Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
125 AM EST Thu Jan 14 2021
Valid 12Z Sun Jan 17 2021 - 12Z Thu Jan 21 2021
...Overview and Guidance/Predictability Assessment...
A deep cyclone should be exiting New England by the start of the
period on Sunday, with upper troughing shifting into the East.
Southern stream energy across the South Sunday-Monday may induce a
weak low pressure area near Texas, while upper level ridging in
the West gets replaced by a shortwave entering the region by
Monday. The guidance shows good agreement on this overall pattern
through day 4, with only minor timing differences in various
features.
As the Western U.S. shortwave drops into the Southwest by day 5
(Tuesday) is when things start to get chaotic. Most of the models
have been consistent for the past day in showing robust
amplification of this shortwave, potentially resulting in a closed
low over the Desert Southwest on Tuesday. Thereafter, models begin
to diverge on whether this system continues steadily eastward into
the Southern U.S. and weakens (like the ECMWF and to some extent
the CMC) or lingers a few days over the Southwest (GFS). This
appears to be dependent on the strength of ridging out over the
Pacific and additional shortwave energy rounding the top of it
into the Pacific Northwest. The ensembles tend to favor a slower
progression of this system, but maybe not quite as intense as the
last couple of runs of the GFS would suggest. Run to run
variability on the details of this system are high and given the
day 6-7 time frame, weighting towards the ensemble means on this
one is preferred.
The WPC forecasts were derived from a mostly deterministic blend
of the GFS/ECMWF days 3-5, quickly transitioning towards the
ensemble means by days 6-7. Did continue to include small
contributions from the GFS/CMC late period just for some added
definition. The ECMWF was just too fast with the Southwest
low/energy and so was removed from the blend. This maintains good
continuity with the previous WPC forecast as well.
...Weather/Threats Highlights...
Low pressure exiting New England on Sunday will allow for
lingering gusty winds across the Northeast, lake effect snow
showers around the Great Lakes, and snow showers in the central
Appalachians. The next system entering the West should bring light
rain or mountain snows to parts of the Pacific Northwest and
northern Rockies on Sunday, and the central/southern Rockies on
Monday. Precipitation should begin to expand into the Four Corners
regions and west-central Gulf Coast early next week associated
with the amplified system in the Southwest. Greater moisture
transport ahead of this system would increase the potential for
widespread rainfall from East Texas into the Lower Mississippi
Valley/Tennessee Valley Tuesday into Wednesday. Uncertainty is
high in the details of this system, so specific amounts are
impossible to pin down at this point, but there is increasing
model signal for the potential for moderate to heavy rainfall in
this region. Some snowfall is also possible in the mountains of
Arizona and New Mexico.
Temperatures 10 to 15 degrees above normal are possible Sunday
across California/Desert Southwest and the Northern Plains but
should trend cooler next week as upper troughing moves through.
Elsewhere, the Southeast begins the period slightly below normal
with values moving back towards normal or just slightly above
normal the rest of the week.
Santorelli
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