Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
145 AM EST Sun Nov 17 2019
Valid 12Z Wed Nov 20 2019 - 12Z Sun Nov 24 2019
...Heavy rain possible for portions of the Southwest through
midweek...
...Pattern Overview and Guidance/Predictability Assessment...
Amplified and moderately progressive northern stream flow is
expected to persist across the CONUS through the medium range,
with a tendency for periods of split flow as energy separates and
cuts off south of the strongest westerlies. An upper-level trough
is forecast to exit the eastern U.S. on Wed as a deepening coastal
low moves quickly into the Canadian Maritimes. A second northern
stream wave should quickly follow on its heels, moving a low
pressure system and trailing cold front across the Midwest/Great
Wed night/Thu and the eastern U.S. by Fri. Farther west, northern
stream energy is forecast to dig southward along the West Coast
Wed, and models/ensembles show consensus that the feature should
eventually close off into an upper-level low, which should then
slowly progress eastward across the Southwest. This feature
continues to be the biggest forecast problem of the medium range,
as solutions differ on exactly how far south/west the feature will
dig before moving east, and also on the speed with which it moves
eastward (which in turn introduces further complexities and
potential interactions with a northern stream wave). The GFS has
consistently been quicker to move the upper low eastward, with the
ECMWF on the slow side of the spread, and the CMC along with
ensemble means in the middle. Taking a look at the hemispheric
pattern, the broader pattern shows very little in the way of
significant flow blocking, with things remaining largely
progressive from Asia across the North Pacific to North America.
Additionally, teleconnections associated with persistent positive
height anomalies becoming established across western Canada favor
colder conditions/lower heights across the eastern half of the
CONUS, suggesting that the flow should have a tendency to remain
somewhat progressive. Given these considerations, decided to lean
away from the slowest ECMWF solution and follow more closely with
the 12Z CMC, which seemed to represent a compromise. Given the
favored timing, this idea would favor the development of a wave of
low pressure along the trailing frontal boundary across the lower
Mississippi Valley by Fri night-Sat which would then move
northeastward into the Ohio Valley/central Appalachians by Sat
night-Sun.
The WPC forecast was initially based on a blend of the 18Z GFS and
12Z ECMWF/CMC during days 3-5 (Wed-Fri). After that time, given
the above train of thought, a blend of the 12Z CMC and 12Z
ECENS/NAEFS ensemble means was used, with increasing weight place
on ensemble means through time.
...Weather Highlights/Threats...
Showers and perhaps a few thunderstorms are possible across the
Southern/Central Plains and mid-Mississippi Valley Wed-Thu in
association with the cold front crossing the region. Rain and
areas of snow associated with this system are expected to spread
into the Great Lakes/Northeast Thu-Fri. Farther west, as the
trough digs along the West Coast and the upper low develops,
moisture will be drawn northward into the southwestern U.S.,
potentially including moisture associated with the future remnants
of Tropical Storm Raymond. Widespread and potentially heavy rains
will be possible across much of the Southwest Wed-Thu, with some
areas expected to receive at least a couple inches of rainfall.
Snow will be possible at the higher elevations from the southern
Great Basin to the central/southern Rockies. The wave of low
pressure expected to develop across the lower Mississippi Valley
Fri-Sat will bring the potential for a large area of overrunning
rain from the Gulf Coast/lower Mississippi Valley, eventually
reaching the Southeast, Appalachians, and Mid-Atlantic Fri-Sun.
Model spread remains fairly high, but in the scenario described
above, rainfall totals of a couple inches would be possible from
the lower Mississippi Valley to portions of the Southeast.
Below average temperatures are expected to overspread much of the
western and central U.S. Wed-Fri with the upper low digging into
the Southeast and the cold front crossing the central/eastern U.S.
High temperatures across the West are expected to be 5 to 15 deg F
below normal through this time period. While the central U.S.
should initially be warm (highs 5 to 10 deg above average) on Wed,
cooler temperatures will arrive in the wake of the front, with
highs 5 to 15 deg below average Thu-Sat.
Ryan
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards 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