718
FXUS02 KWNH 030759
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
359 AM EDT Wed Sep 3 2025

Valid 12Z Sat Sep 06 2025 - 12Z Wed Sep 10 2025


...Overview...

An amplified pattern will be in place across the CONUS by the
weekend with an upper low over Ontario anchoring troughing across a
vast portion of the central and eastern states. This will favor
below average temperatures behind multiple cold fronts along with
multiple rounds of showers and thunderstorms along the leading
boundary through the Southern/Central Plains to 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 Storm Lorena 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.S. Lorena
as well as other features like shortwaves in the vicinity.


...Guidance/Predictability Assessment...

The latest suite of guidance is in generally good agreement with
the upper low/trough and continues to have typical spread
shortwaves rounding the base of the trough that would effect the
timing and position of frontal boundaries and QPF over the weekend.
To the west, an upper-ridge will initially be in place over the
northwestern U.S./southwestern Canada with an upper low meandering
in the northeastern Pacific. Periodic shortwave energy ejecting
from the low may pass across the West under the ridge, and
potentially further east into the central U.S., before the upper
low begins to progress eastward over the West by early next week.
Downstream, the deeper upper low/trough anchored over Ontario is
projected to lift northeastward with the potential for shortwave
energy along the backside of the system to drive another frontal
system southward through south-central Canada and into the north-
central U.S. early next week.

The wrench in the forecast revolves around T.S. Lorena and its
future track and potential influence on precipitation in the
Southwest/South-central U.S. The majority of solutions keep the
cyclone offshore where it will linger or dissipate; however, the
GFS is more progressive in nature and brings it into northwest
Mexico. However, despite the uncertainty on some of the
track/timing specifics, most guidance agrees some energy/moisture
influence will eventually shift northeastward into the Southwest
to South-central U.S., leading to increased precipitation chances.
A frontal system on the southwest side of the Ontario upper-low as
well as the influence of potential short-wave energy emanating
from the western U.S. could also impact precipitation chances. Not
surprisingly, QPF has various timing/location/total differences
across the guidance, so the updated forecast remains a bit
conservative in terms of amounts, but a general theme of increased
moisture/precipitation across the Southwest to South-central U.S.
is agreed upon and the ceiling could be higher.

The WPC preferred blend utilized a multiple model start that
slowing included the ECMWF ensemble and GEFS means through the
extended period.


...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 will become modified as moisture
in association with current T.S. Lorena is drawn into the
Southwest/Southern states. This will result in PW values rising to
well above normal. A vast area will have an elevated threat for
heavy rainfall and local flash flooding within this very moist
environment. Instability looks to be particularly high in the
Desert Southwest and would not be surprised to need Slight Risks
eventually there. For now, a broad Marginal covers most of the
West/Southwest. 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 and
southeastern Arizona/southwestern New Mexico), 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 tropical cyclone track. Generally some
precipitation looks to expand into parts of the Plains with a
chance for heavier rain this weekend into early next week.

Showers and thunderstorms are expected to fire up along and ahead
of the leading cold front as it advances through the Central and
Southern Plains. Rainfall continues to look like it will 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.
Although the latest guidance show a fair amount of spread on where
the heaviest amounts will focus, there is a growing signal for the
Southern Plains into the Lower Mississippi Valley therefore
supports the need for a Marginal Risk for Day 5. The influence of
Lorena could have an impact on this potential as well. The
lingering frontal boundary(s) in combination with Gulf moisture
could also lead to some heavier rainfall along the Gulf Coast into
early next week.

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. The lingering boundary will lead to
storm potential along the coastal Southeast into early next week
and there is the potential for some heavier rainfall here as well.


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 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.

Campbell/Putnam


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









$$