Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
149 AM EST Tue Dec 25 2018

Valid 12Z Fri Dec 28 2018 - 12Z Tue Jan 01 2019

...Overview...

Significant winter storm in the short range will exit through the
Great Lakes/Northeast Friday as its attendant cold front sinks
into the Gulf of Mexico. In the west, another upper trough and
surface cold front will push into the Southwest/Four Corners
Fri-Sun as another Pacific system comes ashore WA/OR Sunday.
Lingering front in the Gulf will have surface waves of low
pressure move along the boundary through the Southeast Sun-Tue
with mostly rain but perhaps some northern periphery and higher
elevation snow.


...Guidance Evaluation/Preferences...

Deterministic blend sufficed to start the period given the good
overall agreement with the exiting system through southern Ontario
Friday. To the west, the large disagreement between the GEFS/ECMWF
ensembles has narrowed but not disappeared -- first over the
northeast Pacific and then into the Southwest. Preferred the
12Z/24 ECMWF-led consensus (with the Canadian and FV3-GFS) that
allowed for a more separated Pacific upper trough to push into
WA/OR quicker than the 12Z/18Z GFS (which was consolidated/slower
with the northern stream). Downstream, trend seems to favor a
weaker and not totally closed off upper low Sunday over NW Mexico
south of the AZ/NM border which yields something in between the
quicker previous runs of the ECMWF and much slower runs of the
GFS. The 12Z/24 ECMWF/Canadian and FV3-GFS offered a good base to
couple with the ensembles which maintained good continuity from
the previous shift. With upper ridging centered over the Bahamas
(and no reason to abate) continued troughing is favored in the
west into the start of next week and the new year (Tue Jan 1).
Northern stream will continue to zip along the US/Canadian border
and the system moving through Idaho Sunday will move into the
Great Lakes around next Tuesday.


...Weather Highlights/Hazards...

Locally heavy rainfall in the Southeast will be ongoing Friday as
the snow winds down over the Upper Midwest/Great Lakes (and exits
the Northeast) while the low and front move to the east.
Concurrently, the system will sink through the Southwest with more
dynamically-driven snow and some lower elevation rain into the
Four Corners region (especially New Mexico). Next Pacific system
enters late Saturday into Sunday with widespread but mostly light
to locally modest rain/snow through much in the interior west
continuing on Monday. That surface low will move along the
US/Canadian border and drag its cold front to the south and east.
All the while, front in the Gulf will linger/waver and send a
couple low pressure centers along it through the Southeast which
will spread some rain over AL/GA/SC/NC but perhaps to the
Mason-Dixon line -- likely rain but some elevation snow.

Temperatures will be cooler than average in the West and Plains
behind the departing Great Lakes storm, with coldest anomalies
(10-20F below average) likely focused over the High Plains and
vicinity. Ahead of it, much above average warmth (plus 10-30F
anomalies) will be drawn into areas east of the Mississippi Valley
Fri/Sat until the cold front moves through. With reinforcing
troughing into the West, temperatures will stay below average from
the Great Basin/Rockies into the Plains, especially over newly
snow-covered areas. Cooler air will filter into the East
post-FROPA but only to near average for late December.
Central/southern Florida will stay under the influence of
subtropical ridging and continue with above average temperatures.


Fracasso


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