Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
145 AM EST Wed Nov 27 2019
Valid 12Z Sat Nov 30 2019 - 12Z Wed Dec 04 2019
...Major Winter Storm to impact the Northern Plains to the
Northeast during the Thanksgiving weekend...
...Pattern overview and Guidance/Predictability Assessment...
The medium range period (Saturday-Wednesday) begins with shortwave
energy ejecting into the Central U.S. inducing cyclogenesis in the
lee of the Rockies. This results in a deep low pressure system and
major winter storm tracking from the north-central Plains to the
Great Lakes this weekend. Additional southern stream energy
rounding the base of this cold core upper low allows for a new
surface low to form off the Mid-Atlantic coast early Monday which
then tracks up the New England Coast through Tuesday. Models and
ensembles continue to show fairly good agreement on the track and
intensity for this storm, with only minor timing and intensity
differences remaining. A general model compromise worked well and
shows good continuity with the previous shift.
Meanwhile, energies dropping southward from the Gulf of Alaska
establishes another trough into or just off the West Coast by day
4/Sunday. For days 3-4, there is good enough model agreement to
warrant a mostly deterministic blend between the latest runs of
the GFS and ECMWF. After this time, there remains question on the
evolution of an upper level low off the West Coast. The latest run
of the GFS (and CMC) show an upper low lingering through most of
the day 5-7 period well off the Southwest Coast, but run-to-run
continuity in the GFS is poor with this. The ECMWF shows better
run-to-run continuity in showing an upper low shifting southward
along the coast and eventually into the Southwest on day 7. The
ensemble means also seem to support this idea of weakening the low
and bringing it inland as well. Therefore, for days 5-7,
increasing weighting of the ensemble means was used with a good
chunk of the ECMWF which is the deterministic model closest to the
ensemble means.
...Weather Highlights/Threats...
A major winter storm with widespread hazardous weather is expected
for parts of the Central and Eastern U.S. over the Thanksgiving
holiday weekend. To the north and west of the surface low, heavy
and significant accumulating snow is likely from the Dakotas to
the Upper Great Lakes on Saturday, with accumulating snow likely
shifting into much of the Northeast Sunday into Monday. High winds
will also accompany this storm, making for difficult and hazardous
travel across much of the region. To the south, locally heavy
rainfall is possible along the attendant cold front as it shifts
eastward across the Deep South/Ohio Valley/Mid-Atlantic states
this weekend. Severe thunderstorms are also possible across the
Lower Mississippi Valley on Saturday as indicated by the latest
medium range SPC severe outlook.
In the West, expect another round of heavy mountain snows across
the higher terrain of northern and Central California, with rain
in the lower elevations as well as up the Pacific Northwest Coast.
More rain should also shift into parts of Southern California by
Tuesday as well.
Temperatures remain cold across the West Saturday into Sunday with
daytime highs 10 to 20 degrees below normal in some spots.
Temperatures should moderate back towards normal (though still
slightly below) through the rest of the period. Ahead of the cold
front in the Central U.S., warmer than average temperatures are
expected Saturday, but should quickly get replaced with below
normal temperatures by Monday which also sweep into the East
Tuesday/Wednesday following the cold frontal passage.
Santorelli
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