Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
154 AM EST Sun Dec 30 2018

Valid 12Z Wed Jan 02 2019 - 12Z Sun Jan 06 2019

...Overview...

A positively tilted trough stretching from the Great Lakes to the
Four Corners region will move eastward to the Southeast and
Tennessee Valley by the end of the week. A closed mid-level low
will be over the Four Corners on Wednesday--moving across the
southern Plains by Thursday before making it to the Southeast.
During this transition, moving from the Plains to Southeast, where
model differences become apparent. Meanwhile, an upper ridge will
build across the western U.S. during this time. By the weekend,
the pattern will become more zonal for the eastern U.S. as a
trough in the Pacific begins to approach the West Coast. Overall,
expect an active pattern for the Southeast and Mid-Atlantic
through the first portion of the week--while precipitation will
increase across the West Coast by the weekend.


...Guidance Evaluation/Preferences...

In the beginning of the medium range period, Days 3-4 (Wed/Thu),
the 12Z GFS, 12Z ECMWF, and 18Z FV3-GFS are in fair agreement. The
timing of the trough moving across the southern Plains lines up
well. The 12Z UKMet was quite slow and much stronger with the
closed low over the Four Corners, whereas the 12Z CMC was too
fast--thus both of these models were not used in the blend. By Day
4/Thu, there were already some differences with the ECMWF and
GFS/FV3, with the ECMWF showing a closed low over the Mississippi
Valley and GFS/FV3 having a more elongated trough. The ECMWF has
been more consistent with this feature especially compared with
the GFS. By Day 5/Fri, the GFS still shows energy with the
elongated trough over the Southeast as the ECMWF lifts the trough
northward over the Mid-Atlantic. The GFS is also more amplified
with the upper ridge over the Intermountain West/Great Basin due
to an incoming closed mid-level low--it keeps the closed low
intact off the coast of British Columbia while the ECMWF has it
becoming an open wave moving across the western Canadian
provinces. Once the weekend approaches, differences once again
show up with the incoming trough in the Pacific in both detail and
timing--with the GFS faster and the ECMWF showing a closed low.
The 12Z ECMWF ensemble mean and 18Z GEFS mean are more consistent
with the main features overall from Day 5 and beyond.

A blend of the 12Z GFS/18Z FV3-GFS and 12Z ECMWF was used for Days
3/4, with more weight on the ECMWF due to its consistency. For
Days 5-7, used a blend of the 12Z ECMWF ensemble mean and 18Z GEFS
mean along with the 12Z ECMWF and a little bit of the 12Z GFS.


...Weather Highlights/Hazards...

Precipitation will begin to increase across the western and
central Gulf Coast by midweek as the mid-level trough approaches
the Plains. At the surface, a low will move northward along with a
warm front. This will promote convection across the Gulf Coast and
Southeast. By Thursday, precipitation is expected to decrease
across the western Gulf Coast while continuing across the
Southeast and central Gulf Coast as the surface low lifts into
these regions. When the frontal system moves north and east into
the Mid-Atlantic and Northeast on Thursday and into Friday,
precipitation will follow. By the weekend, precipitation should be
winding down for the eastern U.S. The highest QPF totals will
stretch from west-central Mississippi eastward into the Carolinas
on Wednesday and Thursday. Farther west, expect an increase in
precipitation by Thursday for the Pacific Northwest--with rain and
higher elevation snow for the Olympics/Washington Cascades and
portions of the northern Rockies. On Saturday and Sunday, a
frontal system associated with the trough off the West Coast will
bring rain/higher elevation snow for California and the Four
Corners.

The southern Plains will be the coolest area in the beginning of
the medium range period--with high temperatures being 15-20
degrees below normal. The northern Plains and Upper Midwest will
be above average for Wednesday/Thursday, with temperatures
expected to be 15-20 degrees above average. The warm conditions
will shift to the Mississippi Valley and eastern U.S. by the
weekend, with temperatures nearing 10-15 degrees above average.
The western U.S. will stay near normal through the medium range
period.


Reinhart


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