Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1010 PM EST Mon Dec 31 2018

Valid 12Z Fri Jan 04 2019 - 12Z Tue Jan 08 2019

...Overview...
A phasing/strengthening negatively tilted trough stretching across
the northeast Pacific from offshore Alaska to offshore the west
coast of the Lower 48 is expected to help expand/strengthen
mid-level positive height anomalies across the Great Lakes/Midwest
with time, which allows troughing across Atlantic Canada to
strengthen downstream.


...Guidance Evaluation/Preferences...
The guidance shows small differences with the first system
tracking from the Mid-Mississippi Valley across the Mid-Atlantic
States offshore Nova Scotia Friday into Sunday; a compromise of
the 00z GFS/00z ECMWF/00z UKMET should work out fine here. 
However, differences crop up within the details of the negatively
tilted trough offshore western North America, including a system
moving across the southern tier of the Lower 48.  The 18z GFS was
the preferred piece of deterministic guidance with this system as
it best matches the 12z ECMWF/12z bias-corrected NAEFS mean.


...Weather Highlights/Hazards...
A deep layer cyclone Friday and Saturday should promote a quick
shot of moderate to heavy rainfall from the mid-Mississippi Valley
to the Mid-Atlantic and southern New England coasts. Much of the
precip associated with this system should be in the form of rain,
but some changeover to freezing rain/snow is possible on the far
northern edge of the precipitation shield, though any
accumulations appear light at this time.

Out West, the first shortwave on Friday should bring rain/mountain
snows into the Pacific Northwest while the approaching trough this
weekend into early next week spreads precipitation into California
(with heavier precip likely focused along favored terrain in
Southern California) and eventually moving into the Southwest and
Colorado Rockies before the heavy precipitation focus shifts into
the Northwest early next week. Heavy mountain snows are possible
especially for the Sierra Nevadas and higher elevations of Arizona
before shifting into the Pacific Northwest late in the period.

The best chance for below to much below normal temperatures should
be early on across the Four Corners into the Southern Plains where
afternoon highs could be 10-20F below normal. The western U.S.
should stay near normal throughout the medium range period while
temperatures east of the Rockies should be near to above normal.

Roth/Santorelli


WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found 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