Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EST Tue Jan 08 2019

Valid 12Z Fri Jan 11 2019 - 12Z Tue Jan 15 2019

...Overview...

Progressive and amplified flow across the North Pacific will
continue to bring significant storm systems to the West Coast
every couple days during the medium range. A strengthening upper
ridge across the northern Rockies/Great Basin will result in split
flow as these Pacific systems move east, with energy separating
into northern and southern stream components. Northern stream
energy will amplify as it enters broad cyclonic upper-level flow
east of the ridge, across the Midwest/Great Lakes/Northeast. This
setup also results in potential phasing of northern/southern
stream waves and an elevated risk for coastal low development off
the Eastern Seaboard. While this pattern does not favor a
widespread outbreak of cold air, persistent troughing across the
East along with the amplified western U.S. ridge will support
transport of cold polar air south into eastern U.S., setting the
stage for at least some winter weather potential on the northern
side a low pressure system expected to track from the Gulf Coast
to off the Eastern Seaboard by the weekend.


...Guidance Evaluation/Preferences...

Models have generally trended toward a somewhat slower and more
amplified solution with an upper trough expected to approach
California Fri-Sat (days 3-4). Meanwhile, farther east, northern
and southern stream shortwave energy will begin to interact/phase
across the central U.S. during the same time period. As is often
the case in these situations, models show a bit more spread with
the northern stream wave, with the GFS the most amplified solution
and the UKMET among the least amplified. Ensemble means seem to
support a blended/consensus solution here. As a result, a
multi-model deterministic blend (ECMWF/GFS/UKMET) was used as a
forecast starting point for days 3-4.

By day 5 (Sun), models have trended a bit toward a more phased
solution with a more amplified trough across the Ohio
Valley/Appalachians and a somewhat less suppressed low track along
the Eastern Seaboard relative to this time last night. At this
time the GFS along with the CMC continue to show the farthest west
low track along the Southeast/Mid-Atlantic coast on Sun, with the
ECMWF and UKMET approximately 100 miles farther south/east. The
GFS remains on the western fringe of the ensemble distribution,
while the ECENS/GEFS/NAEFS ensemble means are much closer to
something along the lines of the ECMWF/UKMET. Farther west,
additional amplified energy approaches the West coast by Sun, with
a trend a slower/deeper solution with this feature, or perhaps
even cutting the system off west of California through early next
week. Continued variability among solutions and the complex nature
of the phasing process with the eastern system reduces confidence
in any particular deterministic solution. Thus, from day 5 onward
substantially increased weight was placed toward the ECENS and
GEFS ensemble means.


...Weather Highlights/Hazards...

Expect the potential for multiple rounds of heavy precipitation
(low elevation rain and mountain snows) across the West Coast
during the forecast period as a rapid succession of Pacific
systems impacts the region. As shortwave energy reaches the
southern Plains/lower Mississippi Valley by Fri-Sat, areas of
heavy rain will be possible, with precipitation spreading east
into the Southeast/southern Mid-Atlantic by Sat night/Sun. Cold
air to the north of the system could result in areas if wintry
precipitation across portions of the Mid-Mississippi Valley on Fri
and across areas from the Ohio Valley to the southern/central
Appalachians and Mid-Atlantic by Sat-Sun. The northern extent of
winter weather potential Sat-Sun across the Mid-Atlantic region
remains highly uncertain given the factors described above. As the
system tracks northeastward in the Atlantic Ocean Sun-Mon, New
England may face a threat for winter weather but this will again
hinge largely on the eventual track of the system and how close it
remains to the coastline.

Above average temperatures will persist through the forecast
period across the north central U.S., with max temp anomalies in
the +10 to +15 deg F range (even warmer for some areas) through
much of the period. The coldest temperatures will be across the
Northeast, where temperatures as cold as 10 to 20 deg below
average will be possible from Fri through early next week.


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