012
FXUS02 KWNH 090703
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
202 AM 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 reasonably progressive pattern is forecast next week as a series
of shortwaves progress across the country. An initial shortwave
should exit the Northeast by Wednesday as another deeper trough
moves into the Central U.S. and eventually the East late week. This
may bring some much needed precipitation to parts of the Mid-
Atlantic to Northeast. Out West, an atmospheric river is forecast
to impact parts of the Northwest around mid-week bringing heavy
rain and snow threats, with additional rounds of precipitation
later in the week with possible trough reloading along the coast.


...Guidance/Predictability Assessment...

The first shortwave exiting the Northeast shows good agreement it
should accelerate northward through the Canadian Maritimes, but the
12z ECMWF was an outlier in showing cut off energy lingering off
the coast and eventually combining with the next system. That next
system shows better agreement today compared to previous runs on
the amplitude and timing as it moves into the East mid to late
week, but lots of uncertainty on possible cut off energy off the
Northeast by next weekend. A deeper trough will cross the West
Coast around Thursday, and there are questions on timing of this
system as it moves across the West Friday-Saturday. The CMC is
quickest to bring the trough inland, while the ECMWF deepens the
system pulling it farther south into southern California and thus
much slower to move the energy inland. Prefer a blend of the
ensemble means for now which are a nice middle ground solution. The
evolution out West has implications on timing of a downstream
ridge into the East next weekend.

The WPC blend for tonight favored a general model blend for the
first half of the period, gradually transitioning to a slight
majority blend of the ensemble means (with the ECMWF and GFS) late
period. Overall, this maintained good continuity with the previous
WPC forecast.


...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,
excessive rainfall and flooding is possible and so a marginal risk
is included on both the 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 mid-week. 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 precipitation
to the eastern half of the country during at least parts of the
Tuesday- Thursday period. Minor model disagreements in the trough
position/evolution do cause some spread for the positioning 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. The surface low track associated with this trough
near the Great Lakes could spread moderate to high winds especially
over the waters but possibly for some land areas like Michigan`s
Upper Peninsula.

While system progression will provide some day-to-day variability,
much of the central to eastern half of the country 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, and
finally in the east- central U.S. Thursday behind the secondary
cold front. 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.


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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