Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 PM EST Mon Dec 06 2021
Valid 12Z Thu Dec 09 2021 - 12Z Mon Dec 13 2021
...Back-to-back heavy mountain snows expected across the Cascades,
Sierra Nevada, Intermountain West, and Rockies...
...Overview...
A generally progressive pattern is expected to persist through the
medium range period with a tendency for a ridge to become more
dominant across the South by early next week. A couple of
synoptic scale systems moving through this progressive pattern
will be of concern. A low pressure system forecast to track
through the central Plains to the Great Lakes will likely bring a
swath of wintry weather across the interior western U.S. late week
to the Great Lakes by the weekend, while heavy rain is possible on
Saturday across the Tennessee Valley ahead of a cold front with
timing uncertainties. Over the Pacific Northwest, the first wave
of heavy mountain snows late this week will likely be followed by
another wave of moisture which has the potential to deliver
heavier snowfall amounts for the higher elevations.
...Guidance/Predictability Assessment...
The latest model guidance this morning exhibits reasonably good
agreement with the overall large-scale pattern described above,
with increasing uncertainties across the South by early next week.
For the low pressure system forecast to develop over the central
Plains on Friday, models have generally increased the amplitude of
the shortwave dipping into the western U.S., resulting in a
stronger/deeper low to develop with a slightly faster forward
motion. There has been a tendency to push the associated cold
front faster toward the east through the eastern U.S. during the
weekend as well. Both the GFS and the ECMWF have attempted to
develop a frontal wave over the southern Appalachians during the
weekend in some previous runs although their ensemble means do not
support such development. By early next week, ensemble means
generally support the establishment of a ridge across the South.
However, the latest (12Z) ECMWF develops an anomalous upper low
over the Deep South amid significant amplification of the frontal
wave over increasing amplitude of the upper-level pattern.
...Weather/Hazards Highlights...
As the trough deepens in the West, precipitation will spread from
the Pacific Northwest and Northern Rockies southward to California
into the Great Basin to Central Rockies Thursday and Friday.
Favored terrain in Arizona could also see some enhanced activity.
Generally expect higher elevation snow and lower elevation rain,
with snow levels likely somewhat low as upper troughing amplifies.
Parts of Southern California may see the first rainfall in over a
month. Then as the western upper trough approaches/pushes into the
Plains toward the end of the week, precipitation should track
ahead of a low pressure system and a strong cold front. Some
wintry weather could spread into the Northern Plains and
Midwest/Great Lakes Friday in the cold air north and west of the
low track. Then rainfall may become moderate to heavy over parts
of the east-central U.S. especially by Saturday with the aid of
strengthening Gulf inflow ahead of the strong cold front.
Additionally, heavy precipitation chances are forecast to increase
in the Pacific Northwest starting around Friday night or Saturday
and continuing through the weekend as a potential atmospheric
river event sets up, though uncertainty remains with the
progression/southward extent of moisture, and will continue to be
monitored.
Ahead of the trough and cold front, mild to warm temperatures are
forecast to be in place in the central U.S. Thursday and Friday,
with highs currently forecast to rise into the 80s in portions of
Texas and the Lower Mississippi Valley, which could approach or
set daily records as the temperatures should be 15-30F above
normal. These warm temperatures will push into the Ohio/Tennessee
Valleys and Eastern Seaboard on Saturday, while temperatures
moderate to near normal behind. The western U.S. can expect below
average temperatures especially Friday and Saturday, with the
Central Great Basin in particular seeing temperatures 10-20F below
normal. Then, as ridging builds across the lower 48 by early next
week, temperatures should return to 10-20F above normal across the
northern two-thirds of the Plains by next Sunday and Monday.
Kong/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