Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1100 AM EST Wed Jan 16 2019

Valid 12Z Sat Jan 19 2019 - 12Z Wed Jan 23 2019

...Major Central to Northeastern U.S. Arctic Blast/Winter Storm...

...Overview...

For the holiday weekend, a low will spread snow to the
north-central U.S. and Northeast, rain to the south-central U.S.
and Southeast, and mixed precipitation in between. Another upper
low is forecast to move through the West on day 4-5 (Sun-Mon),
which will deepen a trough over the south-central U.S. once again
by Tues-Wed.


...Guidance Evaluation/Preferences...

Deterministic and ensemble model guidance was in reasonably good
agreement for days 4-5, so the WPC forecasts were derived from a
multi-model blend of the 00Z ECMWF and UKMET, 06Z GFS, and NAEFS
and ECMWF ensemble means. By day 5, the trough centered in the
Rockies began to have some model disagreement between the slower
Canadian and ECMWF solutions and the faster GFS, which continues
into days 6-7. The 00Z GFS especially was considerably faster than
the GEFS mean with this feature. Thus leaned away from that and
more toward the ECENS and operational ECMWF, along with the NAEFS.


...Weather Highlights/Hazards...

Frigid arctic air spills across the central then eastern U.S. as
the northern stream amplifies around a Hudson Bay upper vortex.
Underneath, amplified southern stream mid-upper level trough
approach/height falls and favorable coupled upper jet support will
spawn cyclogenesis from the south-central to eastern U.S. over MLK
Jr. weekend. Deepening Gulf of Mexico then Atlantic moisture will
advect inland in advance and around the developing storm and
overrun a sharp arctic front. The heavy snow event will begin in
the short range in the Central Plains, and will spread eastward on
day 4 from portions of the Middle Mississippi Valley through the
Midwest/Ohio Valley, Northern Appalachians, and northern
Mid-Atlantic/Northeast. Some locations could see blizzard
conditions in the tight pressure gradient between the surface low
and a strong arctic high. Meanwhile, heavy warm-sector
rains/strong convection move through the
South/Southeast/Mid-Atlantic and a transition zone between snow
and rain is likely to include significant sleet and freezing rain.
Please consult WPC QPF and Winter Weather Forecasts for a
probabilistic assessment of the significant winter weather threat.

Upstream, Pacific storm systems will reload with new focus as a
heavy precipitation threat into the Pacific Northwest. Wintry
weather spreads/moderates inland across the north-central Great
Basin/Rockies later in the holiday weekend and then across the
central U.S. early-mid next week. There is a signal for increased
mid-upper level trough amplitude/separation over the south-central
U.S., but there is more uncertainty with the specifics of this
flow and associated surface system response. However, most
guidance supports an increasing lead Gulf moisture inflow and
precipitation pattern with additional potential for wintry weather
on the cooled northern periphery heading into Tue/Wed. 
 
Tate/Schichtel


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