002
FXUS02 KWBC 241858
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 PM 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, a positively tilted
shortwave aloft over the Intermountain West will produce heavy
snow in higher elevations of the central Rockies in particular.
Eastward progression of this feature and trailing northern stream
energies pushing southeast from Canada will ultimately develop a
large scale mean trough over the eastern half or more of the
country while a Northeast Pacific upper ridge drifts toward the
Pacific Northwest coast. 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 some 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...

The latest guidance cycle presents similar themes as before. These
include good agreement on the large scale pattern evolution toward
an amplified eastern trough and Northeast Pacific ridge nearing the
Pacific Northwest, with a weak southern stream upper trough/low
offshore California. However, ahead of the developing eastern upper
trough the guidance continues to have difficulty in resolving
important details for low pressure affecting the East during the
busy holiday travel period spanning the latter half of the week.

Issues with the initial Intermountain West shortwave energy remain
an initial contributor to guidance spread/variability at the
surface farther eastward, with differences greater than usual for
a Days 4-5 forecast and not really improving much if any over the
past 2-3 days. Machine learning (ML) models do not seem to provide
too much added help for this feature, with timing differences
comparable to the dynamical models. The 06Z GFS was somewhat more
questionable with its slow timing though. Latest ECMWF runs have
been consistent with a stronger/faster shortwave.

The next input of interest will be northern stream dynamics
pushing from Canada into the northern Plains by Thursday and then
continuing eastward into the northeastern quadrant of the lower 48
and eastern Canada. Albeit with finer scale detail differences,
there is a more common idea among most dynamical/ML models toward
greater height falls aloft than depicted in the 06Z and new 12Z GFS
runs--specifically within an area encompassing the Great Lakes and
Ontario as of early Friday and extending into the Northeast and
Quebec by Saturday. The 00Z GFS offered a somewhat better
comparison to other guidance aloft.

Needless to say, the combined differences for upper level support
led to lower than average predictability for surface low evolution.
Including the new 12Z run, the ECMWF has been on the northern and
then western side of the envelope (and fairly strong) for surface
development near New England due to its stronger leading
shortwave, with the CMC and ECMWF/CMC ensembles generally leaning
in that direction. The new 12Z ECMWF does trend farther east beyond
New England though. In contrast, most ML models show a more
suppressed low track with development occurring much farther
offshore and leading to minimal effects on the Northeast.

Initial preferences focused on the most common ideas of the
typically higher-predictability aspects of flow aloft, leading to a
Days 3-5 blend consisting of 40 percent 00Z ECMWF and the rest
split among the 00Z GFS/UKMET/CMC. This led to an eastern U.S.
surface evolution closest to a weaker version of the 00Z ECMWF
along with significant tempering of that model`s Northeast U.S. QPF
toward continuity and the 00Z ECens mean. In contrast, NBM
guidance incorporated more than a desired amount of the drier
Northeast/wetter Southeast-Carolinas scenario from the GFS/GEFS.
The overall forecast provided the best continuity possible while
awaiting clearer signals for any greater adjustments that could be
made with some confidence. The forecast trended toward nearly half
model/half ensemble mean input (00Z GEFS/ECens/CMCens) by the end
of the period.

The favored forecast blend represented consensus ideas near the
West Coast, with an average of guidance looking good for the
southern stream upper system off California (hedging away somewhat
from the 00Z ECMWF that looked a tad eastward by Saturday) and the
average among the ML models comparing well for the dynamical
model/mean consensus for the upper ridge nearing the Pacific
Northwest.


...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. Continued model
spread for timing of the upper shortwave maintains some uncertainty
for the precise duration of this activity. Regardless of details,
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, guidance continues to suggest this activity
should be fairly light.

Farther east, as a low pressure/frontal system develops in the
south-central Plains and tracks eastward/northeastward as the week
progresses, precipitation appears likely for a majority 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 and duration/timing. Excessive rainfall outlooks for
Days 4-5 continue to show no risk areas due to the model spread of
low pressure evolution and associated effects on rainfall
coverage/rates. However is still 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.

A surge of Arctic air including one or more reinforcing
waves/fronts will increase the coverage of below normal
temperatures from the northern-central Plains into the East later
this week into next weekend. 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.
The cold temperatures overspreading much of the remaining lower 48
east of the Rockies will be moderated in comparison but still yield
highs of 10-15F below normal from the Midwest/Mississippi Valley
through the Mid-Atlantic and Southeast from Friday onward. The
Midwest to Ohio Valley may trend another few degrees colder by next
Sunday. The southern tier will experience a cooling trend 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 as the northeastern Pacific upper ridge approaches,
from a bit below normal to a bit above normal.


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





$$