Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
129 AM EST Thu Dec 26 2019
Valid 12Z Sun Dec 29 2019 - 12Z Thu Jan 02 2020
...Significant winter storm expected to impact portions of the
Upper Midwest and Upper Great Lakes this weekend...
...Overview and Guidance/Predictability Assessment...
A relatively deep upper-level and surface low are forecast to be
in place across the Midwest Sunday morning (day 3), with a
potential winter weather event for portions of the Upper
Midwest/Great Lakes expected to be ongoing. Model consensus for
this system has improved substantially over the past couple days,
and all deterministic solutions show the deep surface low moving
relatively slowly into the upper Great Lakes by Mon morning and
into Ontario by Tue morning. This is a trend toward a slower
progression of this feature shown by some of the ECMWF yesterday.
Farther east, a triple point low is expected to develop somewhere
across the Northeast on Mon, although models differ on the exact
timing of this feature, but agree that it should remain relatively
week and progressive. Overall the GFS has outperformed the ECMWF
during the medium range with this system, however, so was
reluctant to lean too heavily on the ECMWF and opted to go with a
GFS/ECMWF blend.
Farther west, a similar scenario as seen in at least a couple
instances recently looks to occur again, with nearly all guidance
forecast 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
shows the system continuing a slow drift southeastward through Tue
before eventually coming under the influence of an amplifying
trough farther north, and moving east more quickly. The ECMWF
continues to be a bit less amplified initially, and slower to
close the feature off than the majority of other guidance (and is
even a bit less amplified with the wave on Mon than the ECENS
ensemble mean - a bit of a red flag for the deterministic
solution). 12Z/18Z guidance showed some consensus that a surface
low could develop across the Southern Plains by Wed night/Thu as
the merged upper trough moves east. The 00Z GFS made a significant
change by days 6-7 (Wed-Thu), and is now much slower to move the
trough/upper low eastward across the Southwest/northern Mexico and
merge it with the northern stream trough, with would keep a phased
southern stream low from developing in the manner shown by
previous guidance. Overall, models are fairly inconsistent by that
time frame, so a more ensemble based blend seemed in order.
The WPC forecast was initially based on a blend of the 18Z GFS/12Z
ECMWF during days 3-5, with a shift to majority ECENS/GEFS
ensemble mean weighting during days 6-7.
...Weather Highlights/Threats...
As low pressure develops and begins to deepen moves northeastward
across the Midwest/Great Lakes Sun-Mon, an area of precipitation
is expected to accompany the system. Potentially significant
accumulating snows are expected Sun-Sun night across portions of
the Upper Midwest/Great Lakes. Accumulating winter weather will
also be possible for northern New England Sun/Sun night, likely
continuing into part of Mon before changing to rain. 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 across the Southeast, Appalachians, and
Mid-Atlantic regions through Sun night before the cold front moves
offshore on Mon. 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, potentially spreading farther inland into portions of
the northern Rockies by midweek. As the next cutoff southern
stream system begins to interact with a northern stream trough and
approaches the central U.S. by next Tue-Wed, ample moisture from
the Gulf of Mexico is forecast to be pulled northward into the
system, with the potential for widespread showers and
thunderstorms across portions of the western/central Gulf Coast
and lower Mississippi Valley, with the potential for areas of
heavy rain. Confidence in this aspect of the forecast is a bit
below average at this time, however, given model variability with
the forward speed of the cutoff upper low.
Temperatures are forecast to be well above average across the
central and eastern U.S. on Sun ahead of the low pressure system.
High temperatures of 15 to 25 deg F above average are forecast
across portions of the Ohio Valley, central Appalachians, and
Great Lakes, with a much of the eastern third of the country at
least 10 deg F above average ahead of the cold front. Low
temperatures across these area on Sunday are forecast to be 20 to
35 deg F above average, with a number of record high minimum
temperatures likely. These warm conditions should moderate
somewhat by Mon as they shift to the Mid-Atlantic/Northeast. 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
and portions of the Great Basin and central/southern Rockies
through much of the forecast period.
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