Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
103 AM EST Thu Jan 17 2019

Valid 12Z Sun Jan 20 2019 - 12Z Thu Jan 24 2019

...Major Winter Storm shifts from Great Lakes/Ohio
Valley/Appalachians to Mid-Atlantic/Northeast Sunday into Monday...


...Guidance Evaluation/Preferences...

The WPC medium range product suite was mainly derived from
reasonably clustered 12 UTC ECMWF/GEFS/ECENS guidance. This holds
good WPC continuity. However, latest GFS/GEFS runs trended more
progressive with the flow. A major lead weekend winter storm still
offers high predictability, but the timing/details of an upstream
main southern stream system next week is more uncertain.


...Weather Highlights/Hazards...

Deep Gulf of Mexico and Atlantic moisture will lift over the
eastern U.S. Sunday. Heavy precipitation focuses around a main
low/frontal system, overrunning arctic air to the north. Heavy
snows shift from the Great Lakes/Ohio Valley and Appalachians
through the northern Mid-Atlantic/Northeast, exiting Monday. Some
locations could see blizzard conditions in the tight pressure
gradient between the surface low and the strong arctic high.
Meanwhile, lingering heavy warm-sector rains/strong convection
moves through the Southeast/Mid-Atlantic Sunday, with a transition
zone between snow and rain to include significant sleet and
freezing rain.

Meanwhile, Pacific storm systems will reload with new focus as a
heavy precipitation threat into the Pacific Northwest this
weekend. Wintry weather spreads/moderates inland across the
north-central Great Basin/Rockies later in the holiday weekend and
across the full U.S. northern tier 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 then
Atlantic moisture inflow and precipitation pattern next Tue-Thu
that will include the threat for a widespread swath of snow/ice in
the cooled northern periphery of the large precipitation shield.

Upstream, sharp shortwave digging next Wed/Thu over the West to
the lee of a building/amplified eastern Pacific upper ridge may
provide cooling and risk of a wintry weather swath from the
Northwest to the north-central Intermountain West/Rockies/Plains.
 
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