938
FXUS02 KWBC 311850
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 PM EDT Sun Aug 31 2025

Valid 12Z Wed Sep 03 2025 - 12Z Sun Sep 07 2025


...Overview...

Upper troughing across the central and eastern U.S. will be
reinforced as the medium range period begins midweek, with strong
energy forming an upper low near the Upper Great Lakes Thursday.
This will bring in well below normal temperatures to 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 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 next
weekend. Some monsoonal moisture may increase late week into next
weekend across the West.


...Guidance/Predictability Assessment...

Through the 00/06Z guidance, the majority of models/ensembles
agree on the large scale pattern with the rather stable
ridge/trough orientation. The 00Z Canadian was offset again with
its handling of the upper low (farther south) compared to the
GFS/ECMWF-led consensus. Details remain less clear, including
shortwave energy that may slide around the larger upper low
circulation, in addition to how the southeastern front lifts
northward just off the East Coast. Regardless, a blended solution
sufficed, with increased ensemble weighting (to 50/50) vs
deterministic solutions in time. This helped smooth out the varied
solutions over the eastern Pacific albeit with weaker features
overall.


...Weather/Hazards Highlights...

The cold front dropping into the north-central U.S. ahead of the
strong reinforcing upper trough could see some convection ahead of
it. Models continue to vary on QPF amounts (in addition to
available instability), but with the machine learning first guess
fields showing either a Marginal or Slight Risk over the U.P. into
northern Lower Michigan, re-inserted a low-end Marginal Risk
outline to account for uncertainty. By Thursday, the front should
be slowing in concert with the upper low, while it should also tap
into some Atlantic moisture, possibly influenced by a coastal
system. For the Day 5/Thursday ERO, a Marginal Risk is delineated
across the central to northern Appalachians and surrounding areas
for isolated flooding concerns though it has been rather dry
recently. Orographic enhancement may play a role in producing
heavier rain totals, which can quickly run off over dry soils.
Moderate to heavy rain is forecast across the Interior Northeast on
Friday, then pulling 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 next 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, and Marginal Risks are depicted over those
states despite low deterministic QPF. This stretched into southwest
Utah and Arizona into New Mexico on Day 4/Wednesday and Day
5/Thursday. 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 Southwest, though its path/intensity is
quite uncertain. Rain chances are currently forecast to expand
farther north into the Intermountain West by next weekend.
Convection could make its way into the southern Plains at times
near a wavy/lingering frontal boundary.

Meanwhile, frontal boundaries are forecast to meander over Florida
in an unstable environment with above normal moisture into
midweek, allowing for localized flash flooding potential and a
Marginal Risk in the Day 4/Wednesday ERO. Most model guidance then
limits potential for heavy rainfall to south of I-4 into Day
5/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 next weekend which would
support additional diurnal convection.

The large trough covering the central and eastern U.S. for the
latter half of this week will promote well below average
temperatures. 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 may be around 15-20 degrees below normal in those areas.
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, with even some possible highs in the
40s in the U.P. of Michigan. While record lows should be less
expansive, there may be a concern for frost around Lake Superior
particularly Thursday morning, depending on wind and cloud cover.
Temperatures are forecast to gradually warm closer to normal into
next weekend. 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, some of which may approach monthly
records on Wednesday (rivaling the early September 1988 heat wave
values). HeatRisk shows Major to locally Extreme conditions in the
northern Great Basin on Wednesday and Thursday, with temperatures
decreasing after that.


Fracasso/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



$$