479
FXUS02 KWBC 020756
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
356 AM EDT Tue Sep 2 2025

Valid 12Z Fri Sep 05 2025 - 12Z Tue Sep 09 2025


...Overview...

An amplified upper pattern will be in place as the medium range
period begins Friday, with an upper low over Ontario anchoring
troughing across a good portion of the central and eastern U.S. and
allowing for below average temperatures behind multiple cold
fronts. Rounds of showers and storms are possible with these cold
fronts on the leading edge of the trough across the
southern/central Plains into the Northeast. Lingering ridging
across the interior West could continue to bring above normal
temperatures to the Northwest before a likely cooling trend by the
weekend. Monsoonal/tropical moisture influenced by Tropical
Depression Twelve-E is forecast to come into much of the West late
this week into the weekend for possibly widespread showers that
could cause localized flash flooding. Moisture generally is
forecast to push east into the central/southern Plains early next
week, but details will depend on the eventual track of T.D.
Twelve-E as well as other features like shortwaves in the vicinity.


...Guidance/Predictability Assessment...

Model guidance starts the medium range period in good agreement
with the aforementioned upper low/trough, with typical spread in
shortwaves rounding the trough that could affect frontal positions
and QPF over the weekend. A complicating factor arises in the
forecast with the track of T.D. Twelve-E and its resulting effects
on the pattern. GFS runs have shown the tropical system take a
northeasterly turn into Baja California with remnant energy
continuing over northern Mexico into the southern Plains.
Meanwhile, non-NCEP guidance has tended to favor a more western
track, including the 12Z EC and CMC that lingered the tropical
system just offshore Baja while weakening, and the newer 00Z EC/CMC
that take the system westward away from Baja. Ensemble members
from dynamical and AI-based models show spread with some members
moving northeast and some moving northwest or west, so this does
not yield confidence in leaning toward one solution or another.
Recent NHC forecast tracks have shown an in between solution where
the system is slower to curve northeast than the GFS, but it does
curve and impact Baja. It seems that regardless some troughing will
make its way into the south-central U.S. this weekend into early
next week, but the tropical system would affect its depth and the
resulting rainfall forecast.

Farther west, a Pacific upper low is forecast to spin initial
troughing into northern California by Saturday and lifting through
the Northwest Sunday. Then the low/trough itself will gradually
track eastward toward the West Coast early next week, with some
typical model spread. Meanwhile, watching a shortwave that dives
southeast through central Canada on the western side of the upper
low, reaching the Upper Midwest with more troughing by around next
Tuesday. 00Z models came in with reasonably good agreement with
that feature (after the 12Z CMC was a fast outlier).

The WPC forecast was based on a multi-model deterministic blend
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...

Moisture is forecast to flow into the western U.S. as shortwaves
undercut the mean upper ridge and as troughing edges toward the
West Coast. The monsoonal moisture looks to be influenced by what
is currently T.D. Twelve-E, bringing well above normal precipitable
water values that end up stretching through much of the West.
Large Marginal Risks are in place for Days 4 and 5 in the
Intermountain West as localized flash flooding is possible in this
moist environment. Instability looks to be particularly high in the
Desert Southwest, and would not be surprised to need Slight Risks
eventually there. Currently though the greatest instability axis
(eastern California, southern Nevada, western Arizona) is located
west of where the models focus QPF (more in the Mogollon Rim),
limiting confidence in the placement. Moisture and upper-level
support for rain and storms will shift gradually eastward this
weekend into early next week, spreading into the southern and
central Plains, but the details are dependent on the T.D. track.
Generally some precipitation looks to expand into parts of the
Plains on Saturday with chances for heavier rain by early next
week.

Frontal boundaries moving through the central and eastern U.S.
ahead of the primary trough late this week will provide a focus for
rain and some convection. At this point rainfall mainly looks to
stay below Marginal Risk levels for flash flooding concerns. One
exception could be the central U.S. though, where the front
oriented west-east and instability could lead to training storms.
But model guidance currently shows a rainfall focus anywhere from
Kansas, Oklahoma, Missouri, or Arkansas on Day 4/Friday, so will
hold off on any ERO risk area for hopefully better model
convergence in future cycles. A couple of rounds of rain are
forecast to move through the East Friday and Saturday with the
fronts, and the cool westerly flow atop the Great Lakes could lead
to lake effect rain showers for the weekend. Farther south, fronts
meandering over southern parts of Florida in an unstable
environment with above normal moisture could continue to allow for
localized flash flood potential with urban areas most vulnerable.
Marginal Risks are in place for South Florida and the Keys on Day
4/Friday and for South Florida on Day 5/Saturday (as the moisture
focus pulls a bit away from the Keys) for this potential.

The large trough covering the central and eastern U.S. for late
week will promote well below average temperatures behind multiple
cold fronts. Lows are forecast to be generally around 10-15 degrees
below average for Friday 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 for
North Dakota to around Lake Superior, depending on wind and cloud
cover. Highs could be even more anomalous on Friday across the
Upper Midwest/Great Lakes -- around 15-20 degrees below normal.
This could set a few daily records for low maximum temperatures,
as highs only reach the 50s in some locations of the Upper Midwest
and Great Lakes. Temperatures are forecast to gradually warm
closer to normal into early next week as the colder air mass moves
east and moderates as the upper trough lifts out. Still,
temperatures may not quite get back to typical early September
values. On the other hand, the Northwest can expect one more warm
to hot day Friday before temperatures continue to gradually
moderate there. In the West, the ample cloud cover and shower
coverage will generally yield near to above average lows and near
to below average highs.


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



$$