108
FXUS02 KWNH 230704
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
204 AM 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...

Upper and surface low pressure is forecast to traverse the Great
Lakes/Northeast Tuesday-Wednesday with some modest rain/mainly
higher elevation snow. Quasi-zonal upper flow should be in place
atop most of the lower 48 other than that feature and a
trough/possible small upper low moving into the West Tuesday-
Wednesday. The latter will help spread precipitation across parts
of the West and produce heavy snow in higher elevations through
midweek. The shortwave is forecast to combine with northern stream
energies as it moves eastward, creating a deepening upper trough
setting up atop the east-central U.S. late week. A surface low
pressure/frontal system should develop in response to this trough
but there remains considerable uncertainty in the surface pattern
and sensible weather details. There is some general clustering for
an area of moderate to heavy precipitation across the Tennessee and
Lower Ohio Valleys into the Appalachians and Mid-Atlantic by
Wednesday and Thursday, spreading into the Northeast Thursday-
Friday. Some wintry precipitation may be possible in the northern
part of the moisture shield and in higher elevations. Increasing
coverage of colder than average temperatures is likely as the week
progresses underneath the deepening upper trough and cold surface
high pressure.


...Guidance/Predictability Assessment...

Model agreement appears reasonable at the start of the medium
range period early Tuesday, with a multi-model blend working well
for the Northeast upper/surface low and the existence of shortwave
troughing/possible small closed low just offshore of Oregon.
However, models diverge as soon as Wednesday with the track and
timing of the western shortwave as it pushes eastward. In
particular, the 18Z GFS dug a stronger closed low farther south
into California Wednesday before tracking it east Thursday, causing
it to be slower than the model consensus. Downstream this resulted
in slower formation of a southern High Plains surface low than
other guidance and a more suppressed low track across the Southeast
into later week. Thus the 18Z GFS appeared to be an outlier but it
did have some support from a handful of 12Z AI/ML models
(FourCastNet, Graphcast, and Pangu). Meanwhile the 12Z AIFS was
weaker than anything else with the western shortwave, but the 18Z
was a bit stronger. The WPC forecast supported more of a middle
ground approach with this shortwave and eventual surface low,
favoring the 12Z run of the GFS and the 12Z ECMWF and CMC. The
newer 00Z runs fortunately seem to be along the same lines. So
developing and deepening troughing with an axis atop the east-
central U.S. by later week along with a surface low forming in
response shows some agreement in principle. But even within this
general consensus there is notable spread including in the ensemble
members. For example, there are still some members especially in
the EC ensemble that show a surface low track in the Ohio Valley
around Thursday-Friday like some older models showed. Plus the
sensible weather including precipitation amount and type will be
very sensitive to small differences. So expect changes to the
forecast in future cycles.

The WPC forecast was based on a blend of deterministic 12Z
guidance early in the period, and reduced the deterministic
proportion in favor of the ensemble means to 60 percent Days 6-7 to
minimize deterministic detail differences.


...Weather/Hazards Highlights...

A Pacific moisture plume will sink slowly southward into Tuesday
with some continued modest rain into California, mainly to the
south of areas that were most impacted by the recent significant
atmospheric river. The moisture and a shortwave providing dynamical
support will lead to accumulating snow across the Sierra Nevada
into the Intermountain West higher elevations including the Wasatch
and into the central Rockies. Heavy snow amounts could pile up
especially in the Sierra (1 to 3 feet) and in the Colorado Rockies
(1 to 2 feet). 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 under the 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 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 awaiting better timing and placement
agreement in the model guidance, plus the Thursday/Day 6 period may
end up being the wetter day. 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. The Southwest could also
see above average temperatures Tuesday that push into the southern
Plains Wednesday. A couple of 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. The second surge of cold air is forecast
to make it through much of the lower 48 (in moderated fashion) by
later next week. This will cool the West and southern tier to near
normal, while below normal temperatures are forecast across much of
the Plains/Mississippi Valley to Ohio/Tennessee Valleys to the
Eastern Seaboard by the end of the week.


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



$$