Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Tue Nov 26 2019
Valid 12Z Fri Nov 29 2019 - 12Z Tue Dec 03 2019
...Major winter storm likely for the Thanksgiving Holiday Period...
...Pattern overview and Guidance/Predictability Assessment...
The medium range period (Friday to Tuesday) begins with a broad
upper level low within very amplified troughing over the West
Coast. Shortwave energy rounding the base of the trough ejects out
into the Central Plains by late Friday-early Saturday with a
rather deep upper level low developing in the Northern Plains and
tracking quickly eastward through the weekend. Compared to recent
days, the models have come into much better agreement with this
feature, with some mostly minor timing and intensity differences
remaining. The notable outlier continues to be the UKMET which is
almost a day later to eject the energy into the Plains than the
rest of the guidance. This results in a rather potent surface low,
and potentially major winter storm, tracking from the
north-central Plains to the Upper Great Lakes/Midwest, and off the
Northeast coast by Monday.
Behind this main system, upper ridging builds across the Central
U.S. while reinforced troughing gets established just west of the
West Coast. There is enough run-to-run variability in the
deterministic models with the evolution of various embedded vorts
that an ensemble mean blend is the best approach at this point.
Across the board, this cycle of the WPC progs used a blend of the
GFS/ECMWF with the ECENS/GEFS means. Good agreement with the major
Central U.S. upper low warranted a majority deterministic weight
days 3-5, with increasing usage of the means thereafter. This
maintains good continuity with the previous WPC forecast.
...Weather Highlights/Threats...
The Central U.S. storm will bring a variety of weather hazards to
a large part of the country for the upcoming Thanksgiving holiday
weekend. Heavy mountain snows will remain possible on Friday for
much of the Central and Southern Rockies coming to an end by
Saturday as the system shifts east. As the surface low moves into
the Plains, a swath of heavy to potentially significant
accumulating snowfall is likely north and west of the low from the
Northern Plains to the Upper Great Lakes Friday and Saturday,
shifting into New England on Sunday. High winds may also accompany
this system making for difficult and hazardous travel across much
of the region. Along and ahead of the attendant cold front, heavy
to possibly excessive rainfall is likely from the Southern
Plains/Gulf Coast to the Ohio Valley. The latest SPC severe
weather outlook also shows a 15 percent chance of severe weather
for Friday across the Southern Plains.
The greatest temperature anomalies will focus across the West
through the weekend with daytime highs 20 to 25 degrees below
normal for some but should trend back towards normal (though still
slightly below) by the end of the period. The Midwest to Deep
South are warm Friday and Saturday, with cooler than average
temperatures moving in by late weekend and early next week. Cold
front exiting the East day 6/Tuesday will bring a return to below
normal temperatures.
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