431
FXUS02 KWBC 041904
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
304 PM EDT Thu Sep 4 2025

Valid 12Z Sun Sep 7 2025 - 12Z Thu Sep 11 2025

19Z Update: The overall synoptic scale pattern is well handled by
the 12Z model guidance suite for Sunday, and a multi-deterministic
model blend suffices as a good starting point in the forecast
process, followed by gradually increasing contributions from the
ensemble means. Some differences in a shortwave trough over
central Canada late Sunday work their way downstream across the
Upper Midwest and Great Lakes on Monday, but otherwise good
agreement elsewhere. Looking ahead to the middle of the week, the
GFS and GEFS mean become more amplified with the upper trough over
the Southeast U.S. compared to the non-NCEP consensus, and also
more amplified with the upper ridge developing across the Plains.
There is some potential for an Omega block type pattern to develop
next week with upper troughs situated over both the West and East
Coasts, and a ridge over the Plains. A lingering stationary front
just off the southeast coast will likely keep scattered to numerous
showers across much of Florida, and a Marginal Risk has been added
for Sunday and Monday for the eastern coast of Florida. The
previous forecast discussion is appended below for reference.
/Hamrick
-----------------

...Overview...

The medium range pattern begins Sunday with an upper trough axis
over the east-central U.S. promoting cooler than average
temperatures. A frontal boundary ahead of the trough is forecast to
push offshore of the East Coast but linger over the western
Atlantic for some coastal showers in the Carolinas and south into
Florida with the front meandering across the peninsula. On Sunday
especially, the back end of the front curling into Texas could
cause some heavy rainfall and localized flash flooding there. In
the West, an upper low looks to slowly move inland and bring
multiple days of modest rain to the Northwest along with cooler
temperatures. Upper ridging is forecast over the Rockies into the
central U.S. as the week progresses in between the two troughs, for
warming temperatures.


...Guidance/Predictability Assessment...

Model guidance is in fairly good agreement with the larger scale
pattern described above. Embedded shortwaves in the flow show
typical spread that can influence sensible weather -- one such
feature is a potential shortwave in the southern Plains that ECMWF
runs are particularly strong with on Sunday, which increases its
QPF and is really the main model that would support the Slight Risk
ERO on Sunday. But especially the newer 00Z models have the depth
of the northeastern trough pretty agreeable and it lifting toward
midweek, while the 12Z CMC was weaker with the southern stream
troughing in the Southeast by midweek (but 00Z CMC improved).
Guidance also shows good consensus with the timing and depth of the
upper low pushing toward the West Coast (after the 12Z ECMWF was a
little hesitant to close off the low early in the week). There is
increasing spread with the evolution of the western low/trough by
Wednesday-Thursday, as model differences with possible influence
from a British Columbia shortwave causes some phasing differences,
but the discrepancies between models are not too large for a Day
6-7 forecast. Thus the WPC forecast began with a multi-model
deterministic blend, and 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 7 amid
increasing spread.


...Weather/Hazards Highlights...

Some enhanced moisture left over from Lorena will be in place atop
the southern Plains into Sunday, while a front stretches west from
the Gulf into Texas. This combination along with instability in
place could allow for some locally heavy rain, and will show a
Slight Risk across the Edwards Plateau to south-central Texas in
the Day 4/Sunday ERO, with a broader Marginal stretching through
much of Texas into the southern High Plains and Colorado. Rainfall
amounts should generally decrease by Monday but some showers are
still possible, and showers and storms could reach into much of the
Plains to Upper Midwest with a tongue of instability near another
front. Consider this activity less than a 5 percent chance of flash
flooding for no ERO risk on Day 5/Monday for now. Central U.S.
convection could continue as the week progresses.

Farther east, the frontal system over Florida and northeast in the
western Atlantic will provide a focus for moisture to pool,
leading to at least scattered diurnal convection each day. Moisture
levels look to decrease slightly over Florida compared to the
short range period, but the saturated grounds from rainfall before
Sunday warrant the addition of Marginal Risk areas for both Sunday
and Monday from roughly Cape Canaveral to the greater Miade-Dade
metro area. Rainfall across the Carolinas into Georgia is rather
uncertain with how much convection may occur onshore or offshore on
any given day. Small shifts in the frontal boundary could produce
significant differences in the rainfall amounts.

The upper low pushing through the eastern Pacific toward the
Northwest will produce multiple days of rainfall next week,
expanding eastward with time from the Pacific Northwest/northern
California into the northern Great Basin and Intermountain West.
This cool season type of rainfall should be marked by moderate
rates that are generally not enough to cause flooding concerns. The
lowering heights aloft yielding cooler temperatures could produce
snow for the highest peaks.

Cooler than normal temperatures are likely to continue into early
next week across the Plains and Midwest underneath the broad upper
trough, with anomalies around 10-15 degrees. This brings highs in
the 70s as far south as parts of the southern Plains, while lows
could reach the 30s in the Dakotas to Minnesota and potentially
cause frost concerns (partially dependent on wind) Sunday morning.
The cooler airmass should moderate as it moves into the East
Tuesday and beyond, while the central U.S. sees a warming trend as
upper ridging moves overhead. Meanwhile, the West can expect
average to above average minimum temperatures especially in the
Northwest, but high temperatures will cool considerably under the
upper low that approaches the West Coast and Great Basin through
next week.


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






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