293
FXUS02 KWNH 220720
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
219 AM 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
pattern by late week. 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. Precipitation is
likely to spread across the central Great Basin/Intermountain
West/central Rockies including potential for heavy snow in higher
elevations. A forming low pressure/frontal system is forecast to
help focus moisture farther east by midweek and beyond. Currently
expect moderate to heavy precipitation across the Tennessee and
Lower Ohio Valleys into the Appalachians by Wednesday and Thursday,
and some precipitation may be in the form of snow in higher
elevations and farther north across the Great Lakes and interior
Northeast, with questionable precipitation types in the Mid-
Mississippi and Upper Ohio Valleys. Increasing coverage of colder
than average temperatures is likely as the week progresses
underneath the deepening upper troughing and cold surface high
pressure.


...Guidance/Predictability Assessment...

Model guidance is in fairly good agreement regarding the overall
pattern next week but varies with the details of smaller scale
features. A shortwave in the north-central U.S./south-central
Canada is one example. The 12Z ECMWF was more aggressive with a
secondary shortwave (behind a reasonably agreeable one progressing
along the U.S./Canada border from northern Minnesota toward Lake
Superior and eastward) dropping southeast toward North Dakota
Tuesday-Wednesday than other deterministic models and AI/ML
guidance, with some impacts on the surface low track. The more
recent 00Z run of the ECMWF seems to be in better alignment.
Farther west, the 12Z UKMET was on its own holding onto a shortwave
just offshore northern California into Wednesday morning while
other models brought that energy farther inland into the West. How
much that energy digs, as well as its interactions with northern
stream shortwaves, will impact the depth and positioning of the
deepening broad trough into later week. The 12Z CMC held troughing
back into the central Great Basin into Thursday unlike other
guidance that was more progressive. The forming surface low and
frontal system on the southern/eastern side of the upper trough
show some notable spread, but the 00Z ECMWF and CMC happened to
agree on a low track across the Southeast Thursday and turning
north, reaching offshore of NYC or so by Friday morning.
Uncertainties
with flow upstream from this trough will also have to be resolved
in future forecasts.

The WPC forecast was based on a blend favoring the deterministic
12Z EC/CMC and 18Z GFS with lesser proportions of the GEFS and EC
ensemble means and the UKMET early on. Gradually reduced the
proportion of deterministic models in favor of the ensemble means,
with the latter over half by Days 6-7 given variations with the
individual models.


...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 added a Marginal Risk to the Day
4/Monday ERO for those areas. 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 is
forecast to move through the east-central U.S. Monday ahead of a
cold front as a relatively weak surface low 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. 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
highest probabilities of notable snow are into the central/northern
Appalachians into northern Pennsylvania and the interior Northeast
by Thanksgiving Day, possibly impacting travel. Continue to
monitor forecasts as details are likely to change.

Temperatures are forecast to be warmer than average in the
southern tier stretching 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. 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.


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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