Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
208 AM EST Tue Dec 07 2021
Valid 12Z Fri Dec 10 2021 - 12Z Tue Dec 14 2021
...Winter weather is likely for the northern Plains to
Midwest/Upper Great Lakes behind a surface low Friday, while heavy
rain is expected in the east-central U.S. along its associated
cold front...
...Heavy precipitation is forecast to spread southward along the
West Coast late this week into early next week...
...Overview...
A generally progressive and active weather pattern is forecast to
occur through early next week, as a mid-upper trough pushes from
the Intermountain West early Friday toward the East by Sunday,
while a surface low and potent cold front track ahead of it. A
swath of wintry weather is likely to spread from the Central
Rockies into northern portions of the Plains to the Midwest/Upper
Great Lakes on Friday behind the low, while heavy rain is possible
late this week across the east-central U.S. along and ahead of the
cold front with some timing uncertainties. Meanwhile, slow-moving
and reloading troughing in the eastern Pacific will give
persistent moist inflow to the West Coast, with heavy mountain
snows likely that should spread southward as the period progresses.
...Guidance/Predictability Assessment...
The medium range period begins Friday with quite good model
agreement with the overall large-scale pattern, despite some minor
differences in shortwave energy within the main trough with its
axis tracking through western and central parts of the U.S.
through the end of the week. The track of the surface low pressure
system moving quickly from the central High Plains into the Upper
Great Lakes has also been in good agreement for a few model cycles
now, though the timing of the cold front extending southward from
the low remains somewhat variable. A multi-model deterministic
blend was able to be used early in the period.
By Sunday, differences begin to arise in the East that amplify
Monday-Tuesday, particularly with whether or not northern and
southern streams will separate depending primarily on distribution
of energy within the trough. This is highly uncertain and has had
quite a bit of run-to-run variability within and between model
guidance. With the 12/18Z (yesterday) model cycle, which the WPC
forecast is based on, trends seemed to be for more separation of
the streams and a southern stream low that is slower to exit the
Southeast. This was shown by the 12Z ECMWF, CMC, and UKMET
(through its period), with the CMC and EC ensemble means also
showing stream separation. The GFS and GEFS runs appeared to be
basically alone in their more phased and faster track of the
trough eastward into the Atlantic. Considering typical biases of
the model suites, the WPC forecast was based on an intermediate
solution but slightly favoring the slower side of the
guidance--mainly a blend of the EC and CMC ensemble means with
some component of the in between deterministic CMC, and not
favoring the 12Z ECMWF that was slower than basically every
ensemble member but also not favoring the fast GFS runs. However,
the incoming 00Z deterministic models including the CMC and ECMWF
are showing faster movement of a more phased trough offshore,
indicating high uncertainty continues. Expect forecast changes
with this feature as models still vary.
Upper troughing is forecast to drop southward from Alaska across
the eastern Pacific over the weekend and into early next week.
While model guidance agrees upon the existence of this feature and
its slow drift southward, there is plenty of variety in how
amplified it could be at any given time, which affects the moist
onshore flow axis into the West Coast and the extent/amounts/type
of precipitation. At this point, a similar model blend previously
described worked for this feature as well--with deterministic
models early on quickly transitioning to a blend heavy on the EC
and CMC ensemble means. In between these troughs, model agreement
is reasonably good for upper ridging to build in across the
central U.S.
...Weather/Hazards Highlights...
The surface low pressure system tracking northeastward into the
Midwest/Great Lakes Friday to early Saturday will likely spread
snow on the backside from the Central Rockies into northern
portions of the Plains and into the Midwest, with moderate to
locally heavy snow possible. The trailing cold front associated
with this low will track through the eastern half of the U.S. and
spread widespread rain with embedded thunderstorms along and ahead
of it. There is potential for heavy rain amounts with
quick-hitting high rainfall rates that could cause flooding/flash
flooding issues for portions of the Lower Mississippi, Tennessee,
and Ohio Valleys on Friday into early Sunday. Then, how much rain
falls along the Eastern Seaboard will depend on the timing of the
upper trough and cold front exiting into the Atlantic, which
remains uncertain. Meanwhile, persistent onshore flow is likely
initially in the Pacific Northwest and northern Rockies and then
could track south into California by early next week. Enhanced
totals are likely along favored terrain, with heavy mountain snow.
Ahead of the trough and cold front, mild to warm temperatures are
forecast to be in place in the central U.S. on Friday, with highs
currently forecast to rise into the 80s from portions of Texas to
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 much of
the central U.S. by Sunday and lasting/warming further though
Tuesday.
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