Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
203 PM EST Tue Nov 23 2021
Valid 12Z Fri Nov 26 2021 - 12Z Tue Nov 30 2021
...Overview...
A relatively stagnant upper-level pattern is expected in the
medium range period, with mean troughing for the eastern half of
the U.S. and quasi-zonal flow shifting to ridging by the weekend
across much of the West, except for a southern stream low
gradually making its way across northern Mexico and the Southwest
as it weakens. This pattern should be relatively dry across much
of the contiguous U.S., with the exception of waves of
precipitation over the Pacific Northwest through the period, as
well as some precipitation coming in across eastern trough,
including a round of moderate to heavy lake effect and interior
Northeast snow Friday into Saturday.
...Guidance/Predictability Assessment...
Guidance continues to show above average agreement on the large
scale flow pattern described above particularly through the
weekend, and a multi-model deterministic blend of the 00Z ECMWF,
CMC, UKMET, and 00Z/06Z GFS was used early in the period. Greater
differences arise with energy digging through the western side of
the trough serving to reload it. The first shortwave with
significant differences between models shows up in the Great Lakes
around Sunday, with another possibly diving toward the Northern
Plains around Monday or Tuesday. The latter shortwave spins up a
closed 500mb low across the Great Lakes region in the 00Z ECMWF by
Tuesday; a hint of this is shown by the 00Z GFS and a weaker
version is shown by the new 12Z ECMWF. Ensemble guidance shows a
bit of a split with GEFS members having more ridging across the
north-central U.S. and EC ensemble members showing more troughing.
So uncertainty is still high with these features. At the surface,
there is still a possibility of a surface low to spin up and
deepen over the Atlantic Monday into Tuesday, but the timing and
strength of the low vary. Ensemble means are weaker, as expected,
with the low but do provide a nice middle ground solution at this
time, particularly the 00Z GEFS and EC means that were right on
top of each other. Incoming 12Z guidance appears it may be more
clustered with this low. The WPC forecast for these features
shifted from a deterministic blend early on toward utilizing more
of the GEFS and EC mean solutions as the period progressed.
Some differences between models also remain with the track and
timing of the southern stream mid-upper low tracking northeastward
from Baja California into the Southwest. 00/06Z GFS runs linger a
closed low longer than other guidance, but all guidance has a
general weakening trend as the period progresses. Additionally,
the precise orientation of an atmospheric river event this weekend
into British Columbia and the Pacific Northwest just north of the
low amplitude ridging remains in question--GFS have generally kept
the greater integrated water vapor transport confined to western
Canada, while the 00Z ECMWF brings it into the Pacific Northwest.
This will have an impact on how much rain impacts the Olympics and
northern Cascades versus the bulk of the precipitation staying
north in British Columbia, increasing uncertainty. For these
features, a multi-model blend sufficed to minimize differences and
produced a middle ground solution at this point, and will continue
to monitor.
...Weather/Hazards Highlights...
As the period begins Friday, a cold front exiting the East Coast
will bring westerly flow to the Great Lakes region and the
Northeast, favorable conditions for lake effect and interior
Northeast (particularly higher elevation) snow through Saturday.
Moderate to locally heavy snow amounts downwind of Lake Ontario,
the Adirondacks, and northern New England are possible. Meanwhile,
a plume of moisture/atmospheric river coming into the Pacific
Northwest could bring another round of moderate to locally heavy
rains to parts of western Washington, with the heaviest totals
likely along favorable terrain. Lighter precipitation should spill
over into northern parts of the Rockies as well this weekend but
quickly dry out early next week. Then another round of
precipitation is forecast for the Pacific Northwest by around
Tuesday. The remainder of the country should remain fairly dry
through the weekend and into next week. A weak system moving
through the Ohio Valley into the East may bring some light
precipitation to the region but nothing looks particularly
impactful at this time.
A building trough over the East should keep temperatures below
normal through the entire five day period with the greatest
anomalies (-10 to -15F) likely on Friday into Saturday from south
Texas/lower Mississippi Valley to the Ohio Valley and
Mid-Atlantic. Meanwhile, amplified ridging will promote a warming
trend across the West and into the Plains with several days of +10
to +20 anomalies (locally higher) likely, especially across parts
of the northern and central High Plains Sunday and Monday.
Tate/Santorelli
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, 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