Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1059 AM EST Sat Jan 19 2019

Valid 12Z Tue Jan 22 2019 - 12Z Sat Jan 26 2019

...Overview...

Latest guidance continues so show agreement in regards to the
amplified pattern with a ridge over the eastern Pacific/West Coast
and a trough dominant over the central and eastern U.S. This
pattern will give way to cold, Canadian air spilling into the
northern tier--making its way down into the middle Mississippi
Valley and portions of the central Plains. This pattern also
suggests higher precipitation chances in the eastern half of the
U.S. while the West Coast should be dry near the upper ridge axis.
Though models were consistent with the overall pattern aloft,
differences in the smaller scale lead to significant differences
at the surface. This impact will affect sensible weather and take
additional time to resolve.


...Guidance Evaluation/Preferences...

Models/ensemble means continue to show significant differences and
trends for various aspects of the forecast--especially with
regards to shortwave specifics and stream interactions. Initially,
on Day 3/Tuesday, models are in line and in general have the
surface low in the same general region of the central Plains.
However, even by Day 4, the 00Z ECMWF begins to diverge from most
of the models at the surface where to place the main surface low
since the ECMWF is slow to lift a shortwave embedded in the
trough. The 00Z and 06Z GFS, 00Z UKMET/CMC are all in agreement to
lift this shortwave quickly from the Great Lakes. Issues continue
to deteriorate further going out in time. The 00Z ECMWF, both
06Z/00Z GFS, and their means still have not come to an agreement
where to place the surface low in the Southeast/Mid-Atlantic by
Day 5/Thu. Up to Day 5, used a general blend of the 06Z GFS/00Z
ECMWF/00Z UKMET/00Z CMC and the 00Z GFS parallel. By Day 6, the
06Z GFS was much stronger with the surface low lifting north of
the Canadian maritimes--thus the WPC blend by Day 6 and 7 used the
00Z GFS in lieu of the 06Z GFS because of this while increasing
the ensemble means.

For the most part shortwave details are sufficiently small in
scale to yield low confidence in specifics, and will be prone to
run-run/day-day variability. Thus expect worse than average
continuity for associated waves/frontal systems.


...Weather Highlights/Hazards...

During Tue-Wed Plains low pressure tracking through the Great
Lakes and into eastern Canada will spread snow from the
north-central Plains and Upper Mississippi Valley through the
central-northern Great Lakes into New England.  The trailing front
should begin to interact with Gulf moisture by Tue night-Wed with
expanding areas of locally moderate/heavy warm sector rainfall. 
Confidence is increasing that a significant wave may form along
the trailing front and spread a broad shield of
precipitation--some heavy--across the south and eastern states at
least through Thu. The track and timing of this wave still remains
uncertain; thus, confidence in remains low where the location of
heaviest rain/snow will fall as well as precipitation types. the
best potential for highest total precipitation remains within an
area from the east-central Gulf Coast/Tennessee Valley/southern
Appalachians northeastward into southern New England.  Best
snow/wintry mix potential will lie to the northwest of that area. 
After passage of this system, one or more waves/fronts may produce
areas of light-moderate snow over some areas east of the Rockies
and cold flow over the mostly ice-free Great Lakes should promote
episodes of lake effect snow.

The forecast looks fairly consistent for the focused band of
shortwave energy and moisture that will bring a period of enhanced
snow and low elevation rain to locations from the Pacific
Northwest into the Northern/Central Rockies Tue-Wed night. Late
week into the weekend additional shortwave energy within
northwesterly flow aloft along with a persistent stationary front
at the surface will provide focus for snow along the northern half
of the Rockies.

Portions of the East will see a very cold start to the period on
Tue (some lows 20F or more below normal over the southeastern
Great Lakes/northern Appalachians) before a brief warm-up ahead of
the approaching front/wave, with some readings up to 10-20F or so
above normal.  Cold air dropping into the east-central U.S. later
in the period may bring temperatures down to at least 15-25F below
normal along the upper and portions of the middle Mississippi
Valley.  Continue to expect moderately above normal temperatures
over the West Coast states after midweek as the upper ridge axis
settles nearby.

Reinhart/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