Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
154 AM EST Mon Dec 03 2018

Valid 12Z Thu Dec 06 2018 - 12Z Mon Dec 10 2018

...Cold pattern over the contiguous U.S. with a significant storm
system likely to affect at least the southern half of the U.S.
late week into the weekend...


...Overview...

Upper-level 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 polar/arctic air into Canada; and
persistent ridging across western North America will favor
transport of cold air southward into the central/eastern U.S. A
number of shortwaves will traverse an active northern stream from
the Midwest to the Northeast during the medium range, with a
strong cold front accompanying the most significant of these waves
across the Plains/Midwest, Ohio Valley, and Northeast Thu-Fri.
Farther south an active southern stream will carry Pacific energy
from near California eastward across the southern states. As this
energy reaches the southern Rockies/Plains, a wave of low pressure
is expected to develop along the polar front near the western Gulf
Coast by Sat. Models and ensembles continue to show varying
degrees of phasing/interaction of this southern stream wave with
additional northern stream energy by the weekend, along with
likely development of a coastal low near the Southeast U.S. coast.
By late in the medium range (early next week) there are some signs
that the pattern may modulate somewhat, with ensemble guidance
suggesting the western North America ridge may begin to shift
eastward toward the Rockies/High Plains, allowing heights to fall
along the West Coast as at least portions that region perhaps
enter a period of stormier conditions.


...Guidance Evaluation/Preferences...

Models continue to handle features in both the northern and
southern streams similarly during days 3-5 (Thu-Sat), and a blend
of the ECMWF/GFS/FV3 served as a good forecast starting point
during this time period. Some differences begin to emerge and
amplify from day 5 onward, particularly with smaller scale
shortwave energy crossing the Upper Midwest/Great Lakes Sat-Sun.
Differences also emerge with the southern stream energy as it
reaches the southern plains/lower Mississippi Valley Sat
night/Sun, with models continuing to show some degree of
variability on timing and amplitude. Given the potential for
phasing interactions between the two streams, uncertainty
continues to be compounded. A comparison of the last few ensemble
cycles does reveal a modest reduction in spread by Sun-Mon with
respect to the evolution of the low pressure system as it crosses
the Southeast states. Based on a consensus of deterministic and
ensemble guidance, a potentially multi-centered low pressure
system is expected to be crossing the coastal Southeast by Sun
morning. After that time, there still remains significant
uncertainty as to whether the low pressure system will track
northeastward along/close to the coast or take a track more out to
sea. Given large variability among the guidance as well as
run-to-run variability, an ensemble mean (ECENS/GEFS) based
consensus solution was used during days 6-7 (Sun-Mon). This
solution depicts a deepening low pressure system off the coast of
the Carolinas Mon morning, fairly close to the position shown by
the 18Z GFS. Along the West Coast, model agreement is better that
the upper ridge should slowly propagate eastward to the Rockies by
Sun-Mon allowing height falls to reach the West Coast. Solutions
differ as to exactly how amplified the trough will be, with some
modest run-to-run variability as to the nature of the system as
well (open wave versus a closed upper low), so the ensemble based
approach is ideal here by days 6-7 as well.


...Weather Highlights/Hazards...

The southern stream system will initially bring locally moderate
to locally heavy precipitation (rain and high elevation snow) to
parts of California and the Southwest. Areas near coastal southern
California and favored terrain over Arizona should see the highest
totals with this event. Farther east, expect areas of heavy
rainfall to develop over the southern plains Thu night-Fri and
then continue eastward into the weekend. Cold air in place on the
northern periphery of the system will likely produce wintry
precipitation in the northern part of the precipitation shield,
starting over the southern plains Thu-Fri and reaching the
Appalachians and East Coast during the weekend. Currently the best
potential for significant snowfall exists from the
southern/central plains into the southern/central Appalachians and
parts of the Mid-Atlantic. Northern stream systems affecting the
northeast quadrant of the nation will be relatively starved for
moisture but lake effect enhanced snowfall will be possible. 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. 

Below average temperatures will prevail across much of the CONUS
during the medium range. The most extreme anomalies should be over
the central U.S. behind the front that drops southward mid-late
week. Some locations may see highs at least 15-20F below average.
The eastern U.S. will generally see highs 5-15F below normal with
some day-to-day variability. The West will begin the period Wed
with highs 5-10F below average, but temperatures will moderate
through the weekend as upper ridging slowly shifts east.
Temperatures may reach several degrees above average across
portions of the central/northern Rockies and the northern High
Plains by Sun-Mon.


Ryan


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