the WPC extended discussion breaks it down well IMO, I highlighted in red the part that's key.
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
148 AM EST Sun Dec 02 2018
Valid 12Z Wed Dec 05 2018 - 12Z Sun Dec 09 2018
...Cold pattern with increasing winter weather potential for
portions of the central/eastern U.S. late this week...
...Overview...
The flow across the CONUS during the medium range will lie in a
transition zone from highly amplified/blocked flow extending from
east Asia across the North Pacific to somewhat more progressive
flow across the Atlantic Ocean. Anomalous ridging across Siberia
will displace cold polar/arctic air toward Canada, and persistent
ridging from western Canada north to Alaska will favor transport
of cold air southward from Canada into the central/eastern U.S. A
number of shortwaves will traverse an active northern stream from
the Midwest to the Northeast during the period, with a strong cold
front accompanying the most significant of these waves across the
Plains, Ohio Valley, and Northeast Wed-Fri. Farther south, an
active southern stream will spread Pacific energy from California
east across the southern states. A wave of low pressure is
expected to develop along the polar front near the western Gulf
Coast by Sat morning in response as the southern stream wave
approaches. Models continue to suggest the possibility for phasing
interaction this southern stream wave with additional northern
stream energy by Sat night/Sun, with potential development of a
coastal low near the Southeast U.S. coast.
...Guidance Evaluation/Preferences...
Models handled both northern and southern stream features
sufficiently during days 3-4 (Wed-Thu) to justify use of a
multi-model deterministic blend (ECMWF/GFS/FV3). Starting on day 5
(Fri), forecast confidence begins a gradual downward slide,
especially surrounding a couple key areas. While most solutions
agree that the southern stream wave should gradually deamplify as
it moves east, differences emerge as to the timing of the
deamplification as well as the timing of the eastward progression.
Uncertainty is even a bit higher in the northern stream by late
week, as models have shown a fairly large degree of variability
and run-to-run inconsistency with a number of smaller scale
impulses traversing the Midwest/Great Lakes. Both of these factors
play a substantial role in any potential phasing of the two
streams, with compounding uncertainty surrounding any such
interaction. As a testament to the large array of possible
solutions, even the usually under-dispersed GEFS becomes quite
dispersed by next weekend, with similar behavior shown by the
ECENS and CMCE ensembles. As a surface low develops along the
western Gulf Coast Fri night, the precise evolution/track of this
feature will hinge entirely on the eventual evolution of these
upper-level features. Possibilities range from a consolidated low
pressure area that moves north toward the Ohio Valley to a more
easterly moving low with energy transferring to a new coastal
system once phasing occurs. Given these considerations, a
transition to majority weighting of ensemble means (ECENS/GEFS)
was shown from day 5 onward, with the solution by day 7 (Sun)
comprised almost entirely of ensemble means.
...Weather Highlights/Hazards...
Systems traversing the northern stream across the Midwest/Great
Lakes will be relatively starved for moisture, but lake effect
enhanced snowfall will be possible. Heavy precipitation (rain and
mountain snow) will be possible for portions of California and the
Southwest Wed-Thu as the southern stream system moves onshore. As
this system reaches the central U.S. on Fri it will begin to
advect deeper Gulf of Mexico moisture northward, which will be
enhanced ahead of a developing surface low in the northwestern
Gulf. The result is expected to be a shield of widespread and
potentially heavy precipitation Fri-Sat from portions of the
southern plains to the Tennessee Valley. Wintry precipitation will
be possible on the northern periphery of this system as a cold
front and strong polar surface high provide an abundant source of
cold air. Precipitation should spread into the
Appalachians/Mid-Atlantic regions by Sat/Sat night as we see a new
surface low potentially developing along the Southeast coast. See
the latest Day 4-7 winter weather outlooks for details on areas
expected to see the greatest winter weather threat in the medium
range.
Temperatures will be below normal across much of the CONUS through
the medium range. The central U.S. will see the largest
temperature anomalies through much of the period, where high temps
are expected to range from 10 to 20 deg F below average.
Surrounding areas from the Rockies to the Eastern Seaboard will
not escape the cold conditions, with highs across a large area
ranging from 5 to 15 deg below average.
Ryan
