Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
213 AM EST Fri Dec 03 2021
Valid 12Z Mon Dec 06 2021 - 12Z Fri Dec 10 2021
...Overview...
The medium range period (Monday to Friday next week) will be
characterized by a broad mean trough across much of the lower 48,
with periods of shortwave energy moving through the trough pushing
low pressure/frontal systems across the country. Rounds of
precipitation are forecast for the West, while rain is likely to
spread into the southeastern quadrant of the country with perhaps
some snow in northern areas, but there is still a good bit of
uncertainty with specific weather impacts.
...Guidance/Predictability Assessment...
The 12/18Z (yesterday) model cycle remains in fairly good
agreement with the large-scale pattern, while aspects of the
smaller-scale details remain quite variable. At the beginning of
the period Monday, these differences include a slower upper trough
over the Midwest in the 12Z UKMET compared to the slightly faster
consensus of other guidance, but other than the UKMET there was
reasonably agreeable timing with a Great Lakes surface low and the
timing of its trailing cold front, maybe just slightly faster than
the previous forecast. Another difference among guidance Monday
was that the GFS runs (and many GEFS ensemble members) continue to
be the strongest with a potent shortwave entering the Pacific
Northwest, which as it dives south into the Intermountain West by
Tuesday has implications for its speed and possible flow
separation. However, at the surface the low pressure system is
well clustered offshore of the Pacific Northwest on Monday.
Meanwhile, southern stream energy near Baja California Monday has
shown a faster trend moving eastward and getting absorbed by
Tuesday compared to slower previous guidance, which is reflected
in the WPC forecast.
By Wednesday, recent guidance generally shows a hint of a positive
tilt to the mean trough, with the exception of the 12Z CMC with
its different phasing of shortwave energy that is not reflected in
its new 00Z run either, so leaned away from that solution. A
notable model difference by Thursday-Friday is the degree to which
energy drops southward through the West as well as its timing. The
GFS and CMC runs show stronger energy creating a deeper Western
trough compared to the ECMWF, which shows a couple rounds of
weaker shortwave energy passing through the mean trough.
Confidence is low with this, given uncertainty with the shortwave
energy stemming from high latitudes, and spaghetti plots of 500 mb
heights show the operational GFS and CMC are within in the
guidance envelope but are on the stronger end of solutions with
troughing in the West. Then in the East, there is uncertainty with
a surface low spinning up along the coastal Northeast late
Wednesday into Thursday, with no good consensus position yet, and
this will have implications for precipitation along the Eastern
Seaboard.
Thus the WPC forecast began the period using a blend of the
12Z/18Z GFS and 12Z ECMWF and CMC, phasing out the CMC from
midweek onward and increasing proportions of the agreeable GEFS
and EC ensemble means (to 60% by Day 7/Friday). This served to
keep proper magnitude of features when there is good model
consensus, and lessened the emphasis on individual models by the
latter part of the period as uncertainty increases.
...Weather/Hazards Highlights...
As a relatively strong surface low pressure system is positioned
across the Great Lakes early Monday, snow is expected there while
strong winds are possible in the tight pressure gradient behind
the low. Conditions will be favorable for lake effect snow behind
the system through Tuesday. Additionally, rain is forecast along
the trailing cold front associated with the low for the Ohio
Valley to south-central U.S., but amounts are likely to be
tapering down by Monday. However, Gulf moisture should increase
again by Tuesday into Wednesday and cause moderate to possibly
locally heavy rainfall from the Lower Mississippi Valley into the
Tennessee Valley and Southeast. Moisture spreading farther north
into cooler air could lead to snow across the northern latitudes,
with relatively higher probabilities from the Great Lakes into
northern New England.
A couple of shortwaves moving across the West will cause
increasing precipitation chances there, with favored windward
terrain seeing the highest amounts. The Pacific Northwest into
much of the Intermountain West and Rockies should see
precipitation Monday and clearing out by Tuesday while chances
ramp up in the Southwest. Then the next round is likely for the
Northwest on Wednesday and spreads into California eastward by
Thursday as energy aloft drops southward. Generally higher
elevation snow and lower elevation rain is forecast, though
varying throughout the precipitation events and perhaps relatively
low snow levels on average given periods of below normal heights
aloft.
Chilly temperatures are likely across the far north-central U.S.
on Monday and Tuesday given snow cover from Sunday's system, with
temperatures 10-20F below normal. Meanwhile the Eastern Seaboard
can expect temperatures about 10-20F above normal on Monday ahead
of a cold front bringing temperatures near average by Tuesday.
Overall temperatures should be within about 10 degrees of normal
in most places Tuesday and Wednesday, before a warmup by Thursday
and Friday from the Plains eastward, with 10-15F above average
temperatures. A cooldown is possible in the Northwest by the
latter part of the week with the potential for troughing aloft
there.
Tate
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