Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1100 AM EST Fri Dec 28 2018

Valid 12Z Mon Dec 31 2018 - 12Z Fri Jan 04 2019

...Overview...

The medium range period (Mon-Fri) features a positively tilted
trough in the West slowly translating eastward as subtropical
upper ridging over the Bahamas retreats into the Atlantic. This
also allows progression of northern stream upper flow along the
U.S.-Canadian border south of an upper low over Hudson Bay.
Initial eastern Pacific/northwestern North American ridging is
forecast to push into the lower 48 with corresponding progression
of the downstream upper trough. This overall pattern evolution
likely results in the heaviest precipitation (mostly in the form
of rain) over the eastern third to half of the country.


...Guidance Evaluation/Preferences...

There remains good enough agreement to warrant a deterministic
model blend (between the 06z GFS/00z FV3-GFS/00z ECMWF) for days
3-4 (Mon-Tues) which clusters well with the ensemble consensus and
previous shift continuity regarding upper troughing moving into
the Southern Plains. Beyond this, differences begin to arise in
both the evolution of the trough (recent runs of some models
depict a closed or nearly closed low forming over the Southeast)
and also how quickly to advance the trough into the East.
Meanwhile, there is some question on how quickly, if at all, upper
ridging into the Western U.S. mid-late week gets broken down (the
00z CMC is by far the strongest in maintaining that ridge) and
also with amplitude and timing of a shortwave moving into the
Pacific Northwest late week and into next weekend. In light of
this, opted to rely much more on the ensemble means late in the
medium range period with only slight contributions from the 00z
ECMWF to add some detail as it seemed to provide a solution most
reasonably in line with the ensemble means. Note that yesterdays
12z ECENS mean was used due to delayed availability of the 00z
run.


...Weather Highlights/Hazards...

A couple of waves over the eastern half of the country will
promote heavy rainfall potential from the Mississippi Valley to
the Tennessee or Ohio Valley and into the Mid-Atlantic and the
Southeast. A trailing wavy frontal boundary making only gradual
progress over the Southeast will likely provide focus for
additional bands of enhanced rainfall from Tues onward, with the
best signal for highest 5-day QPF totals currently extending from
the central Gulf Coast through Georgia. Moisture reaching the
Great Lakes/New England may be in the form of snow, especially
with lake effect activity behind low pressure exiting New England
mid-week. In the Southwest, an initial upper low early in the week
will likely result in another round of precipitation for parts of
eastern Arizona and New Mexico, with the highest potential for
accumulating snows confined to the higher mountains. The Pacific
jet should reach far enough south for the Pacific Northwest to see
an increase of rain/high elevation snows by next Thu/Fri with some
meaningful totals likely to extend at least into the
Olympics/Washington Cascades.

The Northern Plains states will see a couple of very cold days
with temperatures 15-30F below normal followed by a rapid warm-up
by Thu. Many areas from the interior West through the rest of the
Plains will also see a period of chilly/cold temperatures,
generally 10-25F below normal. Cold anomalies should be most
widespread in the Mon-Wed time frame until the trough weakens and
moves eastward. In contrast the East will see above normal
temperatures until cooler air moves in Wed-Thu. Greatest anomalies
should be in the plus 20-30F range for low temperatures on Tue,
while high temperatures should be 10-20F above normal Mon-Tue.
Expect Florida to be the last to see the late-period cooling trend
over the East as the cold front finally moves eastward, while the
West Coast states should experience temperatures within a few
degrees of normal on most days next week.


Santorelli/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