Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
135 AM EST Sat Dec 21 2019
Valid 12Z Tue Dec 24 2019 - 12Z Sat Dec 28 2019
...Overview and Guidance/Predictability Assessment...
Models/ensembles show a persistent upper-level mean trough
generally across the West Coast and Great Basin through the medium
range. Meanwhile, moderately progressive flow will persist
downstream across the central and eastern U.S. as shortwave energy
ejects from the western trough and encounters broad anticyclonic
flow across the south central and southeastern states. A
persistent upper-level low across the Arctic Ocean well north of
Alaska along with weakly positive height anomalies across Canada
will keep the risk of any significant outbreaks of cold air across
the CONUS to a minimum during the forecast period.
A relatively deep upper-level low and associated low pressure
system are forecast to begin be pulling away from the Southeast on
Tue (day 3). Over the past 24 hours, models have converged a bit
toward the slower side of the spread. With the ECMWF remaining
slower than the GFS, leaned somewhat toward the slower ECMWF
solution. Farther west, model timing/amplitude differences
continued to be evident in the guidance with a shortwave reaching
California early Tue, and then deamplifying across the
central/eastern U.S. Wed-Thu (days 4-5) as a relatively weak
frontal wave moves from the central U.S. to the Ohio Valley.
Models show general agreement that another trough or potentially a
closed upper low should approach the West Coast either late Wed or
early Thu. Fairly significant differences remain, however. The
ECMWF has trended toward a much deeper/closed and slowly
progressing system, while the GFS has remained a more open wave,
albeit trending slower/deeper with each consecutive run, and the
00Z GFS at least hinting at the potential for a closed upper low
around the time the feature nears the coast. Given the trends,
preferred to lean more toward the ECMWF since it seemed to be
leading the way on the current trends. During the day 6-7
(Fri-Sat) time period, additional northern stream shortwave energy
appears likely to move across the CONUS northern tier, with
deterministic solutions showing a range of timing/amplitude for
these features, favoring heavier use of ensembles means.
Based on the described considerations, the WPC forecast was
initially based on a blend of the 12Z ECMWF/18Z GFS during days
3-4, with quite a bit more emphasis placed on the ECMWF. For days
5-7, a blend of the ECMWF along with the the ECENS/GEFS ensemble
means was used.
...Weather Highlights/Threats...
Scattered rain/mountain snow showers are expected across the Four
Corners region Tue as a shortwave passes overhead. Increasing
moisture return across the Southern Plains starting Thu night and
continuing through late in the week may produce areas of rain in
an overrunning-type setup north of a frontal boundary. Some wintry
weather may be possible on the northern fringe of this area of
precipitation, from portions of the Central Plains to the Midwest,
although precipitation amounts in those areas look rather light at
this point in time. Rain and mountain snow are expected to become
much more widespread along the West Coast by Wed-Thu, continuing
through late in the week for the Northwest, as the last of the
described troughs and additional smaller scale northern stream
energy move onshore.
Temperatures are forecast to be 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
Tue-Wed, where high temperatures of 15 to 20 deg F above average
will be common. A broad area of the central/eastern U.S. will see
highs 5 to 15 deg above average persisting through the end of next
week. Meanwhile, with persistent upper-level troughing, much of
the West will see temperatures near or slightly 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