Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Mon Dec 20 2021
Valid 12Z Thu Dec 23 2021 - 12Z Mon Dec 27 2021
...Pattern Overview and Guidance/Predictability Assessment...
The large scale pattern continues to show better than average
agreement and predictability. Shortwaves tracking around a very
strong central Pacific into Bering Sea/Alaska upper ridge will
periodically dive into and reinforce an amplified trough whose
axis should be near the West Coast, possibly drifting slightly
inland later in the period. Fast flow downstream--between low
heights over Canada and a ridge tracking over Mexico and the
southern Gulf of Mexico--will carry along ejecting features that
should support a couple surface systems during the period. Expect
the West Coast upper trough to support a broad area of
precipitation over the West for multiple days with areas from
western Oregon into the Sierra Nevada likely to see the highest
five-day totals. Also of note, a strong temperature contrast will
set up between much above normal temperatures over and east of the
central/southern Plains and well below normal temperatures over
the extreme northern Plains. Even with the above average
confidence in the large scale pattern, embedded details have lower
predictability so some specifics for the lower 48 forecast remain
uncertain.
The first major feature of note will be a West Coast shortwave
that ejects eastward after early Thursday and likely produces a
Plains through southeastern Canada/Northeast U.S. system around
Friday and Christmas Day/Saturday. Over recent days there has
been a fair amount of variability for the character of this energy
near the West Coast and how quickly it will continue onward. An
average of 12Z/18Z guidance eventually trending toward a mix of
models and means provided a good intermediate solution early in
the period and held close to continuity later as the 18Z GFS/12Z
ECMWF ended up trending slower than in the previous cycle while
the ensemble means and CMC continue to reflect a faster solution
per continuity. Overall the new 00Z model runs have become less
amplified with the West Coast shortwave as of early Thursday and
differ for details of separate offshore energy, affecting
coverage/timing of precipitation over southern parts of the West.
This change has allowed the GFS and ECMWF to return to a faster
surface progression over eastern North America though.
The aforementioned strategy also worked well for resolving lower
confidence details within and downstream from the persistent West
Coast trough after ejection of the first shortwave. There is a
reasonable signal in the models and ensemble means that another
general area of low pressure should evolve over the central U.S.
around Sunday-Monday.
...Weather/Hazards Highlights...
A broad area of the West will see lower elevation rain and
mid-high elevation snow with a persistent flow of moisture
directed onshore ahead of shortwaves digging into the West Coast
mean trough aloft, with moisture and terrain enhancement extending
as far east as the Rockies. There will be periods when the
precipitation will be moderate or heavy which may increase soil
saturation and sensitivity to local runoff; however uncertainty on
timing and shortwave interactions will temper confidence in
resolving the exact location and intensity of lower elevation
rainfall with respect to local runoff/flooding issues over
California. The Oregon Cascades and parts of northern/central
California are likely to have some of the highest accumulations,
especially heavy snow in the Sierra Nevada. These snows may have
high liquid equivalents in the favored upslope areas of the Sierra
Nevada range. Favored terrain over the Great Basin and
northern/central Rockies will likely see meaningful precipitation
as well. The timing and amounts of precipitation over southern
parts of the West have become more of a question mark with latest
adjustments in the models. Colder air pushing into Northwest may
lead to very low snow levels by next Sunday-Monday while other
parts of the West may also see declining snow levels as the upper
trough axis nudges eastward.
A late week into weekend system should spread precipitation across
the northeastern quadrant of the country. Best potential for some
snow will be across the extreme northern tier including the Upper
Great Lakes and parts of New England. Confidence is only moderate
at best for system details given model variability thus far.
Another area of precipitation (some snow in northern areas) may
develop over the northern and east-central U.S. toward early next
week with another area of low pressure.
The central and southern Plains will see much above normal
temperatures for most of the period with plus 15-25F anomalies.
Highs could even reach or exceed 30F above normal at some
locations on Friday. Flow ahead of the late week system will push
some of this warmth into the East as well. Daily records for
highs/warm lows will be possible where temperatures are most
anomalous. In contrast, temperatures that are 15-30F or more
below normal should settle into parts of Montana and North Dakota.
The West will trend gradually colder with time, leading to some
highs 10-15F below normal by the weekend and early next week.
Rausch/Campbell
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