415
FXUS02 KWNH 250713
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
212 AM EST Mon Nov 25 2024

Valid 12Z Thu Nov 28 2024 - 12Z Mon Dec 02 2024


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


...Overview...

As the medium range period begins Thanksgiving Day, northern
stream and initially southern stream shortwave energy will combine
to create a large scale mean trough over the eastern two-thirds of
the country. Guidance continues to show developing low pressure
ahead of the evolving eastern upper trough but with significant
detail differences affecting sensible weather. Precipitation is
generally expected across the Eastern Seaboard on Thanksgiving,
including some potential for snow in the Interior Northeast
especially in higher elevations, but there is lower than average
confidence in precipitation amounts/placement/type. 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 the weekend. The cold westerly flow
should lead to some possibly heavy lake effect snows by the weekend
behind the low pressure system. Meanwhile in the West, near
average temperatures are expected with just some light
precipitation possible at times.


...Guidance/Predictability Assessment...

While model guidance remains agreeable for the large scale
pattern, embedded shortwaves within the broad eastern trough are
still quite uncertain even as the period begins Thursday, leading
to impactful surface low placement and strength differences. The
control ECMWF runs have been very consistent on the northwestern
side with the surface low track as a stronger/faster shortwave
(atop the Ohio Valley or so, while other models are generally over
the central Plains early Thursday) amplifies the trough more
quickly than other guidance. This northwestern low track does have
ample support from the ECMWF ensemble members (that do seem
underdispersive given the spread in other model suites) and from
the UKMET. Meanwhile other models for the most part are more
suppressed with the low, tracking over the Southeast before the
turn northward across the western Atlantic. These include GFS, CMC,
and AI/ML model runs. GEFS and CMC ensemble members indicate ample
spread. More specifically, the 12Z CMC was not favored for this
forecast because it showed a secondary low becoming dominant and
appearing farther south of other guidance by 12Z Friday. The 18Z
GFS was rather fast with its low track and thus preferred the 12Z
run. The WPC forecast tried to strike a balance between the most
suppressed and the farthest northwest solutions. The GEFS mean
seemed to be a reasonable middle ground and thus favored it in the
model blend even early on. The newer 00Z GEFS mean has jumped north
a bit though, which may indicate a coming trend toward the ECMWF
type low track. The low track will of course yield considerable QPF
differences. The NBM QPF seemed to do a reasonable job in this
cycle for a middle ground forecast; for example it showed some QPF
in the Northeast but lower than the ECMWF.

The early part of the medium range period actually seems to be the
most uncertain, and unfortunately most impactful due to holiday
travel. Models generally agree for the upper trough to broaden into
late week as energy digs through the northern Rockies/Plains and
then move eastward, and the newer 00Z models appear in even better
agreement after some detail disagreements in the 12/18Z cycle.
Brief ridging in the Northwest Thursday should reform over the
weekend, and last into early next week unless CMC runs that are
alone in having a shortwave move through are corrected. Farther
south there is some southern stream troughing with embedded energy
that gradually drifts through the eastern Pacific toward
California. Some model differences are seen with this, especially
by early next week when the timing differences start to make the
pattern out of phase between models. Fortunately the 00Z models
seem to be coming in in better alignment.

The WPC forecast was based on a blend of the deterministic models
and some GEFS mean early in the period, and ramped up the
proportion of ensemble means to over half later in the period to
provide an intermediate solution.


...Weather/Hazards Highlights...

Precipitation is likely to spread across much of the East on
Thanksgiving Day. The ample model spread with the surface low track
significantly limits confidence in the details beyond that,
however. In a general sense, some moderate rain could spread across
the Southeast into the Carolinas near the cold front, with amounts
likely staying below any flooding hazard thresholds. Closer to the
low track, a round of precipitation could move through the Mid-
Atlantic to Northeast. While lower elevation/coastal/southern areas
will most likely stay rain, the Day 4 Winter Weather Outlook
indicates the highest probabilities for snow greater than 0.25"
liquid equivalent across higher elevations of the Interior
Northeast. But the axis and amounts of snow 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. After the surface low exits, the pattern will
become favorable for several days of lake effect snow with cold
westerly flow over the Great Lakes. Localized heavy snow amounts
are possible in the typically favored areas.

Elsewhere, a quieter period is likely for the West precipitation-
wise. Parts of California may see light to locally modest
precipitation at times later this week into the weekend with some
energy aloft, with some light amounts perhaps spilling into the
Southwest. The same goes for the Pacific Northwest into the
northern Rockies. Models are starting to show a general signal for
a round of light to moderate snow across parts of the northern
Plains on Friday into the Mid-Mississippi and Ohio Valleys
Saturday. Over the weekend and early next week, return flow of
moisture could start to come into the western Gulf Coast region for
some shower activity.

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 the weekend. The northern Plains will take the brunt
of the cold air, with highs in the single digits and teens and
lows well 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 across the Midwest, Ohio and
Tennessee Valleys, and Appalachians Friday and Saturday. A
secondary surge of cold air will bring those areas down to 15-20F
below average into early next week. The Eastern Seaboard can expect
gradually cooling temperatures Friday through Monday toward 10-15F
negative anomalies. The southern tier will experience a cooling
trend as well, after a warmer than average Thanksgiving morning in
the Southeast before the cold frontal passage. Another cold front
early next week could even bring the Florida Peninsula down to a
bit below normal. 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.


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







$$