Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 PM EST Wed Dec 25 2019
Valid 12Z Sat Dec 28 2019 - 12Z Wed Jan 01 2020
...Confidence increasing for a significant winter storm to impact
portions of the upper Midwest into the upper Great Lakes Saturday
and Sunday...
...Overview and Guidance/Predictability Assessment...
Since yesterday, the ECMWF has maintained the abrupt switch of
supporting rapid cyclogenesis across the central U.S. this
Saturday (Day 3) in general agreement with the scenario that the
GFS has advertised for multiple days. The Canadian and UKMet
models also support this scenario although they indicate a less
robust and slower-moving system than the ECMWF and the GFS.
Thereafter, models begin to diverge on the subsequent track and
evolution of the cyclone. The ECMWF again makes an attempt to
slow the system down substantially with the associated upper low
lingering across the Great Lakes into the middle of next week,
while the GFS keeps things much more progressive. The UKMET is
slower but more in line with the more progressive GFS idea. The
GFS does show a small trend of slowing down the system in recent
runs. Regarding the forward speed of the trailing cold front, the
differences between the ECMWF and GFS are not as great as with the
positions of the low center itself. Given the superior
performance of the GFS in recent days in terms of forecast
consistency, opted to once again lean toward the GFS for this
forecast cycle. This yields a triple point surface low developing
near the Mid-Atlantic coast and then moving rapidly up the New
England coast Sun night/Mon, with the cold front sweeping off the
East Coast at a slightly slower speed than previous forecasts.
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 then develop into a closed low as it dives
southward along the U.S. West Coast. 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 trending less
amplified and more progressive in the past couple of runs but the
latest 12Z run has resumed a slower and more closed-off solution.
The GFS has again shown a higher degree of consistency with this
system although the 12Z run has switched to a more progressive
solution.
The WPC morning grids for the medium-range was based heavily on a
70% blend of 06Z GFS with 30% of the 00Z ECMWF with increasing
weights toward their ensemble means for the longer range time
frame. In addition, more EC mean was used for Day 3-5 than normal
to handle the uncertainty.
...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 into the upper Great Lakes. Given improved
confidence in this scenario, boosted winter weather outlook
probabilities once again 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 initially
for northern New England 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 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.
Kong/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
Hazards:
- Heavy precipitation across portions of the Central Plains, the
Middle Mississippi Valley, the Great Lakes, the Upper Mississippi
Valley, and the Northern Plains, Sat-Sun, Dec 28-Dec 29.
- Heavy precipitation across portions of the Northeast and the
Great Lakes, Sun-Mon, Dec 29-Dec 30.
- Heavy rain across portions of the Central Plains, the
Mid-Atlantic, the Lower Mississippi Valley, the Central
Appalachians, the Tennessee Valley, the Great Lakes,
the Middle Mississippi Valley, the Northeast, the Southern
Appalachians, the Upper Mississippi Valley, the Southeast,
the Southern Plains, and the Ohio Valley, Sat-Mon, Dec 28-Dec 30.
- Heavy rain across portions of the Southeast, the Southern
Appalachians, the Lower Mississippi Valley, the Southern Plains,
and the Tennessee Valley, Wed, Jan 1.
- Flooding possible across portions of the Southeast.
- Flooding occurring or imminent across portions of the Northern
Plains and the Tennessee Valley.
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Sat-Tue, Dec
28-Dec 31.
- High winds across portions of mainland Alaska, Mon-Tue, Dec
30-Dec 31.
- Much below normal temperatures across portions of mainland
Alaska, Sat-Sun, Dec 28-Dec 29.
- High significant wave heights for coastal portions of the Alaska
Panhandle and mainland Alaska, Tue-Wed, Dec 31-Jan 1.
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