Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
157 PM EST Sat Jan 09 2021
Valid 12Z Tue Jan 12 2021 - 12Z Sat Jan 16 2021
...Overview...
The best consensus of guidance is fairly stable in showing
amplification of the western mean ridge aloft after midweek along
with corresponding deepening of a large scale trough over eastern
North America. This trough will replace an elongated multi-stream
mean trough from eastern Canada into northern Mexico. Another
Pacific shortwave rounding the mean ridge will likely reach the
Northwest by early next weekend and flatten downstream flow enough
to start pushing away the late week trough over the East. Expect
heaviest precipitation during the period to be in the Northwest as
shortwaves/frontal systems head into and around the mean ridge
aloft. Elsewhere, precipitation should be exiting the East early
in the period with some areas of precipitation (mostly snow)
spreading into the Great Lakes and vicinity by late week.
Shortwave energy ejecting from the southern Plains/northern Mexico
could produce one or more areas of precipitation over parts of the
South/Florida but with very low confidence for how much or where
if any occurs.
...Guidance/Predictability Assessment...
The pattern transition toward larger scale features later in the
week should improve predictability relative to earlier time frames
when smaller shortwaves that are typically not well resolved a few
days out in time play a greater role in the forecast. Still there
are some differences of note through the forecast. As of early
day 3 Tue recent guidance trends have been more sheared with
shortwave energy supporting a weak system which the guidance
average now depicts as a mere wavy front. Upstream elongated
trough energy over the southern Plains into Mexico continues to be
problematic in terms of strength and timing of ejection.
Confidence in any particular solution remains below average for
this. Meanwhile differences persist for energy rounding the
upstream ridge. The 00Z CMC/GFS looked particularly fast with
leading energy that they brought into the Great Lakes after
midweek. The 06Z/12Z GFS have returned toward consensus that
places more emphasis on trailing amplifying flow. GFS runs have
been inconsistent with the shortwave reaching the Northwest by
next Sat, 00Z/06Z versions noticeably flatter than other models
and the 12Z run more amplified than the average. ECMWF runs have
been less erratic but show a gradual stronger trend.
An operational model composite represented the best ideas of
guidance early in the period while toning down the most uncertain
details. A trend toward nearly even weight of the 06Z GFS/GEFS
mean and 00Z ECMWF/ECMWF mean maintained consensus and continuity
well over most areas later in the forecast.
...Weather/Threats Highlights...
Heaviest rain and mountain snow in the Northwest will likely
extend into midweek, with strong westerly flow aloft focusing
moisture into the favored terrain along the coastal ranges and
Cascades. Exact location of highest totals is still dependent on
lower predictability shortwave specifics better resolved in the
short range. There should be a brief break as upper ridging
amplifies followed an episode of what should be less extreme
precipitation as another shortwave and frontal system pass
through. The Olympics and northern Cascades could see locally
heavier activity though. Some of the moisture from these events
will extend into the northern Rockies. Precipitation should be
exiting the Eastern U.S. as the medium range period begins on
Tuesday. The associated weak system will bring a trailing front
through/south of the Florida peninsula, with showers possible at
least along the front and possibly later as the front settles
south of the state. Any mid-late week rainfall will depend on
lingering moisture and low-confidence specifics of shortwave
energy that may cross the southern tier of the lower 48. The
deepening upper trough and leading frontal system reaching the
East by Thursday to Friday should bring a combination of synoptic
and lake effect/enhanced snows from the Great Lakes into the
northern/central Appalachians. There could be some rain in the
southern part of the moisture shield along the leading front.
Parts of the Great Lakes/Northeast may continue to see some snow
into weekend.
Continue to expect well above normal temperatures across much of
the northern tier through much of next week into the weekend with
areas from Montana into the Upper Midwest likely to see the
greatest anomalies. Greatest coverage of plus 20-30F anomalies
will likely be from Tuesday into early Thursday. Some of this
warmth will also extend into the central Plains. Upper ridging
will bring a warming trend to the Western U.S. mid-late week with
highs up to 10-15F above normal and possibly daily record highs at
a few locations over California and the Southwest. A cool start on
Tuesday across the South should moderate back to normal by
Wednesday and beyond. The upper trough/front reaching the East
late this week will bring highs down to near or modestly below
normal levels by next Saturday.
Rausch/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