226
FXUS02 KWBC 070758
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Tue Oct 7 2025

Valid 12Z Fri Oct 10 2025 - 12Z Tue Oct 14 2025


...General Overview...

A deep upper low off the Pacific Northwest is expected to turn
toward the coast on Friday and swing east across the Intermountain
West through this weekend. This will draw tropical moisture from
Priscilla and lead to widespread precipitation across The West.
There is even further model support for a coastal storm to develop
off the Carolina Coast Friday night and drift up the Mid-Atlantic
coast through the weekend. In between these systems is persistent
upper-level ridging centered over Texas with an axis that extends
up the Great Plains on Friday and then gradually shifts east to
the Great Lakes by Sunday.


...Guidance/Predictability Assessment...

The opening of the upper low off the Pacific Northwest on Friday
and its eastward progression as a trough across the western U.S. is
one of the main forecast challenges of the period. Confidence is
high on the low center tracking toward the OR/CA border with global
deterministic guidance more progressive than the EC-AIFS (with the
00Z GFS/CMC turning more progressive while the 00Z ECMWF slowed a
bit). This slower progression plays a role in timing and intensity
of precipitation across the West this weekend.

Despite the uncertainty with timing of the trough ejection there is
decent QPF agreement across the Southwest among global guidance for
Friday and Saturday. WPC QPF for the Southwest is a blend of the
01Z NBM with the 18Z GFS/EC-AIFS which tones down the excess bias
correction in the Desert Southwest that comprises 50% of the NBM
for Day 4 and on.

Coastal cyclogenesis off the Carolinas Friday night that drifts
north to at least the southern Mid-Atlantic coast this weekend is
now favored by the EC-AIFS, GFS, and ECMWF while the CMC and UKMET
are not yet on board. This system could become a rather significant
nor`easter with heavy rain, strong wind, and coastal concerns.
A varied mix of the global deterministic models from 12Z/18Z were
used to produce the Days 3/4 forecast with considerable usage of
ensemble means for Days 5-7 with an attempt to make the 500mb and
frontal progs similar to the EC-AIFS which is not available in our
legacy software system.


...Weather/Hazards Highlights...

An influx of tropical moisture ahead of Priscilla and an opening
trough along the West Coast is expected across the Intermountain
West Friday through the weekend. Despite higher QPF on Friday, the
forecast prevalence of cloud cover and limited instability still
warrants a Marginal ERO for Four Corners (including most of
Arizona) in the new Day 4. Lower QPF, but greater forcing from the
approaching trough (along with a second day of moderate to heavy
rain) warrants a Slight Risk for the new Day 5 from the Mogollon
Rim of Arizona to the San Juans of Colorado including some slot
canyons in southern Utah and northwest New Mexico. The tropical
moisture surge continues to overspread the rest of the Intermountain
West on Sunday, particularly with a preferred slower ejection of
the trough from the West Coast, so that threat will continue to be
monitored. Expect mountain snow under the upper trough passage.

Low pressure looks to develop along a stalled boundary off the
Carolina Coast Friday night with a surge of heavy rain slowly
lifting north into the southern Mid-Atlantic through the weekend.
The Day 4 Marginal ERO is maintained along the Carolina Coast with
some marked increases in QPF for Saturday from a full-fledged
Nor`easter. A Slight Risk ERO is raised for the new Day 5 for
eastern North Carolina.

Cool conditions with max temperatures five or so degrees below
normal continue along the Eastern Seaboard Friday. Meanwhile,
ridging over Texas amplifies up the Rockies and Great Plains this
weekend which will maintain above normal temperatures. Cool
conditions are expected over the West behind the cold front once
the low off the Pacific Northwest ejects east.

Jackson


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


$$