance remains quite agreeable with the amplified
synoptic scale pattern anchored by the ridge in the West and trough
in the East next week, with relatively more spread in the smaller
scale features. But models are even showing convergence on the
placement of an initial shortwave/weak upper low over the
northeastern U.S. as the period begins Tuesday, though the 12Z
UKMET was a little east. There are some minor model differences
with a subtle shortwave in the Mid-South around Wednesday, which
does lead to some QPF differences such as higher QPF in the
Southeast from ECMWF runs compared to other models. Meanwhile in
the West, there is reasonable agreement for an upper low near
Vancouver Island to retrograde as it is blocked by the West ridge,
while weak energy makes its way toward northern California into
Wednesday.

Then as a potent vort max drops south through the western side of
the trough around midweek, this will reinforce and deepen the
trough for the central and east-central CONUS. Models are agreeable
with this idea and show more consensus tonight with an upper low
closing off near the Upper Midwest/Upper Great Lakes by Thursday.
However, one trend seen in the newer 00Z guidance is to show the
upper low notably slower than the 12/18Z model runs, as the 00Z
models linger the upper low near the Great Lakes through late week
rather than lifting it toward Ontario and the Hudson Bay. The 00Z
CMC seems to be the southernmost extreme. Will have to keep an eye
on this potential and see if AI/ML models from the 00Z cycle come
in similarly.

The WPC forecast was based on a multi-model blend favoring the 18Z
GFS and 00Z ECMWF and CMC early in the forecast period. As the
period progressed, lessened the proportion of deterministic
guidance in favor of the ensemble means, with the means reaching
about half the model blend by Days 6 and 7 amid increasing spread.


...Weather/Hazards Highlights...

Shortwave energy and a weak surface low along a front could
support storms with elevated rain rates centered over the Mid-South
on Tuesday. A Marginal Risk remains in place in the Day 4/Tuesday
ERO for that region with only minor changes to continuity. Farther
south, frontal boundaries are forecast to meander over Florida in
an unstable environment with above normal moisture, also allowing
for localized flash flooding potential there and Marginal Risks in
the Day 4/Tuesday and Day 5/Wednesday EROs. Additional frontal
boundaries atop Florida may help focus continued diurnal convection
in Florida into later next week, but with some signal for moisture
levels to decrease back toward normal.

Farther north, a cold front dropping into the north-central U.S.
ahead of the strong reinforcing upper trough could see some
convection ahead of it. On Tuesday, locally heavy rain amounts look
to focus around the Upper Midwest, so will continue to denote a
Marginal Risk there in the Day 4 ERO. Instability and moisture are
not too high, but the forcing is strong and the front may be moving
a little slower on Tuesday. On Wednesday, this front continues to
track southeast and lead to rain chances across the east-central
U.S. generally in the Great Lakes and Ohio/Tennessee Valleys. Given
the lack of strong instability and moisture, as well as model
spread in placement of any heavy QPF, did not delineate any
Marginal Risk for this on Day 5 at this time, but will continue
watching. Rain is forecast to push into the East Thursday-Friday
(after a dry period).

Meanwhile, monsoonal moisture is forecast to flow into the
southwestern U.S. underneath the mean upper ridge for scattered
showers and thunderstorms. Marginal Risks are in place across much
of Arizona and New Mexico (and clipping neighboring states) on Days
4/5 for locally heavy rainfall causing flooding concerns
especially in sensitive areas. Additionally, extended the Marginal
Risk into the Sierra Nevada and vicinity on Tuesday and east into
much of Nevada on Wednesday. Despite low deterministic QPF,
moisture anomalies are generally highest there and the weak energy
around northern California could provide a bit of lift. Some rain
and storms could also extend across the southern Rockies to
southern Plains next week.

The large trough covering the central and eastern U.S. next week
will promote below average temperatures. Temperatures around 5-10
degrees below normal will be fairly common east of the Rockies into
Tuesday, but the reinforcing energy coming into the trough will
push a strong cold front into the north-central U.S. Wednesday and
spreading into much of the Plains and Midwest Thursday. Lows are
forecast to be generally around 10-15 degrees below average, while
highs should be around 15-20 degrees below normal. These cool highs
could set daily records for low maximum temperatures, as highs
only reach the 50s in some locations of the Upper Midwest and Great
Lakes. While lows generally do not look record-setting, there may
be a concern for frost around Lake Superior particularly Thursday
morning. On the other hand, the West can expect warm to hot
conditions under the upper ridge. Temperatures will be most
anomalous in the Northwest, with upper 90s and possibly reaching
100F east of the Cascades. Record high minimum and maximum
temperatures are possible, and the HeatRisk rebounds to Major to
locally Extreme in the northern Great Basin.


Tate


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




$$