330
FXUS02 KWNH 240711
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
210 AM EST Sun Nov 24 2024

Valid 12Z Wed Nov 27 2024 - 12Z Sun Dec 01 2024


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


...Overview...

As the medium range period begins midweek, upper energy over the
Intermountain West will produce heavy snow in higher elevations of
the central Rockies in particular, 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 Valley
and Southeast 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, with best chances across the Northeast. The
deepening upper trough and cold surface high pressure will increase
the coverage of colder than average temperatures east of the
Rockies through late week and into next weekend. The cold westerly
flow should lead to some possibly heavy lake effect snows by next
weekend behind the low pressure system.


...Guidance/Predictability Assessment...

Though models agree reasonably well with the formation of an upper
trough atop the eastern U.S. for later week, unfortunately the
associated surface low pressure system track has higher than
typical spread for what is now a Day 4-5 forecast for the latter
part of this week, aligning with busy holiday travel dates.
Variability with the initial Intermountain West shortwave energy
and trailing northern stream flow are notable contributing factors
to the model disagreements. The 12Z UKMET was among the slower
models pushing the West shortwave eastward, but the 12Z AIFS and
some other AI/ML models were similar. The bulk of the other models
agree there should be cyclogenesis in the southern Plains by
Wednesday, but with differ in the low track beyond that. Generally,
ECMWF control runs and many EC ensemble members indicate a farther
northwest track compared to GFS and CMC guidance. But GFS/CMC have
not been very stable with their low tracks. The 12Z GFS and CMC
seemed reasonably agreeable with a track over the Tennessee Valley
and the northern Mid-Atlantic or so on Thursday and ending up near
Cape Cod 12Z Friday, albeit with plenty of spread in their ensemble
members. The 18Z GFS was slower to track the low northeastward but
with a similar path. Since the 12Z GFS and CMC were kind of a
middle ground between the northwestern EC low track and other
models that were more suppressed, leaned toward a low track like
theirs and the similar NAEFS mean, putting the low over eastern
Kentucky Thursday 12Z and near Boston Friday 12Z. This was a
northward shift compared to the previous forecast and was reflected
in the QPF. However, the newer 00Z GFS and CMC have trended toward
a more suppressed low track across the Southeast before the low`s
turn north across the western Atlantic, certainly lessening
confidence in this trend. The occluding low should make its way
into eastern Canada over the weekend.

Behind the trough, upper ridging is generally forecast to form
along the West Coast, but undercut by southern stream energy that
complicates matters. The southern trough/perhaps closed low should
move gradually eastward toward California by next weekend, but at
this time any notable surface features look to dissipate over the
eastern Pacific, but will continue to monitor. A model average
seems reasonable for this. CMC runs are alone in suppressing the
northern side of the northern stream ridge by next weekend, so
preferred the GFS/EC there.

The WPC forecast favored a blend of the 12/18Z GFS and 12Z EC/CMC
and NAEFS mean early in the period, with gradually increasing
proportions of the GEFS/EC/NAEFS mean as the period progressed to
reduce individual model discrepancies.


...Weather/Hazards Highlights...

Shortwave energy moving through the Intermountain West through
midweek will lead to precipitation, including potentially heavy
snow across the Colorado Rockies in particular. Most precipitation
should finally abate in the West for the latter part of this week,
aside from light rain or snow showers. Some showers may approach
California around Friday or so with upper level energy coming in,
but most models do not show any heavy amounts.

Farther east, as a 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/Southeast late
Wednesday-Thursday, though with uncertainty in placement. Continue
to hold off on any ERO risk areas due to the model spread of the
track/timing differences for low pressure and associated effects on
rainfall coverage/rates. But there could be a nonzero risk as
convection ramps 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. Rain may be
generally heavier by Thursday but possibly more progressive as the
low and front move quickly. To the north of the low track and
frontal boundary, snow or ice may be possible, 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 Middle Mississippi Valley Wednesday, the Upper Ohio
Valley Wednesday-Thursday, and the central/northern Appalachians
into northern Pennsylvania and the interior Northeast by
Thanksgiving Day and lingering into 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. Then cold westerly flow over the Great Lakes should lead to
downwind snow next weekend. Localized heavy snow amounts are
possible in the typically favored areas.

Increasing coverage of below normal temperatures will be the trend
later this week into next weekend with a surge of Arctic air. The
northern Plains will take the brunt of the cold air, with highs in
the single digits and teens and lows below 0F in some areas,
equating to 15-25F below normal. But colder temperatures should
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. The Midwest to Ohio
Valley are forecast to see chilly temperatures that are below
average by 10-20 degrees. The cooler air will push through the
southern tier as well, after a warm Wednesday in the south-central
U.S. when highs in the 70s and 80s will be around 10-20F above
normal, and a warmer than average Thanksgiving morning in the
Southeast before the cold frontal passage. Meanwhile, expect
temperatures to gradually warm in the West, from a bit below normal
to a bit above normal.


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



$$