Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
310 PM EST Mon Dec 09 2019

Valid 12Z Thu Dec 12 2019 - 12Z Mon Dec 16 2019

...Developing low pressure system likely bring areas of heavy rain
up the East Coast Friday into the Saturday with wintry
precipitation at first across the interior Northeast and central
Appalachians...

...Moist and fast flow across the West could lead to next low
pressure system developing over the Deep South early next week...


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

Model guidance this morning continues to indicate that a cold core
upper vortex will meander in the vicinity of Hudson Bay through
this weekend. Across the U.S. mainland, the pattern will become
more amplified this weekend as a shortwave exiting the southern
Rockies/northern Mexico is forecast lift toward the northeast and
interact with the northern stream flow over the eastern U.S. The
GFS continues to show a faster motion of this shortwave across the
northern Gulf of Mexico Thursday into Friday compared with the
ECMWF, resulting in a much faster progression of the wave through
the Eastern Seaboard during the weekend.  A 30% blend of the 06Z
GEFS with 70% the 00Z EC mean (together with some of their
deterministic solution) was used to handle this system.  This
yielded a slower forward motion of the system with the associated
moisture pushing further into the interior sections of the entire
East Coast this weekend. A similar blend of the 06Z GEFS with 70%
the 00Z EC mean was used for Days 6 and 7 to handle the
uncertainty on the back side of the low over the Northeast as well
as a developing low over the Deep South.


...Weather Highlights/Threats...

Models still offer varied low scenarios from the Gulf of Mexico to
the Southeast then up the Eastern Seaboard into next weekend.
Forecast spread and run-to-run variability remain substantial. 
But there appears to be a trend for the associated moisture
pushing further into the interior sections of the entire East
Coast this weekend.  This will likely bring areas of heavy rain up
the East Coast especially for the Appalachians into the
Mid-Atlantic Friday into Saturday.  Subsequent system interaction
with a slow-to-recede lower atmospheric cold air damming offers an
interior snow/ice threat to monitor.  Wintry precipitation can be
expected at first across the interior Northeast and central
Appalachians Friday night into early on Saturday.  As is often the
case, the degree of phasing/interaction between the two initially
separate streams and baroclinic zone/coastal setup will be
determining factors of eventual evolution and any potential for
any deeper coastal storm developments and threats.

Out West, the pattern will become wet across the Pacific Northwest
into later this week as Pacific shortwave energies/height falls
punch onshore. Moderate to heavy snows will develop as the system
penetrates inland, especially over favored terrain of the
north-central Great Basin/Rockies later week through the weekend.
By next Monday, the exiting wave will possibly inject and then
interact with Gulf moisture to spwan a low pressure system
forecast to move through the Deep South.

Kong/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

Hazards:
- Heavy precipitation across portions of California, the Pacific
Northwest, and the Northern Great Basin, Thu-Fri, Dec 12-Dec 13.
- Heavy precipitation across portions of the Northeast and the
Great Lakes, Sat-Sun, Dec 14-Dec 15.
- Heavy rain across portions of the Southeast, the Lower
Mississippi Valley, the Southern Appalachians, and the Tennessee
Valley, Mon, Dec 16.
- Heavy rain across portions of the Northeast, the Central
Appalachians, the Tennessee Valley, the Mid-Atlantic, the Southern
Appalachians, and the Southeast, Fri-Sat, Dec 13-Dec 14.
- Heavy snow across portions of the Central Rockies, the Northern
Plains, the Northern Rockies, and the Northern Great Basin,
Thu-Fri, Dec 12-Dec 13.
- Heavy snow across portions of the Central Great Basin, Fri-Sat,
Dec 13-Dec 14.
- Heavy snow across portions of the Central Rockies, the Central
Plains, and the Central Great Basin, Fri-Sun, Dec 13-Dec 15.
- Flooding occurring or imminent across portions of the Northern
Plains.
- Much below normal temperatures across portions of the Great
Lakes, the Upper Mississippi Valley, and the Northern Plains, Thu,
Dec 12.


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