Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
133 AM EST Sun Dec 22 2019
Valid 12Z Wed Dec 25 2019 - 12Z Sun Dec 29 2019
...Overview and Guidance/Predictability Assessment...
An amplified and moderately progressive flow pattern is expected
across the CONUS during the medium range forecast period. A
consolidated vortex across the Arctic Ocean and an active Pacific
wave pattern will keep any significant arctic air outbreaks away
from the CONUS, with many locations, especially across the central
and eastern U.S. experiencing unusually mild conditions for late
December.
Models show good consensus early in the forecast period that an
upper-level trough should amplify along or just off the U.S. West
Coast, with a closed upper low developing in the vicinity of
southern California by Wed night/Thu morning. Models continue to
differ on the timing of this feature as it moves eastward across
the Southwest through Fri, with the ECMWF on the slow side of the
spread and the GFS on the fast side, and the ensemble means
largely in the middle. Farther north, an active northern stream
will transport numerous smaller scale impulses across the CONUS
northern tier, with several waves of low pressure traversing a
frontal boundary across the central U.S. Wed-Fri. Overall, model
consensus was sufficient during days 3-5 (Wed-Fri) to base the
forecast on a multi-model deterministic blend, including the 12Z
ECMWF/UKMET/CMC/18Z GFS, with average forecast confidence.
By days 6-7 (Sat-Sun), confidence decreases as model spread
increases on the specifics of how quickly the southwestern system
moves east into the central U.S., and any potential interactions
with northern stream energy/resultant surface cyclones across the
Plains or Midwest. One area where consensus does exist is on the
idea that the flow across the CONUS should become more amplified
through time, with the setup on day 7 likely featuring an
amplified upper trough, the product of some degree of some
northern/southern stream phasing, somewhere across the central or
eastern U.S. The specifics of such a system, however, are elusive.
Despite rather large deterministic spread by next weekend,
ensemble solutions were somewhat agreeable. Thus, the day 6-7
forecast was based heavily on a blend of the 12Z ECENS/NAEFS
ensemble means, with only very modest amounts of deterministic
guidance used.
...Weather Highlights/Threats...
Rain and mountain snow are expected along the West Coast on Wed,
and continuing into Thu across southern California, as the upper
trough moves onshore. The Pacific Northwest will remain wet
through much of the forecast period with persistent rain and
mountain snow as a number of shortwaves and weakening Pacific
frontal systems affect the region. Moisture return ahead of the
southern stream trough will create an overrunning setup north of a
frontal boundary across the Southern/Central Plains by Thu, with
showers expected to develop. Precipitation should expand in
coverage and intensity across the Southern/Central Plains by Fri
as the upper low moves east across the Southwest, intensifying
moisture return into the central U.S. By next weekend, with
potential development of a low pressure system across the central
U.S., models indicate the potential for a band of moderate to
possibly heavy precipitation from the Southern Plains to the Ohio
Valley/Great Lakes. The details are quite murky by that time, for
reasons described above. Nonetheless, there appears at least some
potential for an area of winter weather on the north side of this
system (despite the relative lack of significant cold air). Once
models get a better handle on the specifics of next weekend's
system, a clearer picture will emerge as to what degree of heavy
rain and/or winter weather threat exists.
Temperatures are forecast to be well above average across much of
the central and eastern U.S. The largest temperature anomalies are
forecast from the Central Plains to the Midwest/Ohio Valley
Wed-Thu, where high temperatures may approach or exceed 20 deg F
above average in some areas. A broad area of the central/eastern
U.S. will see highs 5 to 15 deg above average persisting into next
weekend. Meanwhile beneath the relatively slow moving upper-level
trough, much of the Great Basin and Southwest will see high
temperatures 5 to 10 deg below average.
Ryan
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards 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