135
FXUS02 KWBC 010749
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
349 AM EDT Mon Sep 1 2025

Valid 12Z Thu Sep 04 2025 - 12Z Mon Sep 08 2025


...Overview...

A deep upper low atop Lake Superior and vicinity as the medium
range period begins Thursday will anchor troughing and resulting
below normal temperatures across the Plains, Midwest, and Great
Lakes/Ohio Valley. Rain is likely ahead of the trough axis,
including some moderate to heavy rain totals possible in the
central/northern Appalachians and interior Northeast later in the
week. Meanwhile in the West, upper ridging is likely to continue
and bring above normal temperatures to the Northwest before a
likely cooling trend by the weekend. Some monsoonal moisture may
increase late week into the weekend across the West.


...Guidance/Predictability Assessment...

Model guidance remains in good agreement with the amplified
synoptic scale pattern anchored by the ridge in the West and trough
in the East, with relatively more spread in the details. The
primary upper low`s position continues to converge in deterministic
models and means, and models are also more agreeable now in
dropping a shortwave south through the north-central U.S. on the
western side of the low. The upper low center is forecast to drift
northeast into the weekend and gradually lift troughing out of at
least southern parts of the lower 48. However, there may be
opportunities for reinforcing shortwaves into early next week. The
ECMWF, especially the newer 00Z run, is particularly aggressive
with a shortwave moving through the Midwest, so will continue to
monitor this potential.

Farther west, there is more spread with the general pattern behind
the Intermountain West ridge, especially in the 12/18Z model
cycle. Pacific troughing moves toward the West Coast at varying
speeds, with the CMC the fastest and becoming out of phase and GFS
runs slowest trying to cut off a low farther west. The ECMWF seemed
generally most agreeable with the ensemble means. But 12/18Z
deterministic models were also more aggressive with a shortwave
moving through western Canada over the weekend and depressing the
interior West upper ridge. Ensemble means, though of course
averaged from the ensemble members and thus can be flatter, were
not in favor of this shortwave feature and maintained more ridging.
Fortunately it seems that the incoming 00Z model suite is becoming
more agreeable with the trough placement in the Pacific, and also
weaker with the Canadian shortwave so that it gets shunted around
the northern side of the ridge.

The WPC forecast was based on a blend of the 18Z GFS, 12Z ECMWF,
and 12Z UKMET 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 half the model
blend by Day 6 and more Day 7 amid increasing spread.


...Weather/Hazards Highlights...

Convection is likely ahead of the cold front moving through the
northeastern quadrant of the U.S. ahead of the upper low. The front
slowing and tapping into some Atlantic moisture, possibly
influenced by a coastal system, could lead to localized flash
flooding concerns. For the Day 4/Thursday ERO, a Marginal Risk is
delineated across the central to northern Appalachians and
surrounding areas for isolated flooding concerns. While antecedent
conditions are generally dry, heavy rain can sometimes quickly run
off over dry soils. Orographic enhancement may play a role in
producing heavier rain totals. Looped the central Appalachians into
this issuance of the Marginal Risk since there should be similar
if not more instability for convection there. The front pushing
east into Friday will shift the convective potential east,and will
show a Marginal Risk for the interior Northeast for the Day 5 ERO,
before rain pulls away Saturday. Some rain may wrap around the
Midwest/Upper Great Lakes late week as upper energy moves through,
and the cool westerly flow atop the Great Lakes could lead to lake
effect rain showers into the weekend.

Monsoonal moisture is forecast to flow into the southwestern U.S.
underneath the mean upper ridge, promoting scattered showers and
thunderstorms. Moisture anomalies are highest into parts of
California and Nevada on Thursday and gradually expanding eastward
into late week. Marginal Risks are in place for eastern California
into the central Great Basin and Southwest on Day 4/Thursday, and
covering much of the Intermountain West farther north and east too
on Day 5/Friday. There is low deterministic QPF over the northern
areas despite favorable conditions, but even the QPF increases late
week into the weekend. The track of an Eastern Pacific tropical
system that is likely to form (see the NHC outlooks) could also
affect the moisture levels in the West, though its path/intensity
is quite uncertain. Rain chances are currently forecast to expand
even farther into the Intermountain West by the weekend. Convection
looks to make its way into the southern Plains at times near a
wavy/lingering frontal boundary, but with plenty of spread in the
details.

Meanwhile, frontal boundaries are forecast to meander over Florida
in an unstable environment with above normal moisture, allowing
for localized flash flood potential with urban areas most
vulnerable. Most model guidance then limits potential for heavy
rainfall to south of I-4 into Day 4/Thursday as the front
approaches from the west. South Florida in particular could see
some lingering moisture astride the wavy frontal boundary late this
week into the weekend which would support additional diurnal
convection. Thus show a Marginal Risk in the Day 4 ERO across
central to southern Florida and a Marginal limited to South Florida
on Day 5/Friday.

The large trough covering the central and eastern U.S. for the
latter part of this week will promote well below average
temperatures behind multiple cold fronts. Lows are forecast to be
generally around 10-15 degrees below average for Thursday through
the weekend across much of the Plains, Midwest, and Great Lakes
south into the Ohio/Tennessee Valleys. Lows reaching the 30s may
cause concerns for frost around Lake Superior, depending on wind
and cloud cover. Highs could be even more anomalous on Thursday and
Friday across the Upper Midwest/Great Lakes, around 15-25 degrees
below normal. This could set daily records for low maximum
temperatures, as highs only reach the 50s in some locations of the
Upper Midwest and Great Lakes, with even some possible highs in the
40s in the U.P. of Michigan. Temperatures are forecast to
gradually warm closer to normal into early next week as the airmass
moves east and moderates as the upper trough lifts. On the other
hand, the West can expect warm to hot conditions under the upper
ridge continuing into late week. 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 but may be more extensive on Wednesday
(now in the short range). HeatRisk shows Major to locally Extreme
conditions in the northern Great Basin through Thursday, with
temperatures decreasing after that.


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


$$