Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Sat Dec 18 2021
Valid 12Z Tue Dec 21 2021 - 12Z Sat Dec 25 2021
...Pattern Overview and Guidance/Predictability Assessment...
From Tuesday into Christmas Day/Saturday, most guidance continues
to show that impulses rounding an impressively strong central
Pacific upper ridge should provide periodic reinforcement of a
deep upper trough aligned just off the West Coast. This trough
will likely contain one or more upper lows, and a persistent flow
of moisture ahead of the trough axis will likely produce a
multi-day period of significant precipitation over parts of the
West Coast (northern/central California in particular) with
meaningful amounts also extending across the West into the
Rockies. Southern Canada/northern tier U.S. flow will carry along
a couple waves with modest precipitation before lower 48 flow
becomes more dominated by what energy may eject from the eastern
Pacific trough. Farther south, a southern stream system tracking
from northern Florida or vicinity into the Atlantic may bring some
rain to the Southeast early in the forecast period. Expect a
broadening area of above normal temperatures over the southern
two-thirds of the central U.S. while the extreme northern Plains
will tend to see below normal temperatures.
For the eastern Pacific upper trough the 12Z/18Z models and means
have improved their relative clustering thanks to the 12Z ECMWF
finally adjusting to a more compatible solution versus the
previous couple runs that were questionably progressive with Gulf
of Alaska energy that digs southward Tuesday-Wednesday. The
current consensus for the mean trough aloft, as well as the best
precipitation focus over northern/central California, compare well
to teleconnections relative to the upstream ridge's positive
height anomaly center. However for specific day-to-day details
there is still a fair amount of ensemble spread and complexity
over interaction among features and their exact position. This
includes not only the initial offshore upper low and the
aforementioned Gulf of Alaska energy but yet another strong bundle
of energy that may drop into the mean trough late in the week. In
the new 00Z model runs the CMC has strayed significantly for some
details but the GFS/UKMET remain fairly close to consensus. The
ECMWF has gone back to a faster scenario.
The progressive nature of flow to the east of the Rockies may
continue to cause some trouble for resolving specifics of
individual waves/fronts. Recently there has been improved
definition of a northern Plains wave that tracks across the upper
Great Lakes and southeastern Canada Tuesday onward but
continuity/clustering have been lacking for trailing features
whose dynamic support are increasingly likely to come from energy
that ejects from the eastern Pacific upper trough. Meanwhile
agreement and to some degree continuity continue to be poor for
the precise structure, timing, and track of the southern stream
system expected to be over or near northern Florida as of early
Tuesday and continue into the Atlantic thereafter. New 00Z model
clustering has generally gravitated to the slower side of the
prior envelope, closest to recent GFS runs.
A blend of 12Z/18Z models from Tuesday into early Wednesday
eliminated the UKMET after the latter time due to straying fast
with the western Atlantic system and gradually trended toward a
late-period solution of 40 percent total 18Z GEFS/12Z ECMWF means
with remaining input coming from the 18Z GFS and 12Z ECMWF/CMC.
...Weather/Hazards Highlights...
Expect the western U.S. to see a broadening area of rain and
mountain snow downstream from the persistent upper trough just
offshore the West Coast, with some of this activity likely to be
moderate to heavy. There are still question marks for some of the
details due to uncertainty over individual systems and their
interaction, but guidance signals and teleconnections for the mean
pattern are consistent in showing northern and central California
as having the highest precipitation potential through the
period--including heavy snow over Sierra Nevada/Shasta areas.
This region should see the most consistent moisture flow and
experience southern/western facing terrain enhancement. It may
take into the shorter-range time frame to asses the location and
intensity of lower elevation rainfall with respect to local
runoff/flooding issues. Effects from the multi-day accumulation
of precipitation will also require monitoring even if it is not
intense on any particular day. Some moisture may extend as far
south as southern California and Arizona by mid-late week, while
the Interior West and Rockies should see meaningful precipitation
as well.
Low pressure tracking from over or near northern Florida into the
Atlantic will spread rain across portions of the Southeast on
Tuesday and possibly into Wednesday. Some locations over the
Southeast/Florida could see moderate to locally heavy rainfall.
Some uncertainty persists for the intensity and northward extent
of rainfall though latest trends are increasing the potential for
meaningful totals over parts of the extreme Southeast. A leading
northern tier system should spread mostly light snow from the
extreme northern Plains eastward through the Great Lakes and New
England during Tuesday-Wednesday. The system's accompanying front
should be relatively dry. Any trailing features could produce
some precipitation but again likely with fairly light amounts.
Above normal temperatures focused over the central/south-central
High Plains on Tuesday will steadily expand eastward across the
southern two-thirds of the central U.S. with time and gradually
become more anomalous. Through midweek highs in this region will
tend to be 10-20F or slightly more above normal. Toward the end
of the week there should be more locations with highs 20-30F above
normal and some record highs could be challenged over the southern
Plains by Christmas Day/Saturday. On the other extreme, the far
northern Plains should see below normal temperatures except for
perhaps being near normal Wednesday. A stronger push of cold air
may reach Montana and the Dakotas next weekend. The unsettled
pattern over the West should lead to near or above normal lows and
near to below normal highs.
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