Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Sun Nov 24 2019
Valid 12Z Wed Nov 27 2019 - 12Z Sun Dec 01 2019
...Several Major Storms for the Thanksgiving Holiday Period...
...Guidance/Predictability Assessment...
The latest models and ensembles show better than average forecast
clustering/predictability regarding the evolution of a shortwave
trough exiting the Central Rockies at the end of the short range
period, and steadily deepening as it lifts into the Great Lakes to
begin the medium range period on Wednesday. This should spin up a
rather impressive cyclone at the surface which tracks from the
Great Lakes to the Northeast on Wednesday-Wednesday night. The CMC
is a hair faster with the upper shortwave/closed low as it exits
the Northeast early Thursday and so a blend of the better
clustered GFS/ECMWF with minor contributions from the ensemble
means was used, which also maintains good continuity with the
previous WPC forecast.
The biggest forecast differences in the medium range is with the
evolution of highly amplified troughing along the West Coast as it
slowly inches inland and shortwave energy eventually ejects into
the Central Plains around day 6 to day 7. Through day 5, models
show reasonable agreement/clustering, but its after this time
where timing and intensity differences begin to arise. The GFS
(and to some degree the GEFS mean) are much quicker to eject
energy from the Southwest into the South-Central Plains, depicting
a wrapped up closed upper low over the Central Plains by as early
as day 6. Most other models and ensembles hold the energy back a
bit, not depicting a closed upper low to emerge until the system
reaches the Midwest on day 7. Given the uncertainty at the longer
range time scale, opted to go for a blend of the ECENS/GEFS means,
along with some percentage of the ECMWF which is better in line
with the ensemble means. Regardless of timing though, the models
do agree that another potent storm system could affect portions of
the north-central U.S. by the end of the period potentially with a
very similar track of the earlier storm days 3-4.
...Weather Highlights/Threats...
Moderate to locally heavy precipitation is likely Tuesday into
Wednesday on the north and west side of the surface low from the
Central Plains to the Upper Midwest. Some accumulating snows may
be possible on the northern edge of the precipitation shield. As
the storm lifts into the Northeast on Wednesday, wintry
precipitation is possible across northern/interior New England. A
period of high winds appears likely to accompany this system as
well.
Meanwhile, heavy mountain snows are likely across the Great Basin
and Rockies Wednesday-Friday with snow levels expected to drop
significantly. Another round of potentially heavy rainfall with a
threat of runoff problems are also especially likely across
Southern CA and the Southwest (including burn scars) by mid to
late next week. Heavy rainfall is also possible from portions of
the Southern Plains to the lower Ohio Valley as multiple shortwave
impulses round the base of the Western U.S. trough and tap some
mid-upper level moisture from the tropical east Pacific. The
northwest edge of this precip shield across eastern New
Mexico/Texas Panhandle may also see accumulating snowfall with
surface high pressure and sufficiently cold air diving southward
across the Plains. With the next potential storm system winding up
in the Central U.S. into next weekend, accumulating snows
(potentially heavy or significant) are possible to the north and
west of the surface low from the Northern Plains to the Upper
Mississippi Valley next Friday and Saturday.
The biggest temperature anomalies will focus across the Western
U.S. through much of the period with daytime highs averaging 10 to
20 degrees below normal. In response to this trough, ridging
builds across the south-central U.S. which should bring a return
of above normal temps (anomalies +5 to +10 degrees) to this region
by the end of the period. In the East, above normal temperatures
on Wednesday should moderate back to normal or slightly below
normal at the end of the week.
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