973
FXUS02 KWBC 191857
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 PM EDT Sat Oct 19 2024

Valid 12Z Tue Oct 22 2024 - 12Z Sat Oct 26 2024

...General Overview...

The weather pattern across the continental U.S. on Tuesday will
feature an upper ridge and surface high over the Eastern U.S.,
accompanied by above average temperatures. An incoming trough from
the Midwest and a stronger trailing northern tier U.S./Canada
trough will then dislodge this ridge while the associated cold
front will herald the arrival of cooler weather towards the end of
the week. The front will have limited moisture to work with so
rainfall amounts from the Great Lakes to the Northeast will be
generally light. Splitting elongated trough energy should produce a
Pacific storm system expected to approach the Pacific Northwest
late this week, but with uncertain timing/strength/track. This
system and possibly a trailing front on Saturday should bring a
return of rain and high mountain snow from northern California to
Washington state. Ahead of the Pacific system, expect mean ridging
aloft and above normal temperatures over the Interior West/Rockies
and then into the central U.S.

...Guidance/Predictability Assessment...

Models and ensembles continue to have difficulty figuring out some
details resulting from the splitting of energy initially within a
Canada into eastern Pacific elongated upper trough. While
there are still differences with specifics of the northern energy
that traverses the northeast quadrant of the lower 48 as a fairly
vigorous shortwave around Wednesday-Thursday, this feature has
generally been better behaved in the guidance than the closed low
expected to form over the Pacific (then lift northeastward as it
opens up) or smaller scale energy that may pinch off near Vancouver
Island or the far Northwest U.S. and then drop southeastward.
Behind these uncertainties, there is actually better than average
agreement for Bering Sea/Aleutians flow that should quickly develop
Northeast Pacific troughing toward the end of the week.

At the moment there is no clear clustering and seemingly as much
spread as in recent runs for where the eastern Pacific low will
close off plus its northeastward timing as it opens up, as well as
for one or more smaller scale pieces of energy that may originate
from near/offshore the Pacific Northwest/Vancouver Island. CMC runs
remain on the faster side of the spread for the Pacific system,
with latest GFS runs tending to lean closer to the CMC than the
slow ECMWF mean runs. Latest machine learning (ML) models reflect a
lot of this spread as well, while the average surface low track has
generally trended a bit southward versus 24-36 hours ago--but still
within a range of the Pacific Northwest to Vancouver Island. There
is also a little better agreement today that the opening of the
upper low should support just moderate depth for the surface low.
Overall prefer a solution closest to continuity, which is in the
middle to faster half of the spread, while waiting for clearer
clustering. This timing difference affects the pattern over the
central U.S. by next Saturday.

Interestingly, the 00Z ML models have become more defined with
lingering Pacific Northwest/Vancouver Island energy that ultimately
translates southeastward like ECMWF/UKMET runs. Such a solution
would produce a more defined surface wave and even a trailing cold
front over the central U.S. versus a broad southerly gradient as
seen in most other guidance. The favored blend yields merely a
wavy warm front at this time, with a more prominent consensus
required to adjust toward the alternate scenario.

The updated forecast started with a blend of the 00Z/06Z
operational models (though splitting ECMWF input with the 12Z/18
run) with least weight to the CMC for the first half of the period.
Then the forecast added some 00Z ECens/CMCens means (GEFS remaining
questionably weak for the Pacific system) and WPC continuity to
complement the 06Z GFS and two ECMWF runs for the rest of the
period to maintain reasonable stability given persistent guidance
spread.

...Weather/Hazards Highlights...

The cold front crossing the Great Lakes and into the Northeast
Wednesday into early Thursday will likely have just enough
moisture and lift to produce some mainly light to moderate showers,
mainly north of the Interstate 70 corridor. It should continue to
remain dry and mostly sunny for areas farther to the south across
the Mid-Atlantic and the Southeast U.S., where the very pleasant
fall weather will continue for a while. There are no risk areas in
the Day 4 and Day 5 excessive rainfall outlooks. Uncertain small-
scale shortwave energy could produce some pockets of precipitation
from the northern Rockies southeastward mid-late week but with
very low confidence in existence or other details.

The pattern will become favorable for the return of moisture to
the Pacific Northwest late this week into the weekend. However
there is continued spread for a system expected to develop over the
eastern Pacific by midweek and lift northeastward thereafter. Rain
and high elevation northern Cascades snow could commence as early
as late Thursday per the fast side of the spread, or take until
late Friday or so on the slower side. Behind this system, Northeast
Pacific upper dynamics and an associated surface front should help
to maintain precipitation through Saturday. Currently this looks
more like a broad precipitation event rather than a focused
atmospheric river, but some locally enhanced totals will be
possible. Lighter precipitation may reach the Northern Rockies.

Temperatures will feel more like September across areas from the
central U.S. into the Northeast Tuesday into Wednesday, with highs
running 10-20 degrees above average for late October and possibly
even a little higher over parts of the Upper Midwest/northern Great
Lakes on Tuesday. A cold front pushing rapidly eastward from the
northern Plains will bring more seasonable conditions after
midweek. From late week into the weekend, a building Western U.S.
upper ridge that pushes eastward ahead of the Pacific storm system
should promote a warming trend first over the Intermountain
West and Rockies/western High Plains and then covering much of the
central U.S. where many areas should see highs 10-20 degrees above
normal. The West Coast states should be within a few degrees on
either side of normal aside from a brief warmer period over parts
of California around midweek.

Rausch/Hamrick


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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