Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1104 AM EST Mon Dec 31 2018

Valid 12Z Thu Jan 03 2019 - 12Z Mon Jan 07 2019

...Overview...

Medium range period begins with a deep closed low over the
Southern Plains progressing east and northeastward to off the
Mid-Atlantic coast by Day 5 (Saturday), while at the same time
additional waves of energy slide through near the U.S./Canadian
border in the northern stream. Out West, the first shortwave
crosses into the Pacific northwest later this week, with a digging
trough approaching and possibly moving inland days 5 and beyond.
This overall pattern would favor heavy rainfall in the Eastern
U.S. through day 5, with the focus shifting to the West Coast by
days 5-7.

...Guidance Evaluation/Preferences...

Albeit minor timing differences, there finally seems to be a
better consensus on the evolution of a deep closed low moving
across the South, lifting towards the Mid-Atlantic and off the
Atlantic coast by this weekend. Overnight, the latest two runs of
the GFS began shifting towards a slower/deeper solution which has
continually run-after-run been depicted by both the ECMWF/UKMet
the past day or two. The latest runs of the CMC and the FV3-GFS
continue to be on the fastest side of the guidance envelope.
Comparing spaghetti plots of the ensembles from yesterday and
today continues to show a trend towards a slower progression and
the WPC blend for this set of progs favors a majority
ECMWF/UKMet/GFS blend for this system. This resulted in a slightly
slower and deeper upper level and surface low compared to the
previous WPC shift.

As the trough/closed low exits the East on Day 5, another trough
in the eastern Pacific approaches the West Coast. There continues
to be some disagreement both on structure and timing with this
system as a whole. On day 6, the past two runs of the GFS hold
onto a rather deep closed low off just off southern California,
while the ECMWF (and to some degree the Canadian) have been
consistent with showing just an elongated trough. The
trough/energy moves into the Southwest around Day 7, but the
timing on this is inconsistent. The ensemble means generally
follow suit with their deterministic counterparts (with obvious
suppression), but a general blend between the the ECENS/GEFS/ECMWF
seemed to be a good starting point, with slight weighting towards
the ECENS/ECMWF just simply due to better run to run consistency.

...Weather Highlights/Hazards...

Closed low latter half of this week should promote heavy rainfall
potential from the lower Mississippi Valley to the central
Atlantic coast. Models have been fairly consistent in showing the
heaviest axis of rainfall near a developing surface low tracking
from the Central Gulf Coast to the Carolinas. Much of the precip
associated with this system should be in the form of rain, but
some changeover to slow is possible on the far northern edge of
the precipitation shield, though any meaningful accumulation
potential remains low. Out West, the first shortwave on Friday
should bring rain/mountain snows into the Pacific Northwest while
the approaching trough this weekend spreads precipitation into
California (with heavier precip likely focused along favored
terrain in Southern California) and eventually moving into the
Southwest. Heavy mountain snows are possible especially for the
Sierra Nevadas and higher elevations of Arizona.

A look at temperature forecasts in the medium range show much
above normal temperatures possible across the Northern Plains on
Day 3, with near to above normal temperatures persisting across
roughly the eastern half of the country. The only chance for below
to much below normal temperatures should be days 3-4 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.

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