Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM EST Sun Dec 19 2021
Valid 12Z Wed Dec 22 2021 - 12Z Sun Dec 26 2021
...Pattern Overview and Guidance/Predictability Assessment...
Latest models and ensembles continue to show only slow evolution
of the upper pattern through the Wednesday-Sunday period.
Impulses flowing around the north side of a very strong
central/east-central Pacific upper ridge should reinforce a mean
trough whose axis should gradually drift from the eastern Pacific
to the West Coast or very slightly inland. Fast flow will prevail
downstream, between low heights over central Canada and an upper
high forecast to migrate eastward across Mexico and the southern
Gulf of Mexico. This pattern will favor multiple days of
widespread precipitation over the West with the Sierra Nevada
likely to see the highest five-day totals. Systems to the east of
the Rockies will initially have dynamic support from southern
Canada/northern tier U.S. flow and then become more dependent on
features ejecting from the West Coast trough. Much above normal
temperatures will prevail over the southern half to two-thirds of
the Plains with some of this warmth extending eastward with time.
Meanwhile the far northern Plains should see a pronounced colder
trend by next weekend.
Guidance through the 12Z/18Z cycles still displayed some important
detail differences for features within the eastern Pacific/West
Coast mean trough with eventual implications on the forecast
across the lower 48. During mid-late week the primary issue has
been the character of digging energy that replaces an initial
short-term upper low. GFS runs had been eager to close off
another offshore upper low whose energy takes longer to eject
inland, thus leading to a slower system reaching the East by next
weekend. Aside from one ECMWF run, that model has tended to be
more open with the shortwave with faster ejection--but generally
not as fast as latest CMC runs. Ensemble means have been
providing a good middle ground among these options. By next
weekend the means end up gravitating toward the 12Z ECMWF idea of
a well-defined system reaching the Great Lakes on Christmas
Day/Saturday and the Canadian Maritimes on Sunday. The fast
nature of the mean flow over most of the lower 48 would seem to
favor this idea versus the slower 12Z/18Z GFS as well. The new
00Z GFS has made a significant adjustment toward the majority
cluster, now showing more north-south elongation of the mid-late
week energy instead of a slow upper low and then faster ejection
thereafter. Later in the period the GFS/ECMWF and their means
compare fairly well for the West Coast trough as additional energy
reinforces the feature, with differences well within typical
error/guidance spread for forecasts that far out in time.
Early in the period over the East, consensus has been stable in
recent runs for the system crossing southeastern Canada and
northeastern U.S. around midweek. However guidance is still
having some difficulties in resolving the details of the system
expected to be just off the North Carolina coast early Wednesday
and continue over the Atlantic thereafter, favoring an
intermediate approach.
The operational model blend used to start the forecast for
Wednesday began to incorporate the 18Z GEFS/12Z ECMWF means by day
5 Friday and then reached 60 percent total ensemble weight by day
7 Sunday. The forecast excluded the CMC by the latter half of the
period and 12Z/18Z GFS weight was kept low enough to minimize
influence of its slower handling of late week West Coast energy.
...Weather/Hazards Highlights...
A broad area of the western U.S. will see rain and mountain snow
during the period, supported by a persistent flow of moisture
ahead of an eastern Pacific/West Coast upper trough. Some of this
activity will likely be moderate to heavy though it will take
additional time to resolve the finer details due to uncertainty
over individual systems and their interaction. However guidance
signals and teleconnections for the mean pattern still suggest
that northern and central California should have the highest
precipitation potential over the five-day period--including heavy
snow over Sierra Nevada/Shasta areas. The greatest liquid
equivalent totals should be over favored terrain along the Sierra
Nevada. It may still take into the shorter-range time frame to
asses the location and intensity of lower elevation rainfall with
respect to local runoff/flooding issues over California. Effects
from the multi-day accumulation of precipitation will also require
monitoring even if it is not intense on any particular day. Some
moisture should extend as far south as southern California and
Arizona after midweek while favored terrain over the Great Basin
and northern/central Rockies will see meaningful precipitation as
well.
Low pressure tracking away from near the North Carolina Coast may
be accompanied by a little rain early Wednesday while passage of a
southeastern Canada system and a weak upstream feature may produce
light snow over the upper Great Lakes and northern New England.
Latest guidance is signaling a more organized system could produce
snow or rain depending on latitude between the northern Plains and
Great Lakes/Northeast, though differences in evolution keep
confidence low for resolving coverage and intensity of
precipitation at this time.
Above normal temperatures over the southern two-thirds of the High
Plains on Wednesday will expand eastward across the Plains and
into the Mississippi Valley later in the week, with increasing
coverage of highs at least 15-25F above normal. Such readings
will likely persist over the southern half of the
Plains/Mississippi Valley through next weekend. A few record
highs may be possible over Texas during this warm period. Some of
this warmth should also reach the East with highs reaching 10-20F
above normal by the end of the week or next weekend. On the other
hand the far northern Plains should see a pronounced colder trend
during the weekend as temperatures drop to 15-30F below normal
over parts of Montana and North Dakota. The unsettled pattern
over the West should lead to near or above normal lows and near to
below normal highs through the entire week. Western highs should
gradually trend cooler with time as the axis of the eastern
Pacific upper trough reaches at least as far as the West Coast.
Rausch
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