Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 PM EST Sat Nov 20 2021
Valid 12Z Tue Nov 23 2021 - 12Z Sat Nov 27 2021
...Potential heavy rainfall for portions of the Southern Plains
and Lower Mississippi Valley may impact travel for the
Thanksgiving holiday...
...Overview...
The medium range period will begin Tuesday rather amplified in the
upper levels, with a trough axis near the West Coast, a deep
trough in the East, and ridging in between. The eastern trough is
forecast to develop a closed upper low around Wednesday and spin
over the Atlantic along with its associated surface low, likely
far enough east not to impact the Northeast U.S. significantly
from midweek onward. The trough in the West will move eastward as
the week progresses, and it is becoming more likely it will 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. on Wednesday into Thanksgiving Day.
...Guidance/Predictability Assessment...
00Z/06Z model agreement was fairly good with the departing trough
across the East, with some lingering questions regarding timing
when the closed mid-upper low develops and placement of surface
lows. The bulk of model guidance agrees that these features will
mainly cause marine and southeast Canada threats, with the system
far enough from the Northeast U.S. during the medium range period
not to cause significant impacts.
In the West, guidance continues to show plenty of run to run
variability regarding placement and strength of shortwaves moving
through the mean trough, which leads to differences in timing and
progression of the separation into northern and southern streams
during the latter half of the week. The 00Z/06Z model cycle
generally flipped back to a solution with more separation of
streams, with many deterministic models (though not the 00Z GFS)
and even the 06Z GEFS mean and 00Z EC mean at times indicating a
southern stream closed low over northern Mexico that lingers there
Friday and Saturday. The 00Z deterministic ECMWF showed this 500
mb closed low farthest west, along with a good portion of the EC
ensemble members, pushing the 00Z EC ensemble mean trough/low
farther west compared to CMC and GEFS ensemble solutions that are
not as slow to move the southern stream trough/low. The stream
separation does cause sensible weather differences as well--the
cold front ahead of the northern stream trough could move faster
with more separation, and QPF in these solutions is lower over the
south-central U.S., possibly because the pattern holds back more
Pacific moisture. The updated WPC forecast did follow the trend
toward a closed low and attempted to take a middle ground solution
in placement between the slower EC suite and the faster guidance.
Given the variability between models and from run to run, there is
still considerable uncertainty with this evolution. Currently
incoming 12Z guidance remains split, with the ECMWF and GFS still
showing considerable stream separation (with the new 12Z EC a bit
faster than the 00Z run and the 12Z GFS aligned with its 06Z run),
while the CMC shows a more phased solution.
Overall the WPC fronts/pressures began with a blend of the
deterministic guidance early in the period, lessening the
proportion of the slow 00Z ECMWF as the period progressed,
splitting the difference with the faster 00Z and slower 06Z GFS,
while increasing the proportion of the 06Z GEFS and 00Z EC
ensemble means.
...Weather/Hazards Highlights...
The cold front associated with the Northeast low pressure should
be well off the Atlantic coast by Tuesday with sprawling high
pressure in its wake across much of the eastern third of the
nation. As a result, daytime highs and morning lows Tuesday and
Wednesday could be 10-20 degrees below normal from portions of the
Tennessee/Ohio Valley into the Mid-Atlantic and the Southeast.
Near to below freezing morning lows could extend rather far into
parts of the South, resulting in the potential for the first frost
or freeze for some locations, most certainly impacting sensitive
vegetation. Low pressure lingering off the Northeast Coast may
bring periods of breezy/gusty winds to parts of New England and
combined with already chilly temperatures could make for some
unpleasant wind chills during at least the beginning of next week.
Some light to perhaps locally moderate precipitation is expected
to spread across the Rockies around Tuesday-Wednesday as a cold
front moves across. Then by midweek, the cold front should reach
far enough into the Plains to begin interacting with Gulf moisture
to produce an expanding area of moderate to possibly heavy
rainfall from eastern Texas into parts of the Lower Mississippi
Valley for Wednesday and especially into Thanksgiving Day, with
amounts currently expected to lessen by Friday as the front moves
offshore. Though this update lowered forecast rainfall amounts per
the latest model consensus, stay tuned for forecast updates as
totals could increase again, and the rain could have impacts for a
couple of the busiest travel days of the year. Additional
shortwave energy into the Pacific Northwest may also bring another
round of moderate to locally heavy coastal rain and mountain snows
on Thursday and Friday, which could impact travel as well.
Above normal temperatures ahead of the western front will across
the Plains Tuesday-Wednesday with anomalies +10 to +20F possible.
Much of the country looks to be generally near normal on
Thanksgiving, before modestly above normal temperatures may begin
to spread across the West and High Plains by later in the week.
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