Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1059 AM EST Mon Jan 14 2019

Valid 12Z Thu Jan 17 2019 - 12Z Mon Jan 21 2019

...Heavy precipitation shifts eastward from California through the
West Thu/Fri...
...Frigid air surges to the Central U.S/Northeast to set the stage
for another major central to eastern U.S. snow threat for the
holiday weekend, with potentially heavy rain farther south...


...Overview...

For the beginning of the forecast period, an upper low once again
shifts into the West Coast. Its energy will help create an upper
trough downstream in the central U.S., which will lead to surface
low development in the lee of the Rockies. This surface low is
expected to shift east and transition into a nor'easter, causing a
myriad of hazardous weather across the central and eastern U.S.


...Guidance Evaluation/Preferences...

Overall, guidance was in reasonably good agreement throughout the
medium range period, adding to forecast confidence. Previous runs
of the GFS were much faster with moving the upper trough and
antecedent surface low across the central to eastern U.S. than the
ECMWF suite, but the latest runs have come into closer agreement,
with the GFS slowing down considerably and the EC speeding up a
bit. Speed of the system will depend in part on whether or not the
northern and southern streams will separate or remain combined as
one trough. Other features, such as the upper-level low moving
into the West early in the period as well as another upper trough
or low approaching the Southwest by day 7, also seemed
well-handled. Thus, a blend of the operational GFS/ECMWF and
GEFS/ECMWF ensemble means was utilized throughout the forecast
period.


...Weather Highlights/Hazards...

In the west, the last in an upcoming southern stream series of
inland slamming Pacific systems this week will bring additional
heavy precipitation to mostly California and to a lesser extent
the Pacific Northwest Thu. Heavy valley rains/mountain snows are
likely with guidance now better clustered versus the last few
days, bolstering forecast confidence. With a healthy influx of
moisture, precipitation will work inland through the Intermountain
West/Great Basin/Rockies late Thursday and Friday with windy
conditions and valley rain/mountain snow. Snow levels will fall as
heights lower which will turn some lower elevations over to snow
as precipitation diminishes Saturday. Pacific storm systems reload
farther to the north later for the holiday weekend to then focus
an associated heavy precipitation threat into/across the Northwest.

Meanwhile, downstream surges of frigid Canadian air into the
central U.S. become more significant Thursday through the weekend
as the northern stream amplifies around a wobbling Hudson Bay
upper vortex. Leading lows Thu-Fri spread some modest snows over
the northeast quarter of the nation. Upstream, a more robust
system ejecting from the West organizes over the southern Plains
with increased Gulf of Mexico moisture and lift through Arkansas
Saturday as an arctic front to the north pushes through the
Northeast. As precipitation overruns the cold airmass to the
north, heavy snow will spread out of the central Plains through
the Midwest/Ohio Valley then Mid-Atlantic/Northeast as potentially
heavy warm-sector rains/convection move through the South. There
is an increasing signal in guidance/improved model and ensemble
clustering for a significant and widespread heavy snow/ice event
to the north of a subsequent main low track through the East then
up off the coast. Please consult WPC Day 4-7 QPF and Winter
Weather Outlook for a probabilistic assessment of the coast to
coast winter weather threat.
 
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