Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
208 AM EST Tue Dec 28 2021
Valid 12Z Fri Dec 31 2021 - 12Z Tue Jan 04 2022
...Bitterly cold temperatures to continue Thursday-Saturday across
portions of the Northern Plains...
...Heavy rain across across parts of the Tennessee/Ohio Valley New
Year's Eve/New Year's Day...
...Overview and Guidance...
An amplified ridge will be anchored over the northern Pacific
Ocean through this period, which will continue to support an upper
trough over the western U.S.; this pattern will favor very brisk
temperatures. A separate piece of energy within the larger scale
trough should track a little offshore southern California
Thursday-Friday. The overall pattern will begin to transition by
the weekend as a new ridge builds farther west, leading to a
deepening trough over the Northeast Pacific and renewed moisture
feed into the Northwest U.S. The trough and associated moisture
will track into the central and eastern part of the country. A
much weaker wavy frontal system will depart from the East late
this week.
The latest model guidance continues to have similar patterns and
evolution through the extended therefore choose to take a
multi-model approach; this also helps maintain continuity from the
previous forecast. The WPC blend used the 12Z ECWMF/CMC/UKMET,
12/18Z GFS initially and gradually increased the use of the EC
ensemble means and the GEFS means trough the middle and later
periods.
...Weather/Hazards Highlights...
Lingering showers are expected across parts of the Southeast near
a wavy cold front along with snow over the Northeast near the
parent low pressure. Scattered to widespread showers and
thunderstorms will move across the Plains towards the eastern U.S.
as a low pressure system and associated fronts develops. A plume
of deep moisture will accompany this system and intensify over the
Tennessee Valley; where conditions will be very favorable for
moderate to heavy rainfall to occur. There will be an elevated
risk for local flooding concerns as this region has had recent
soaking rains to lower local flash flood guidance. Expect some
wintry weather in the northern side of the precipitation
shield--from parts of the Northern/Central Plains through the
Midwest, Great Lakes, and Northeast. The intensity and storm
track, along with any waves along the boundary, will determine the
the location and amounts.
Heavy mountain snow will spread across the Pacific Northwest and
Northern Rockies Thursday ahead of an approaching shortwave.
Abundant moisture will be directed onshore southern California
with the upper trough/possible closed low positioned offshore.
Moderate to locally heavy rainfall may be possible in the lower
elevations; with snow spreading across the higher terrain
especially over the Colorado Rockies with the merging of northern
and southern stream upper systems. A third system nearing the
Pacific Northwest will usher in an additional round of heavy
mountain snow by the weekend. Lower elevation rain could become
moderate to heavy from the Pacific Northwest into northwestern
California by early next week.
The frigid airmass over the northern tier should persist through
at least Saturday with the daily readings averaging 20 to 35
degrees below average for the end of the year. A few locations in
far northern North Dakota and Montana may stay below 0 F even into
next weekend, despite temperatures moderating slightly as the week
progresses. Gusty winds will create some dangerous wind chill
values of below 0 F. Temperatures are also likely to be below
normal by about 5-15 degrees across the rest of the western U.S.
through Sunday. Warm and mild weather is expected across the
Southeast and Mid-Atlantic into Sunday, with daily temperature
readings of 15 to 25 degrees warmer than normal from the Southern
Plains to the Ohio Valley and east into the Mid-Atlantic region.
Campbell/Rausch
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
Hazards:
- Heavy precipitation across portions of California, the Central
Great Basin, the Pacific Northwest, and the Northern Great Basin,
Sun-Mon, Jan 2-Jan 3.
- Heavy precipitation across portions of the Southwest, Thu-Fri,
Dec 30-Dec 31.
- Heavy rain across portions of the Lower Mississippi Valley, the
Central Appalachians, the Tennessee Valley, the Middle Mississippi
Valley, the Mid-Atlantic, the Southern Appalachians, the
Southeast, and the Ohio Valley, Sat-Sun, Jan 1-Jan 2.
- Heavy snow across portions of the Pacific Northwest and the
Northern Great Basin, Thu, Dec 30.
- Heavy snow across portions of the Great Lakes and the Upper
Mississippi Valley, Sat, Jan 1.
- Severe weather across portions of the Southeast, the Middle
Mississippi Valley, the Lower Mississippi Valley, the Ohio Valley,
and the Tennessee Valley, Sat, Jan 1.
- High winds across portions of the Central Rockies, the Central
Plains, the Northern Plains, and the Northern Rockies, Thu, Dec 30.
- Much below normal temperatures across portions of the Upper
Mississippi Valley, the Pacific Northwest, the Northern Rockies,
the Northern Great Basin, and the Northern Plains, Thu-Sat, Dec
30-Jan 1.
- Much below normal temperatures across portions of the Central
Plains, the Middle Mississippi Valley, the Great Lakes, the Upper
Mississippi Valley, and the Northern Plains, Sat-Sun, Jan 1-Jan 2.
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
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/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml