Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1100 AM EST Sun Dec 16 2018

Valid 12Z Wed Dec 19 2018 - 12Z Sun Dec 23 2018

...Periods of heavy precipitation expected to continue over the
Northwest through next weekend...

...Late week storm likely brings widespread wind and rain for much
of the eastern U.S....


...Overview...

Some details remain in question but there is good
agreement/continuity with the general idea that a rapidly
building/sharpening ridge over the West Wed-Thu will support
downstream digging of shortwave energy into an amplified trough
over the Mississippi Valley by Thu.  The sharpening trough should
then close off near the southern Appalachians as a significant
surface cyclone will likely move up the Appalachians on Friday. 
Meanwhile, incoming Pacific shortwave energy should dampen the
western ridge, leading to a noticeably less amplified overall mean
flow with the hint of a split flow pattern just inland from the
West Coast by next Sunday.  The progressive/low amplitude nature
of flow in this time frame will likely create difficulties in
resolving shortwave specifics.   


...Guidance Evaluation/Preferences...

Global models and ensembles are all coming into agreement for a
significant cyclone forecast to impact much of the eastern U.S.
Friday to Saturday, with the storm track slowly trending farther
and farther inland.  Apt to use more of the 00Z ECMWF solution for
the sea-level pressure fields for this time-frame, which depicts a
double-barrel low pressure system moving up the Appalachians in
favor of a larger separation between the upper-level cut-off low
to the southwest and the triple-point low to the northeast.  For
Days 3 and 4, and further out for Days 6 and 7, mainly a consensus
of the 00Z ECMWF/EC mean and the 06Z GFS/GEFS was used, which is
keeping good continuity with the previous WPC forecasts.

...Weather Highlights/Hazards...

One wave of heavy rain/higher elevation snow over the Northwest
during the short range period should be tapering off by next Wed. 
Then low pressure likely to track just offshore the Pacific
Northwest into western Canada Thu-Thu night should bring another
episode of significant precipitation.  This event may have
somewhat more limited extent of heaviest activity, most likely
focused over the Olympics and northern Cascades, but still expect
some moisture to reach into extreme northern California.  There
may be a period of fairly strong winds if the surface low is on
the deeper side of the full guidance spread.  Expect lighter and
more scattered precipitation Fri followed by periods of mostly
light-moderate rain/snow during the weekend.

The storm system expected to affect the East should spread a broad
area of moisture over most of the eastern half of the Lower 48
during the latter half of the week, with chilly cyclonic flow
supporting Lake Effect/terrain-enhanced precipitation during the
weekend.  While some details of system evolution have changed over
the past day, best guidance signals for highest rainfall totals
for this event continue to exist over the Florida Peninsula and
North Carolina coast.  Other relative maxima may exist with
Atlantic inflow across the northern half of the East Coast and to
the northwest of the storm track.  This should be a fairly warm
system for the time of year, with most snow confined to the
western periphery and then over the Great Lakes and vicinity after
system passage.

Expect the greatest temperature anomalies to be of the warm
variety over the central U.S. during Wed-Fri, with some areas in
the northern Plains seeing min temperatures in particular
exceeding 20F above normal.  Such anomalies may reach daily
records for warm mins.  Meanwhile the eastern U.S. storm may pull
in a brief surge of warmth to the Mid-Atlantic/Northeast late in
the week.  The Pacific origin of supporting energy for this storm
should keep cold sector temperatures from reaching much lower than
normal, though some cooler readings may be possible over the
Southeast around Fri-Sat.  The overall flatter trend for flow
aloft should promote moderation of temperatures toward normal over
many areas of the country by next Sun.

Kong/Rausch


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