Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 PM EST Fri Feb 05 2021
Valid 12Z Mon Feb 08 2021 - 12Z Fri Feb 12 2021
...Northern Plains to Midwest Arctic outbreak gradually expanding
to the south and east...
...Pattern Overview and Guidance/Predictability Assessment...
Models and ensembles still advertise a pattern favorable for
extremely cold conditions persisting over the central U.S. through
next week with gradual expansion of the cold air farther
south/east with time. The West will see more moderate
temperatures but with a general cooling trend. During the
forecast period the eastern half of the lower 48 should see
multiple precipitation areas of various types, with a general
tendency for the rain-snow line/possible icy mix transition zone
to push farther south by late in the week.
Consensus continues to show a blocky Canada pattern holding a deep
upper low just north of the U.S.-Canada border through at least
midweek and possibly beyond, while mean ridging persists over the
eastern Pacific. These features will help to maintain cyclonic
mean flow over the continental U.S. next week. Teleconnection
relationships seem to provide some background on the developing
guidance spread that becomes significant over some areas by late
in the week. Multi-day mean charts at the D+8 time frame
generally highlight two positive height anomaly centers, one over
the eastern Pacific and another just northeast of Hudson Bay. The
Pacific center favors positively tilted troughing over the West
and moderately above normal heights over about the southeastern
third of the CONUS (along with near/moderately above normal
temperatures in the East). In contrast the core of positive
anomalies near Hudson Bay would favor moderately below normal
height anomalies over the East (and below normal temperatures over
the southern two-thirds of the region), though with agreement on
some degree of troughing over the West. Questions include how
quickly the southern Canada upper low begins to depart/shear out
as initial northern Canada ridging weakens and flow tries to
adjust toward some composite of the telecon relationships
(GFS/GEFS runs slowest, 00Z ECMWF fastest but prior run slower),
along with the relative influence of the Pacific/Canadian
features. For the late week forecast, current uncertainty favors
a multi-guidance/multi-run approach to yield a deterministic
solution that maximizes flexibility depending on how future runs
adjust/cluster.
As for other details, guidance offers some significant differences
in northern tier U.S. upper flow and associated surface evolution
over and near New England by midweek. Lack of agreement among
fast GFS runs/12Z CMC, slow UKMET runs, and the deep 00Z
ECMWF--and low predictability for specifics of the fast
flow--recommend an intermediate solution for the time being.
Meanwhile there is reasonable agreement with weak shortwave energy
filtering through the East Pacific ridge aloft early-mid week but
with a general consensus for it to produce lighter precipitation
than a number of GFS runs have been showing over parts of the
West. The late period blend reflects some degree of west-central
U.S. troughing that would expand moisture over the
southern/eastern U.S while keeping the forecast between the GFS
and 00Z ECMWF extremes.
...Weather/Threats Highlights...
Terrain/upslope snow over the northern Rockies and vicinity should
begin to slowly taper off on Monday as the main shortwave energy
slides eastward to support frontal wave development over the
Northeast mostly on Tuesday with some effects possibly lasting
into Wednesday. This should lead to a band of potentially
meaningful snow from the Midwest into the Northeast from Monday
into Tuesday. Over the Midwest/Lower Great Lakes the guidance has
decreased totals somewhat over the past day, while models have
been conflicted over how the Northeast wave will evolve, so
precise effects remain uncertain. Meanwhile, a stationary front
across southern Florida as of early Monday will lift northward and
likely provide a period of enhanced rainfall from the eastern Gulf
Coast region to the southeastern U.S. coast. By later next week,
there is an increasing chance for Gulf moisture to return and
expand in coverage across the southern tier states and possibly
the East in an overrunning pattern north of a stationary front
across the northern Gulf. Depending on details of upper flow
there is also potential for this front/embedded wave(s) to lift
farther northward over the East. With the cold air in place
farther inland, a swath of wintry weather is possible from
portions of the southern tier into the Northeast later next week
but with lower confidence on the specifics.
The forecast is fairly consistent in showing coldest anomalies of
at least 20-30F below normal early in the week from the
northern-central Plains into the Midwest, with some readings
possibly as low as 30-45F below normal over parts of Montana.
During the rest of the week this core of cold air will remain in
place and steadily push to the south and east. Temperatures
reaching 20-35F below normal may extend into the southern Plains
and much of the Mississippi Valley by later in the week but
uncertainty in the eastern U.S. pattern lowers confidence in
exactly how cold the East will be at that time. Most locations
west of the Rockies will see a general cooling trend. Northern
areas should start out near to moderately below normal and trend a
bit colder, while the Great Basin/Southwest start the weak
moderately above normal and then decline toward normal (or
slightly below in parts of the Great Basin).
Rausch/Kong
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are 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