174
FXUS02 KWBC 021920
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
320 PM 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 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...

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. 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. Meanwhile, guidance is also in
generally good agreement that the deep upper low/trough anchored
over Ontario will begin 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.

T.S. Lorena remains the main complicating factor during the
forecast period with respect to its future track and potential
influence on precipitation in the southwest to south-central U.S.
The latest 00Z and 06Z runs of the GFS remain most aggressive in
bringing a stronger system eastward over Baja California and
northwestern Mexico, while much of the other global
deterministic/mean guidance has the storm lingering/dissipating
along or offshore the coast. The suite of EC AI guidance shows
varying solutions, with at least one hinting at a more easterly
track, and AI ensemble data shows a 50/50 split in solutions as
well. However, despite the uncertainty on some of the track/timing
specifics, most guidance agrees some energy/moisture influence will
eventually shift northeastward into the southwestern 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 southwestern to south-central
U.S. is agreed upon and the ceiling could be higher.

The updated WPC forecast begin with a composite blend of the
deterministic guidance while quickly incorporating at least a small
part of the ensemble means given differences on the specifics on
shortwave timing/placement, with the means eventually representing
half the blend by the end of the period. The 00Z GFS was favored
over the 06Z run because the forecast for Lorena was slower and
less aggressive which was in better agreement with the other
guidance.


...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.S. Lorena, 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 and
southeastern AZ/southwestern NM), 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.S. Track. Generally some precipitation looks to
expand into parts of the Plains with a chance for heavier rain
this weekend into 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
Texas, Kansas, Oklahoma, Missouri, or Arkansas on Day 4/Friday and
Day 5/Saturday, so will hold off on any ERO risk area for hopefully
better model convergence in future cycles. 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/Friday and for South
Florida on Day 5/Saturday (as the moisture focus pulls a bit away
from the Keys) for this potential. 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 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.

Putnam/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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