Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1057 AM EST Sun Jan 27 2019

Valid 12Z Wed Jan 30 2019 - 12Z Sun Feb 03 2019

...Arctic airmass expected to bring record breaking temperatures
and dangerously cold wind chills to the north-central U.S...


...Overview...

An amplified flow regime is expected to persist across the CONUS
through the medium range, but with some changes to the structure
by next weekend. A deep upper low expected to be centered across
the Great Lakes on day 3 (Wed) will support rapid progression of a
few shortwaves from the northern plains to the Northeast. One
shortwave and deepening low pressure system will exit the
Northeast on Wed while a secondary wave/front crosses the Ohio
Valley quickly moving toward the Eastern Seaboard. Farther west, a
compact but energetic Pacific upper low appears likely to reach
the West Coast on Wed and then dig into the Great Basin/Southwest
by Thu-Fri, reaching the southern plains as an open wave by the
weekend, when another vigorous Pacific trough reaches the West
Coast. The large upper low should begin to lift out by Thu-Fri,
opening much of the CONUS up to Pacific shortwave energy, with
periodic amplification of progressive northern stream energy,
likely bringing another frontal system into the central U.S. by
next weekend.

...Guidance Evaluation/Preferences...

Models consensus has improved over recent days with a consistent
picture of the deep vortex crossing the Great Lakes by mid to late
week evident across the guidance. Additionally, solutions seem to
have settled toward favoring the compact upper low reaching the
West Coast on Wed and digging into the Southwest after that. A
blend of the 00Z ECMWF/CMC and 06Z GFS represented this consensus
well, and served as a forecast starting point through much of the
forecast period during days 3-5 (Wed-Fri). Significant model
differences begin to emerge during days 6-7 (Sat-Sun). Timing
differences develop with the next amplified wave reaching the West
Coast, with the GFS much faster to bring the trough inland
relative to the ECMWF/CMC. At this time, WPC favored the slower
solution here. In the northern stream, a number of solutions
suggest amplification of a shortwave across the Canadian prairies
next weekend, with an associated surface low passing along the
U.S./Canada border toward the Great Lakes. Model consensus was a
bit better here and ensemble members showed reasonable clustering
with respect to the surface low track, even with some
timing/intensity variability evident in deterministic solutions.
Given these considerations, the forecast during days 6-7 continued
to emphasize the ECMWF/CMC but shifted considerably more emphasis
toward the ensemble means (ECENS/GEFS), with majority weight place
on the means by day 7.


...Weather Highlights/Hazards...

The deepening low pressure system exiting New England on Wed could
produce heavy snow for portions of northern New England. A
secondary cold front crossing the central/eastern U.S. in the wake
of this leading system will usher a frigid arctic air mass into
much of the central/eastern U.S. Temperatures across a large area
from the northern plains and Midwest to the Ohio Valley are
forecast to be 30-40 deg F below average on Wed. High temperatures
will remain well below zero across most of the Midwest on Wed,
with low temperatures surpassing -30 deg for some areas across the
northern plains/Upper Midwest. A number of record lows and record
low max temperatures will likely be broken both Wed and Thu. A
tight pressure gradient between the arctic surface high and the
deepening low pressure system across New England on Wed into Thu
will result in potentially gusty winds for much of the
Midwest/Great Lakes, with dangerous wind chills surpassing -40 deg
F for many areas - which can produce frostbite on exposed skin in
a matter of minutes. The air mass will moderate some as it moves
east by Thu, with max/min temps from 20-30 deg F below average
spreading into the Mid-Atlantic/Northeast. A number of temperature
records may also be in jeopardy on Thu across these areas.
Temperature will quickly moderate by late week into next weekend
as the upper low quickly lifts northeastward into Canada, allowing
temperatures to gradually return closer to seasonal norms.

Along the West Coast, the arrival of a couple Pacific systems will
keep precipitation (rain and mountain snow) fairly widespread,
starting in the Northwest and gradually shifting south across
California. Rain/snow will gradually spread into the Great
Basin/Rockies by next weekend as the second/larger upper trough
moves inland.


Ryan


WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation and winter weather outlook
probabilities 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