277
FXUS02 KWBC 091859
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 PM EST Sat Nov 9 2024

Valid 12Z Tue Nov 12 2024 - 12Z Sat Nov 16 2024

... Another atmospheric river to bring heavy rain and mountain
snow threats to parts of the Northwest mid next week...

...Overview...

A fairly progressive pattern is forecast next week. Upper
troughing over the East to start the period Tuesday will push into
the Atlantic into midweek while another trough tracks from the
Intermountain West to the central U.S. Tuesday-Wednesday and into
the East around Thursday, bringing some light to moderate much-
needed precipitation for the Midwest to Mid-Atlantic/Northeast.
Upper ridging is likely on either side of these troughs, promoting
some warmer temperatures. Out West, an atmospheric river is
forecast to impact parts of the Northwest around midweek bringing
heavy rain and snow threats, with additional rounds of
precipitation later in the week with trough reloading along the
coast.


...Guidance/Predictability Assessment...

Model guidance remains reasonably agreeable on the large scale
progressive pattern, and exact timing and amplitude of the troughs
and ridges are the most uncertain aspects of the forecast. The
first trough shows okay agreement as it moves through the East, but
as it tracks offshore there are model differences with an upper
low perhaps closing off atop the western Atlantic by Wednesday or
Thursday. Model differences with the next trough (the one that
starts over the West Tuesday) may be more impactful for sensible
weather. Namely, GFS and GEFS based guidance are quicker with the
trough axis or at least farther east with its eastern side for
Tuesday-Thursday, leading to QPF pushing eastward through the
Midwest to Northeast more quickly than shown by the non-NCEP
guidance. The WPC QPF made an attempt to slow down the NBM QPF that
looked more like the GFS/GEFS, but may have ended up too light
with amounts than will actually verify. The new 12Z GFS may be in
slightly better alignment with the other guidance than its previous
runs. Meanwhile the new 12Z CMC and ECMWF have the depth of the
trough a little deeper into the South compared to previous runs,
which also slows the trough down a bit, so will continue to monitor
this possibility.

Timing of the next trough moving through the West later week also
shows some timing inconsistencies, and a couple of rounds of energy
shaping the trough differ. Generally preferred a middle ground of
the slower EC/EC mean and faster GFS/GEFS mean.

The WPC forecast used a multi-model deterministic blend early in
the forecast period, and then decreased amounts of the
deterministic guidance (GFS and UKMET first, then CMC and ECMWF) as
the period progressed in favor of the GEFS and EC ensemble means.


...Weather/Hazards Highlights...

The main concern for weather hazards will be with a modest
atmospheric river set to begin impacting the Pacific Northwest by
Tuesday and may last a few days. Heavy Rainfall and mountain snows
are expected, and with an AR expected in the short range period
creating wetter antecedent conditions, excessive rainfall and
flooding are possible. Marginal Risks remain in place for both Day
4/Tuesday and Day 5/Wednesday for parts of the Pacific Northwest
and farther south into northern California as the AR slowly shifts
southward midweek. Accumulated snow may become heavy in the
Cascades as the week progresses, with modest snow amounts into the
northern Rockies.

The upper trough affecting the West early in the week will
progress eastward and bring some areas of much needed but modest
precipitation to the eastern half of the country during at least
parts of the Tuesday-Thursday period. Continued model disagreements
in the trough position/evolution do cause some spread for the
positioning and amounts of precipitation on individual days, but on
a broad scale, modest precipitation should affect parts of the
Plains and Mississippi Valley Tuesday, spreading into the
Midwest/Great Lakes and Ohio/Tennessee Valleys to Appalachians
Wednesday, and toward the Eastern Seaboard Thursday. Most
precipitation looks to fall as rain given the warmer than normal
temperatures, until some ice and snow are possible in higher
elevations of the Interior Northeast mid- to later week. Models are
starting to show some potential for rain to focus near the central
Gulf Coast on the southern side of the upper trough and front by
Wednesday, but this is still uncertain. Winds may be gusty behind
the associated low pressure and frontal system in the northern
Rockies to northern Plains Tuesday-Wednesday.

While system progression will provide some day-to-day variability,
most of the lower 48 to the east of the Rockies should see above
normal temperatures much of the week. Temperatures are forecast to
moderate behind a cold front across the Great Lakes region Tuesday
and over much of the Eastern Seaboard Wednesday, while the Plains
and Mississippi Valley remain mild, with milder temperatures
stretching into the east-central U.S. by late week under the
trough. Meanwhile lingering snow cover in the central/southern
Rockies will promote cooler than normal highs there. The West
otherwise should generally near to a bit below normal, with
increasing coverage of below normal temperatures (especially highs)
by late week.


Tate/Santorelli


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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