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

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

...Guidance Evaluation/Preferences...
A blend of the non-06z GFS guidance (00z ECMWF, 00z Canadian, and
00z UKMET with the 00z GFS parallel/FV3 as a reasonable
alternative) sufficed given the good overall agreement with the
systems moving through the West and downstream eastern ridge which
maintained good continuity from the previous shift.  With time,
00z ECMWF and 00z bias-corrected NAEFS ensemble means were
included into the pressures/500 hPa height preference mix.


...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 in the
wake of a cold frontal passage but with near average temperatures
for late December considering the above average mid-level heights.
Central and southern Florida will stay under the influence of
subtropical ridging and continue with above average temperatures.


Roth/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