344
FXUS02 KWBC 231859
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 PM EST Sat Nov 23 2024

Valid 12Z Tue Nov 26 2024 - 12Z Sat Nov 30 2024


...Messy Thanksgiving travel possible in the eastern half of the
U.S., including potential central/northern tier winter weather...


...Overview...

A southeastern Canada/New England system will produce some modest
rain/mainly higher elevation snow Tuesday into Wednesday. Behind
this low, quasi-zonal upper flow should should prevail across most
of the lower 48 while an initial trough/possible small upper low
near the West Coast becomes embedded within the westerlies. This
shortwave will help spread precipitation across parts of the West
and produce heavy snow in higher elevations through midweek, and
then combine with northern stream energies farther eastward as the
large scale pattern settles into a mean trough over the eastern
half or more of the country with an upstream ridge over the
Northeast Pacific. Guidance continues to show developing low
pressure ahead of the evolving eastern upper trough but with
significant detail differences affecting sensible weather.
Currently the best potential for moderate to heavy precipitation
exists across the Tennessee and Lower Ohio Valleys into the
Appalachians and Mid-Atlantic during a portion of the mid-late week
period. Some wintry precipitation may be possible in the northern
part of the moisture shield and in higher elevations. East of the
Rockies, the deepening upper trough and cold surface high pressure
will increase the coverage of colder than average temperatures as
the week progresses.


...Guidance/Predictability Assessment...

Latest guidance continues to agree fairly well for the large scale
pattern but ongoing disagreements/variability with initial West
Coast shortwave energy and trailing northern stream flow maintain
considerable spread for eastern U.S. low pressure by the latter
half of the week. In addition, differences exist for a southern
stream trough/upper low that may approach California toward the end
of the week (affecting precipitation coverage/amounts over
California and the Southwest) while there is also spread for the
longitude of Northeast Pacific ridging.

Even including the new 12Z runs, guidance remains far apart for
specifics of the initial shortwave near the West Coast. Continuing
its theme from the 00Z cycle, the UKMET is on the slow extreme
while the new 12Z CMC has jumped to the fast side. The new 12Z
ECMWF has adjusted faster to a less extreme degree, and both of
those new 12Z runs now deepen low pressure more quickly with a
track into New England and then Quebec (ECMWF farther northwest).
This differs from what has been a recent majority cluster depicting
a somewhat farther southeast track, or previously in some guidance
runs an Ohio Valley low followed by secondary coastal development.
In contrast the 00Z CMC eventually strayed to the suppressed side
over the Atlantic. The 12Z UKMET is also quite suppressed due to
its slow West Coast shortwave. Meanwhile, latest machine learning
(ML) guidance has at least been trimming its weak/suppressed side
of the spread late this week while still showing a subset of
solutions a little northwest of the dynamical ensemble means and
GFS runs/00Z ECMWF. By early Saturday the ML runs generally favor
the system reaching somewhere in the vicinity of Nova Scotia (or a
little west/north/east), with an average depth in the 980s mb. A
blend of the 06Z GFS/00Z ECMWF and ensemble means provided a
reasonable intermediate approach given the spread and guidance
variability, with reasonable continuity aside from typical minor
timing adjustments.

Along the West Coast late in the period, guidance has been
gradually becoming a little better defined/clustered with the upper
trough/low and weak surface system offshore California, with an
average of (minus the farther south 12Z UKMET as of the end of its
run early Friday) looking reasonable. Farther north, some
noticeable differences arise with the longitude of the upper ridge
axis. Consensus keeps it farther offshore than the 12Z UKMET, but
there is a minority portion of the ML model envelope that would
bring the ridge a little farther east than seen in most of the
dynamical models/means by next Saturday. It will be worth watching
for any such trends in the dynamical guidance.

