Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
157 AM EST Wed Dec 25 2019
Valid 12Z Sat Dec 28 2019 - 12Z Wed Jan 01 2020
...Significant winter weather event possible for portions of the
Midwest and Upper Great Lakes Saturday and Sunday...
...Overview and Guidance/Predictability Assessment...
Models have rapidly come into much better consensus over the past
24-36 hours on the evolution/progression of an upper low which is
forecast to have already begun the process of phasing with a
northern stream shortwave across the central Rockies/High Plains
on Sat (day 3). At least during the Sat-Sun time period the
majority of guidance has now come around to the idea which the GFS
has shown for multiple days of a phased and highly amplified
trough/upper low with a relatively deep surface low expected to
move from the Central Plains Sat morning to the Upper
Midwest/Great Lakes Sun morning. Models begin to diverge during
the day on Sun, with the ECMWF again making an attempt to slow the
system down substantially with a forecast upper low lingering
across the Great Lakes into the middle of next week, while the GFS
keeps things much more progressive. The 12Z UKMET was somewhat of
a middle ground, although a closer to the more progressive GFS
idea. Given the superior performance of the GFS in recent days in
terms of forecast consistency, opted to once again lean toward the
GFS in this forecast cycle. In this solution, a triple point
surface low should develop across the Northeast by Sun night/Mon,
with the cold front quickly sweeping off the Eastern Seaboard by
that time.
Farther west, a similar scenario as seen in at least a couple
instances recently looks to occur once again, with nearly all
guidance forecasting a shortwave to separate from an active North
Pacific jet and dive southward along the U.S. West Coast as it
amplifies, with development of a closed upper low expected on Mon.
Most guidance show the system continuing a slow drift
southeastward through Tue before eventually falling under the
influence of an amplifying trough farther north and moving east
more quickly by the end of the forecast period/middle of next
week. The ECMWF was a bit less amplified initially and slower to
close the feature off, although it has been rather inconsistent
from run-to-run. The GFS has again shown a higher degree of
consistency with this system, and a blend of the GFS along with
the ensemble means was the preferred solution by late in the
forecast period.
The WPC forecast during days 3-5 was based heavily on the 18Z
GFS/12Z UKMET solutions along with some influence from the
GEFS/ECENS ensemble means. Starting on day 6 (Tue), increased
weight was place on the GEFS/ECENS ensemble means, along with some
continued use of the GFS through day 7 (Wed).
...Weather Highlights/Threats...
As low pressure develops and begins to deepen across the Central
Plains on Sat and moves northeastward into Sun, an area of
precipitation is expected to develop and expand northward and
eastward across the Plains and eventually the Midwest/Mississippi
Valley. Sufficient cold air may exist for precipitation to fall as
a wintry mix/and or snow in the northern portion of the
precipitation area. The greatest potential for significant winter
weather Sat/Sat night appears to be in a band across portions of
the Upper Midwest and Upper Great Lakes. Given improved confidence
in this scenario, boosted winter weather outlook probabilities for
these areas. By Sun/Sun night, some potential winter weather may
continue for portions of the Upper Midwest/Great Lakes on the west
side of the low pressure system, while accumulating winter weather
will also be possible for portions of the Northeast and especially
northern New England. Please see the medium range winter weather
outlook products for more details on the winter weather threat.
Farther south along the cold front, rain is expected from the
lower Mississippi Valley to the Appalachians and Mid-Atlantic
regions Sat through Sun night before the cold front quickly moves
offshore by Mon morning. Across the Pacific Northwest, as the
shortwave moves onshore early next week, rain and mountain snow
are forecast to develop across the coastal ranges and Cascades in
Oregon and Washington.
Temperatures are forecast to be well above average across the
central and eastern U.S. Sat-Sun ahead of the low pressure system.
High temperatures of 10 to 20 deg F above average are forecast
across a large area of the central/eastern U.S., with highs
forecast to exceed 20 deg above average across portions of the
Mid-Mississippi and Ohio Valleys as well as the lower Great Lakes.
The cold front forecast to arrive by Sun-Mon 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 over the weekend, 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