Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
145 AM EST Tue Dec 24 2019
Valid 12Z Fri Dec 27 2019 - 12Z Tue Dec 31 2019
...Overview and Guidance/Predictability Assessment...
A mid/upper-level shortwave and the associated frontal system are
forecast to cross the Northeast on Fri (day 3). Model consensus
was good with this system and no major differences were noted. The
most significant forecast issue during the medium range continues
to be the progression/evolution of an upper-level low initially
across the northern Baja Peninsula early Fri, which is then
forecast to move east across the Southwest and toward the central
U.S. by Sat. After several days of strikingly different solutions
among the guidance with respect to the forward progression of the
system, and resultant differences in interaction/phasing with
northern stream energy across the central U.S., some trends have
finally emerged that may shed some light on the eventual outcome.
The ECMWF along with a number of ECENS members have increased the
forward speed of the system, although not to the point of the GFS
yet. The ECMWF (along with the UKMET/CMC to some degree) are still
slow enough with their progression that the system misses its
chance for any significant interaction with the northern stream
wave, and continues to progress more slowly eastward across the
lower Mississippi Valley/Southeast as a cutoff upper low. The GFS,
on the other hand, has remained steadfast in its idea of the
southern stream wave reaching the Southern Plains by Fri night,
where it begins an interaction with a northern stream shortwave,
resulting in the development of a relatively deep low pressure
system across the Plains and Midwest Sat-Sun (days 4-5). This
system bears quite a few similarities to a similar system several
weeks ago for which the GFS significantly outperformed much of the
other guidance in the medium range, the origin of which (for both)
seems to be a shortwave initially over the data sparse North
Pacific, south of the Aleutians. Given these factors, opted to
lean heavily toward the GFS (18Z run) during days 3-5, along with
a gradually increasing component of the ECENS/GEFS ensemble means
as well (both of which provided a significant degree of support to
an idea similar to the GFS). Following this idea, the phased
system should cross the Northeast late Sun/Mon, with the cold
front moving off the Eastern Seaboard by Monday morning.
Models continue to show an additional trough/upper low digging
southward along the West Coast Sun-Mon, with most deterministic
solutions showing another cutoff low near/across southern
California by Mon. Deterministic solutions are quite similar with
the timing and evolution of this feature, although a look at
ensemble members shows that spread is perhaps a bit higher than
would be indicated by deterministic guidance alone. Given some
general consensus among the guidance, a gradual trend toward
heavier use of ECENS/GEFS ensemble means continued through days
6-7 (Mon-Tue).
...Weather Highlights/Threats...
The upper low crossing the Southeast on Fri is expected to
mountain snow to portions of the Southwest and southern Rockies.
Ahead of the upper low, an overrunning setup across the
Southern/Central Plains is expected to result in development of an
area of precipitation, which should expand northward and eastward
across the Plains and eventually the Midwest/Mississippi Valley
through time as the eventual low pressure system spins up. Locally
heavy rain will be possible for some areas, while in the northern
portion of this precipitation shield, sufficient cold air may
exist for precipitation to fall as a wintry mix/and or snow. The
winter weather potential appears to be focused in a band from the
southern Rockies to the central High Plains on Fri, extending into
the Midwest/Upper Great Lakes by Sat. Given at least somewhat
improved confidence in this scenario, boosted winter weather
outlook probabilities across the Midwest/Great Lakes Sat/Sat
night. Please see the medium range winter weather outlook products
for more details on the winter weather threat. As the system
continues east, winter weather is possible across portions of the
Northeast Sun/Sun night in the cool air mass north of the warm
front, with rains expected farther south along the cold front
across the Appalachians, Mid-Atlantic, and Southeast.
Temperatures are forecast to be well above average across the
central and eastern U.S. Fri-Sun ahead of the eventual low
pressure system. High temperatures of 10 to 20 deg F above average
are forecast across a large area. The cold front forecast to
arrive over the weekend will bring an end to the warm temperatures
from west to east, with temperatures returning to near seasonal
norms. Meanwhile, given persistent lower heights across the
interior western U.S., highs are forecast to be 5 to 10 deg below
average from the Great Basin to much of the Southwest Fri-Sun,
with temperatures moderating somewhat by early next week.
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