Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
154 PM EST Sun Nov 21 2021
Valid 12Z Wed Nov 24 2021 - 12Z Sun Nov 28 2021
...Potential heavy rainfall for portions of the Southern Plains,
Lower Mississippi Valley, and Pacific Northwest may impact travel
for the Thanksgiving holiday...
...Overview...
An amplified pattern should prevail through much of the medium
range period (Wed-Sun) beginning with a trough axis moving through
the interior West, a closed low off the Northeast Coast, and
ridging in between. The developing closed low off the Northeast
coast may linger for a few days but likely remaining far enough
east not to impact the Northeast U.S. significantly. Out west,
guidance continues to advertise the trough will likely split into
two separate streams, with southern energy digging into Mexico and
northern stream energy sliding across the Northern Plains and
Great Lakes. The latter may eventually combine with a shortwave in
central Canada to form a closed low over eastern Canada next
weekend. The southern stream energy and moisture flowing into a
frontal system could produce locally heavy rain across the
south-central U.S. during the late week.
...Guidance/Predictability Assessment...
The 00Z/06Z guidance continues to show the leading trough/closed
low linger near the Northeast before pulling away from the coast
further out into the Atlantic Ocean. This feature will mostly
impact the Canadian maritime region. Meanwhile another longwave
trough moves into the central CONUS and transitions to a split
flow pattern. The northern stream is expected to continue to track
across the Mississippi Valley to the Mid-Atlantic and Northeast
regions while the southern stream gets hung up near the Baja
Peninsula for a few days. The southern stream energy continues to
trend weaker in the models however this leads to uncertainty on
when energy slides southeast into the Southern Plains. The CMC is
the quickest to bring the energy into the Southern Plains next
weekend and is quite generous with the associated QPF. The
GFS/ECMWF have slower timing, but also differing placements of the
maximum QPF. As noted on the last cycle, the ensemble means
continue to support the parent ECMWF/GFS solutions. The northern
stream has had persistent, run-to-run consistency and clustering
with the evolution as the low closes off near eastern Canada,
likely a result of stronger shortwave energy dropping in behind it
from western Canada. The next shortwave should begin impacting the
West by Saturday-Sunday. The CMC is much slower with the shortwave
as it drops southward from Alaska late this week and quickly
becomes out of phase with the GFS, ECMWF, and the ensemble means
showing strong ridging over the West Coast on day 7 rather than
troughing like the better consensus would suggest.
The WPC model blend consisted of 00Z CMC/ECWMF/00Z and 06Z GFS and
the their ensemble means through the extended period - weighting
of the means increased through the middle and later periods. This
combination helps maintains continuity with the previous WPC
forecast.
...Weather/Hazards Highlights...
Moderate rainfall, possibly heavy at times, is expected over parts
eastern Texas into the Lower Mississippi Valley on Wednesday and
into Thanksgiving Day as a cold front passing through the Southern
Plains. The best chance for heavy rainfall may be Thursday along
the western Gulf Coast where instability should be greatest and at
least a localized flood threat may develop. Less intense and more
scattered rain may extend northward along the cold front into
parts of the Ohio Valley Wednesday-Thursday and eventually the
Mid-Atlantic/Northeast on Friday. The degree of risk for excessive
rainfall will be dependent on the how quickly the cold front
progresses; if it slows or stalls the risk for local flooding
concerns will elevate.
Shortwave energy may help direct an elongated plume of moisture
into the Pacific Northwest to produce terrain-enhanced, moderate
to heavy rain and mountain snows across eastern portions of
Washington and Oregon on Thanksgiving Day. Unsettled conditions
will persist across the West through the weekend, with the
precipitation weakening in intensity for a few days. The system to
follow has the possibility of ushering in another round of locally
heavy rains to the Pacific Northwest.
For temperatures across the CONUS, sprawling high pressure in the
East on Wednesday should continue well below normal daytime highs
and morning lows, particularly for locations in the Southeast and
Mid-Atlantic states. Daily low reading are expected to be near or
below freezing. Temperatures may briefly warm back towards normal
on Thursday and Friday but could trend chillier again next weekend
as the next cold front moves through. Through the extended
forecast, locations across the West are expected to trend warmer,
which will spill over into the Plains next weekend. Daytime highs
for the northern and central High Plains could be 10 to 20 degrees
above normal by next Sunday.
Campbell
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