Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Thu Nov 18 2021
Valid 12Z Sun Nov 21 2021 - 12Z Thu Nov 25 2021
...Potential continues for a strong cold front and significant
storm to affect the East next week...
...Overview...
Latest model and ensemble guidance remains similar for the
evolving large scale pattern through Thanksgiving while displaying
typical spread and run-to-run variability for the details. A
Pacific upper ridge moving into the West/Plains during the first
part of the calendar week will support an amplifying eastern U.S.
trough whose axis should reach near the East Coast by early
Tuesday. It is still up for debate where one or more embedded lows
may close off over the Great Lakes or southern Canada early in the
week but there is a more common theme of an upper low over or just
offshore New England by Wednesday. Thereafter, how quickly the
upper low departs will depend on the strength and westward extent
of North Atlantic upper ridging. The evolution aloft and
potentially complex surface low pressure/frontal evolution may
bring a heavy rain threat to northern New England in the warm
sector and a period of well below normal temperatures, brisk to
strong winds, and lake effect/terrain enhanced snow behind the
strong cold front. Continue to monitor forecasts as details may
change. Farther west, the combination of a Pacific shortwave
reaching the West by Tuesday and an opening upper low initially
southwest of California should yield a positively tilted upper
trough aligned from the Northern Plains into northwestern Mexico
by Thanksgiving. These features and energy rounding the upstream
ridge may produce periods of precipitation over the Pacific
Northwest while rain may expand/intensify along and north of the
western Gulf Coast on Thanksgiving as Gulf moisture interacts with
a Plains front.
...Guidance/Predictability Assessment...
For the evolution across eastern North America, over recent days
the ensemble means (and model runs closest to them in principle)
have provided a fairly steady anchor for the general forecast with
fairly modest adjustments over consecutive runs. Latest GEFS/ECMWF
means have achieved similar solutions for the East Coast upper
trough by early Tuesday by way of the ECMWF mean nudging faster
and GEFS mean somewhat slower. Both the operational models and
means have recently shown a trend for a sharper/deeper trough in
response to a slower/stronger trough reaching the West and
stronger intervening ridge. Models have been waffling on if/where
an embedded upper low may form between Sunday and early Tuesday.
However there is a decent consensus that an upper low should reach
along or just offshore the Mid-Atlantic or New England coast by
later Tuesday or Wednesday. Thus far any stray southern solutions
for this upper low (some ECMWF runs, 18Z GFS) have reverted back
to the north toward the larger clustering in subsequent runs. A
building upper ridge over the North Atlantic and Greenland poses
the risk of slowing down the system for a period of time. Latest
GFS runs have started to show this and the new 00Z ECMWF has
trended much slower late in the period. At the surface, there is
reasonable continuity with low pressure consolidating over the
Upper Great Lakes/southeastern Canada Sunday-Monday. After that
the forecast gets more complicated as some models show a frontal
wave tracking northward over the western Atlantic while one or
more other waves evolve over or reach areas from over/east of New
England into the Canadian Maritimes, in response to the upper
dynamics. Enough upper ridging to the north could keep this system
near New England through or beyond midweek. Many of these details
have lower predictability several days out in time, especially
since the supporting energy is still over or near Alaska at this
time.
The 12Z/18Z GFS, 12Z ECMWF/CMC, and the ensemble means provided a
reasonable cluster of guidance for the upper trough reaching the
West by Tuesday (as mentioned, a little deeper/slower than seen
yesterday; UKMET runs have been slow) and the eastern Pacific
upper low opening up to the south, as well as for the overall
trough that continues eastward through the rest of the period.
However by Thanksgiving there is a fair degree of uncertainty over
how energy will be distributed within the trough. Also new 00Z
runs add other options, such as a stronger upstream shortwave
reaching the West in the GFS and much more flow separation in the
CMC.
Based on guidance comparisons through 12Z/18Z, the updated
forecast incorporated the 12Z/18Z GFS, 12Z ECMWF, and some 12Z CMC
for the first half of the period and then transition to a
model/ensemble mean blend. GFS input tilted more toward the 12Z
run once the 18Z version strayed south of consensus with the upper
low near the East Coast, while the 12Z GEFS mean provided a
slightly stronger solution than the 18Z run for the system near
New England.
...Weather/Hazards Highlights...
Consolidating northern tier U.S./southern Canada low pressure will
bring a leading front and stronger trailing front into and through
the East early in the calendar week. Precipitation (mostly rain)
will develop along and east of the Mississippi Valley on Sunday.
Most rain should be light to moderate but locally more intense
activity may be possible over parts of the Ohio/Tennessee Valleys.
As the front and possible waves head into New England, recent
trends have lowered precipitation totals somewhat over
western/southern areas but maintain the potential for some heavy
rainfall over Maine, aided by Atlantic inflow. Snow is still most
likely in the cold air behind the system, to the lee of the Great
Lakes and along westward-facing terrain from the central
Appalachians northeastward. There is still enough uncertainty in
system evolution to allow for other possibilities regarding
precipitation types and amounts. Meanwhile confidence is higher in
a period of brisk to strong winds from the Upper Midwest through
the East Sunday into Tuesday and then possibly lingering over the
Northeast thereafter. Across the northern tier the combination of
low temperatures and wind would produce cold wind chills.
The Pacific Northwest into northern Rockies should see mostly
light to perhaps locally moderate precipitation during the first
half of next week with a shortwave/cold front arriving from the
Pacific. The combination of this energy aloft and the
opening/progression of the upper low southwest of California may
spread some moisture into other parts of the West by
Tuesday/Wednesday, but still with modest confidence for
precipitation coverage/amounts. Around Thanksgiving the western
front may reach far enough into the Plains to begin interacting
with Gulf moisture to produce an expanding area of rain along and
north of the western Gulf Coast. Some of this rain may become
heavy.
The strong cold front crossing the East early in the week will
bring well below normal temperatures that should be most
pronounced between Monday and early Wednesday when there should be
decent coverage of highs/morning lows 10-15F below normal, with
some pockets of highs possibly 15-20F below normal over southern
areas. Temperatures may fall below freezing rather far into the
South. Readings should rebound toward normal by Thanksgiving.
Portions of the Rockies/Plains will see a brief warm period early
in the week with some plus 10-15F or so anomalies. A cold front
moving from the West into the Plains during Tuesday through
Thanksgiving will bring moderately below normal highs in its wake.
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
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