Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
150 AM EST Thu Nov 25 2021
Valid 12Z Sun Nov 28 2021 - 12Z Thu Dec 02 2021
...Overview and Guidance/Predictability Assessment...
Mean troughing in the East will be reinforced several times by
shortwaves and maintained throughout much of the medium range
period (Sunday-Thursday). Elsewhere, an amplified ridge across the
west should weaken as a couple of shortwaves move through and into
the central U.S., which should also help to flatten the overall
upper pattern some by mid to later next week.
The guidance continues to show reasonably good agreement for this
large scale flow pattern but offers plenty of uncertainty still in
the details. Energy reloading through the western side of the
trough in the East on Sunday-Monday continues to show some timing
and magnitude differences, which would affect a surface low
deepening off the Northeast coast. The 18z yesterday (and todays
00z) GFS continues to be strongest/deepest with the upper low
while the 12z/yesterday ECMWF intially keeps a surface low farther
south along the Mid-Atlantic coast before jumping north and more
in line with the other solutions. The ensemble means seem to offer
a solution clustered closer to the GFS/CMC/UKMET which is what WPC
favored tonight. Regardless, the placement of this surface low
will have impacts for the Northeast including how much
precipitation falls, so this will need to be monitored.
Out West, a weak shortwave into the West Coast around Monday next
week should propagate eastward into the Central Plains and
eventually the East by the middle to latter part of next week. The
last couple of model runs indicate there's plenty of uncertainty
remaining with this system, especially the latter half of the
period mainly timing and intensity and how it interacts with the
Eastern trough already in place. A blend towards the ensemble
means later in the period seemed to mitigate these differences
well until these details can be ironed out in future runs.
...Weather/Hazards Highlights...
The atmospheric river into the Pacific Northwest should be
weakening by Sunday, though some lingering moderate to locally
heavy rainfall will remain possible across the Olympic Peninsula
and far northern Cascades. Westerly flow across the Great Lakes
should continue to support lake effect snowfall with some light to
moderate accumulations possible Sunday into Monday. Models show
the heaviest rainfall associated with the early period Northeast
surface low should mainly remain offshore, but wrap around light
snow or coastal rain may be possible. An upper low camped out west
of Baja California may help funnel moisture towards the Gulf Coast
region, but associated precipitation should be light and scattered
in nature. After Monday, conditions across the CONUS should begin
to dry out for most regions as the flow becomes less amplified.
The exception to this may be across the Pacific Northwest where
troughing in the northeast Pacific should keep a consistent focus
of moisture into the region.
The mean trough over the East should keep temperatures slightly
below normal into Tuesday with some moderation back towards normal
Wednesday and Thursday. Meanwhile, upper level ridging keeps
conditions warm throughout the West and central U.S. the entire
period, with a handful of both record highs and warm overnight
lows possible. The greatest anomalies should be across the
northern-central Plains where daytime highs could be 20 to 30
degrees above normal. As the flow flattens during the late period,
above normal temperatures should also begin expanding into the
Midwest and South.
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