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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1820Z Dec 07, 2019)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
120 PM EST Sat Dec 07 2019

Valid 12Z Tue Dec 10 2019 - 12Z Sat Dec 14 2019

...Much below normal temperatures for the Northern
Plains/Midwest/Great Lakes...

18 UTC Update...

No major changes were made to the ongoing forecast in this update.
The GFS/ECMWF were heavily used in the forecast for days 3-5
(Tue-Thu). For days 6-7 (Fri-Sat), ECENS/GEFS ensemble means
comprised a majority of the forecast, with some continued use of
the deterministic ECMWF as well. Models continue to show support
for the development of a significant wave of low pressure along a
surface front in the Gulf of Mexico by late next week, potentially
tracking northeastward across the Southeast U.S. by early next
weekend. Spread is considerable with respect to the timing and the
eventual track of this feature, however, which supported a
solution close to the ensemble means. Updated winter weather
outlooks this morning showed an expanded region of 10-30 percent
probabilities for days 6-7 across portions of the Mid-Atlantic and
Northeast, associated with this system.

Ryan


Previous Discussion (issued at 0721 UTC)...

...Pattern Overview and Guidance/Predictability Assessment...

A southern stream flow associated with an amplifying Southwest
upper trough early week still offers some uncertainty in terms of
magnitude and ejection timing for the U.S. southern tier Tuesday
into Thursday, but guidance clustering has improved to bolster
forecast confidence.

Overtop, a cold Hudson Bay upper vortex lingers next week. A
notable upper trough rounding underneath amplifies to the central
then eastern lower 48 states Tuesday/Wednesday, with less
amplified troughing/cyclonic flow for much of the rest of week. An
upstream ridge amplifies up the North American West Coast early
week before being weakening midweek by eastern Pacific shortwaves
that cut inland from the Northwest to the snowy Rockies. A favored
and reasonably well clustered GFS/ECMWF and GEFS/ECMWF ensemble
solution suggests  this energy may subsequently act to amplify
central to southern/eastern U.S. upper troughing. This may lead to
organized cyclogenesis to focus moisture and lift to produce a
risk of heavy rainfall across the Southeast. Subsequent system
deepening, northward lifting, and interaction with receding lower
atmospheric cold air damming over the East heading into next
weekend may offer an emerging wintry snow/ice threat to the north
to monitor.   


...Weather Highlights/Threats...

There remains a strong signal for a swath of moderate rainfall
ahead of a main cold front from the lower MS Valley to the
Appalachians/Mid-Atlantic and southern/eastern New England
Tuesday. Activity will shift offshore midweek. Frontal waves and a
dynamic upper trough/jet also supports some enhanced snows on the
cooled northwest periphery of the precipitation shield from
Appalachians to the Northeast. Lake effect snow should also
persist across the Great Lakes in the wake of the cold front which
is particularly cold. Polar plunges are expected behind the cold
front offer temperature anomalies of 15 to 30 degrees below normal
centered over the Upper Midwest with reinforcing shots of cold
air.

Over the West...a building West Coast upper ridge should keep most
of the West dry through early next week beyond some Rockies snows.
The pattern should become more wet and elevation snowy for
especially the Pacific Northwest, but also inland to the Rockies
as shortwave energies round the northern part of the ridge later
week.

Schichtel


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