055
FXUS02 KWBC 261754
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1254 PM EST Tue Nov 26 2024

Valid 12Z Fri Nov 29 2024 - 12Z Tue Dec 03 2024

...Arctic cold will spread across the northern Plains with below
normal temperatures reaching most areas east of the Rockies...

...Multiple days of heavy lake effect snow late this week into
early next week...


...Overview...

A large scale upper trough is expected to encompass much of the
central to eastern United States Friday into the next week as the
the Thanksgiving Day weather system pulls away from New England
early on Friday. In the wake of this system, breezy conditions will
set up and winds will become favorable from the west/northwest for
a long duration lake effect snow event. Some of the snowfall
accumulations could be significant, especially downwind in the
favored locations. Aside from the lake effect snow potential, a
large area of Arctic air will spread across much of the country
east of the Rockies, bringing some of the coldest air so far this
season. Temperatures are likely to fall into the negative teens
across North Dakota, where wind chill readings could be as low as
-15F. Out West, an upper ridge will keep the weather generally
 benign and with moderate temperatures.


...Guidance/Predictability Assessment...

Given the highly amplified flow pattern (deep trough east, ridge
west), the latest model guidance is in reasonably good agreement
from the synoptic level. This resulted in generally above average
confidence in the pattern and led to a greater use of the
deterministic guidance. However, smaller scale features embedded
within the mean flow could have sensible weather impacts. One
example is with a shortwave tracking through the north-central U.S.
on the western side of the trough Friday-Saturday that looks to
bring a streak of light to moderate snow generally from the north-
central Plains into the Mid-Mississippi Valley. The depth of the
shortwave will help determine the axis of the snow, with some model
spread that is reasonable for a subtle feature. Confidence here is
on the lower side, particularly for location as subtle shifts in
the storm track are likely.

The WPC forecast was based on a blend of the deterministic models
early in the period, gradually ramping up the ensemble means to
half by Days 6-7. This maintained good continuity from the previous
forecast (Days 3-6).


...Weather/Hazards Highlights...

A significant Arctic air intrusion will spread across much of the
central and eastern United States, the first of the season. While
not expected to break many temperature records, it will be a
noticeable change across many areas and for some across the
northern Plains, hazardous to dangerous cold temperatures are
expected. Minimum temperatures could bottom out in the negative
teens and combined with the wind, wind chills as low as -15F are
likely across portions of North Dakota. This will pose an increased
risk of hypothermia and frostbite on exposed skin.

A handful of record low max and min temperatures are more likely
farther south across the Ohio Valley into the Appalachians, where
temperatures are forecast to be around 15-20F below average and
stay below freezing for highs in many areas. Temperatures are
likely to moderate across the north-central U.S. into early next
week, while the greatest below normal temperature anomalies are
forecast to migrate over the East in moderated form.

Heavy lake-effect snow is expected through much of the period, as
westerly flow develops snow east of the lakes Friday-Saturday, and
winds may shift to a more northwesterly direction into early next
week for a snow focus southeast of the lakes Sunday-Monday.
Significant accumulations are possible for some of the favored lake
effect locations. While specific accumulations will become clearer
over the next few days, peak amounts are likely downwind of Lake
Erie and Lake Ontario. Travel disruptions are likely especially on
I-90 between Cleveland and Buffalo and I-81 north of Syracuse.

Elsewhere, a quieter period is likely for the West precipitation-
wise. Light to locally modest precipitation is possible at times in
the Pacific Northwest into the northern Rockies. Expect
temperatures in the Rockies westward to be near average late this
week and warm to a few degrees above normal into early next week.
Models continue to show a general signal for a round of light to
moderate snow across parts of the north-central Plains on Friday
into the Mid-Mississippi and Ohio Valleys Saturday, though the axis
is uncertain depending on a shortwave aloft. Meanwhile, some rain
showers are possible in Florida on Friday with a cold front. 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.


Tate/Taylor


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




$$