The updated forecast based on 00Z/06Z guidance started with an
operational model blend with greater emphasis on the 06Z GFS/00Z
ECMWF early in the period, followed by removal of the minority
UKMET component after Wednesday and the CMC after early Friday. 06Z
GEFS/00Z ECens filled in for the removed models and ultimately
reached 40 percent total input by next Saturday.


...Weather/Hazards Highlights...

A Pacific moisture plume, initially with precipitable water values
at least three standard deviations above normal, will sink slowly
southward into Tuesday with some continued rain and high elevation
snow over California. The moisture along with shortwave energy to
the north providing dynamical support/strong westerly flow aloft
will likely focus the heaviest totals into the central-southern
Sierra Nevada. Although this region is south of areas that were
most impacted by the recent significant atmospheric river, guidance
clustering has improved regarding the threat of significant totals
on Tuesday and first-guess excessive rainfall guidance also shows
potential for high enough totals/rates to merit a risk area. Thus
the Day 4 ERO has introduced a Marginal Risk area for the western
slopes of the central-southern Sierra Nevada below the expected
snow level. Some meaningful rainfall is possible farther west along
the California coast but likely with less extreme totals. Expect
significant snow to extend into the Intermountain West higher
elevations including the Wasatch and into the central Rockies as
well. The heavy snow could accumulate to 1 to 3 feet in the Sierra
Nevada and 1 to 2 feed in the Colorado Rockies. Precipitation
should finally abate in the West for the latter part of next week.

Farther east, light to moderate precipitation is likely across the
Northeast into Tuesday with the leading low pressure system, and
breezy westerly flow on the backside of the low will also produce
some lake effect and upslope rain/snow showers. Then as another low
pressure/frontal system develops in the south-central Plains and
tracks eastward/northeastward as the week progresses,
precipitation is likely for much of the eastern half of the CONUS
Wednesday-Friday. Currently the heaviest rainfall axis looks to be
around the Mid-South/Tennessee Valley/Lower Ohio Valley late
Wednesday-Thursday, though with uncertainty in placement. Gave
some consideration to a Marginal Risk for the Day 5/Wednesday ERO
as convection is forecast to ramp up Wednesday evening/night in an
environment with some instability, and the west-east track of the
convection along the surface low may lead to some training of
storms. Plan to hold off on any risk area for now given the
continued track/timing differences for low pressure and associated
effects on rainfall coverage/rates. Snow or ice may be possible
north of the low track and frontal boundary, possibly impacting
travel. The Day 4-7 Winter Weather Outlook indicates some low
probabilities for snow greater than 0.25" liquid equivalent across
parts of the central Plains/Middle Mississippi Valley Wednesday,
the Upper Ohio Valley and the central/northern Appalachians into
northern Pennsylvania and the interior Northeast by Thanksgiving
Day, and lingering across the Northeast and the Great
Lakes/Appalachians on the backside of the low by next Friday. But
the axis of snow (and possibly ice) will be quite dependent on the
uncertain surface low track, and confidence remains below average
regarding specifics of precipitation coverage/type/amounts.
Continue to monitor forecasts as details become better refined over
the coming days.

Expect temperatures to be warmer than average across the eastern
U.S. on Tuesday, but a cold frontal passage by midweek will lead to
highs near or a few degrees below normal. Also above average
temperatures may progress from the Southwest on Tuesday into the
southern Plains by Wednesday, with some plus 10F or greater
anomalies. A couple surges of Arctic air look to impact the
northern Plains through Tuesday and again later week, with highs in
the teens and lows below 0F in some areas, equating to at least
10-20F below normal. North Dakota and parts of Montana will see the
best potential for some even colder anomalies. The second surge of
cold air should eventually overspread much of the lower 48 east of
the Rockies (in moderated fashion), leading to below normal
temperatures across much of the Plains/Mississippi Valley through
the Ohio/Tennessee Valleys to the Eastern Seaboard by the end of
the week. Expect moderately below normal high temperatures to
spread across areas west of the Rockies mid-late week, trending
close to normal by next Saturday.


Rausch/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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