779
FXUS02 KWBC 221857
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
157 PM EST Fri Nov 22 2024

Valid 12Z Mon Nov 25 2024 - 12Z Fri Nov 29 2024


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


...Overview...

The general upper-level pattern next week will start with quasi-
zonal flow atop the lower 48 with embedded shortwaves at various
times and regions, transitioning to a broad deepening trough
aligned over the east-central part of the country by late week. To
the south of an initial trough/upper low near the Pacific Northwest
coast, some moist inflow (though much weaker than the ongoing AR
event) will continue to spread into parts of the West and produce
precipitation early next week. Moisture will likely extend across
the central Great Basin/Intermountain West/central Rockies
including potential for heavy snow in higher elevations.
Progression of West Coast energy and eventual approach of Canadian
flow should support developing low pressure/frontal system that
would help to focus moisture farther east by midweek and beyond.
However current guidance suggests much lower confidence in surface
details versus the overall upper pattern evolution. There is some
clustering toward an area of moderate to heavy precipitation
across the Tennessee and Lower Ohio Valleys into the Appalachians
by Wednesday and Thursday, with increasing spread for
amounts/coverage toward the East Coast. Some snow 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...

Latest guidance continues to show reasonable agreement with respect
to the large scale pattern evolution during next week, but there
are some embedded sensitivities that ultimately affect specifics of
the Midwest/Great Lakes into southeastern Canada system early-mid
week and trailing low pressure that should be over the southern
High Plains as of early Wednesday and then affect the eastern U.S.
later in the week. Based on 00Z/06Z model guidance, an operational
model composite early and then partial inclusion of the 06Z
GEFS/00Z ECens means after midweek provided a reasonable reflection
of consensus ideas plus late week trends near the East Coast.

There is still some uncertainty over the specifics of southern
Canada/northern tier energy supporting the leading Midwest/Great
Lakes system early in the week, with the 00Z ECMWF also differing
from other guidance over eastern Canada (showing a Quebec upper low
not seen in other solutions, and corrected in the new 12Z run). At
the very least, other operational models and machine learning (ML)
guidance suggest that latest GFS runs could be a few mb deep with
the surface low by early Tuesday. The multi-model blend helps to
mitigate these and other detail uncertainties.

While there is good agreement toward an upper trough axis reaching
near the east-central U.S. by late week, the upper shortwave and
surface evolution details are more in doubt. There has been
spread along with run-to-run variability for the timing and
evolution of initial trough/upper low energy near the Pacific
Northwest, and beyond that some differences for trailing northern
stream flow pushing southeastward from Canada. These combine to
yield increasing uncertainty for surface low details over the
eastern U.S./western Atlantic after midweek. In the 00Z/new 12Z
cycles, the most notable trend is in the ECMWF/ECens away from a
scenario of primary low pressure tracking into the Ohio
Valley/Great Lakes followed by secondary coastal development and
toward most GFS runs and a number of other solutions that depict a
somewhat flatter evolution through the Tennessee Valley and then
continuing eastward/northeastward. Individual ensemble members show
even wider spread by early Friday, as do the ML models which range
from one still over the Ohio Valley and at least one other so
weak/progressive that it minimizes precipitation coverage over the
East. Another couple ML models have a wave over the Southeast as of
early Friday. Meanwhile, the 00Z/12Z CMC runs show a deeper
surface low than other dynamical/ML models by early Friday, albeit
with a position not far from the dynamical model/ensemble mean
majority centered close to the northern Mid-Atlantic/southern New
England coast. This dynamical majority cluster seems to offer the
most reasonable starting point given the current guidance spread.


...Weather/Hazards Highlights...

An upper trough/possible closed low and a surface low offshore of
the Pacific Northwest could continue to direct moisture into the
West Coast early next week, though much weaker than the current
atmospheric river. By Monday the highest moisture anomalies look to
focus into southern California, with generally modest rain
expected near coastal areas there, but do not currently expect
flooding concerns with this rain since this area has not been
impacted as much by recent heavy rain. However, some precipitation
is likely into northern California too, which will have much wetter
antecedent conditions after the past few days. Rain amounts of 1"
to locally 2" of rain would not typically cause concerns in
northern California/far southwestern Oregon, but there may be a
chance for some flooding because of saturated soils and above
normal streamflows, so have maintained the Marginal Risk in the
Day 4/Monday ERO for those areas with only minor adjustments per
latest guidance. Meanwhile snow is likely to accumulate across the
Oregon Cascades and especially the Sierra Nevada Monday-Tuesday,
spreading east across 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. Precipitation should finally abate in the
West for the latter part of next week.

Farther east, a round of light to moderate precipitation should
move through the east-central U.S. Monday ahead of a cold front as
low pressure moves through the Great Lakes region. Most
precipitation should be rain other than some northern tier snow.
Lake effect rain/snow showers should continue into Tuesday behind
the low, with a brief period of breezy conditions possible as well.
Then as another low pressure/frontal system develops in the south-
central Plains and tracks eastward as the week progresses,
moisture will stream into the eastern half of the CONUS. Currently
the heaviest rainfall axis looks to be around the Mid-
South/Tennessee Valley/Lower Ohio Valley, though with uncertainty
in placement. North of the low track and frontal boundary, snow or
ice may be possible. Some wintry weather may affect the Middle to
Upper Mississippi and Ohio Valleys along with the Great Lakes
region Wednesday-Thursday, while currently the relatively higher
probabilities of notable snow are into the central/northern
Appalachians into northern Pennsylvania and the interior Northeast
by Thanksgiving Day, possibly impacting travel. There is still
considerable uncertainty over the system`s track and strength, so
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 from the southern
tier into the east-central U.S. on Monday ahead of a cold front.
This front will cool the Ohio Valley by Tuesday and then the East
by Wednesday, leading to highs a few degrees below normal. A couple
of surges of Arctic air look to impact the northern Plains early
in the week 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, even
cooling the southern tier to near or below normal after a warm
early week period with upper 80s in southern Texas.


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







$$