Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1232 PM EST Sun Dec 08 2019

Valid 12Z Wed Dec 11 2019 - 12Z Sun Dec 15 2019

...Much below normal temperatures linger for the Upper
Midwest/Great Lakes Midweek...
...Emerging wet flow over the Northwest...
...Cold air damming lends a wintry risk over the East with
potential low developments... 


18 UTC Update...

Model consensus was relatively good through at least the first
half of the forecast period, and a majority 00Z ECMWF/06Z
GFS-based blend was used during days 3-4 (Wed-Thu). Shortwave
energy crossing the southern tier Thu-Fri along with additional
northern stream shortwave energy moving into the north central
U.S. at the same time will be prominent features in the latter
half of the forecast period, particularly with respect to the
eventual development and track of a low pressure system across the
eastern U.S. As is often the case, the degree of
phasing/interaction between these two initially separate features
will be a determining factor in the eventual evolution. The last
couple runs of the GFS have phased the shortwaves relatively
quickly, resulting in a Gulf of Mexico surface low that tracks
much farther west (across the Appalachians or Ohio Valley)
relative to the less phased ECMWF/CMC/UKMET, which track much
closer to the Eastern Seaboard. At this time, the latter idea is
the preferred one, with the 00Z GFS being the most recent run that
looked more like this. Thus, the forecast during days 5-7
(Fri-Sun) was comprised of gradually increasing weight on ensemble
means (ECENS and GEFS), with some continued use of the 00Z
ECMWF/00Z GFS. Overall, no major changes were made to the ongoing
forecast.

Ryan


Previous Discussion (issued at 0514 UTC)...

...Pattern Overview and Guidance/Predictability Assessment...

A cold Hudson Bay upper vortex lingers next week. A notable upper
trough rounding underneath shifts over the Midwest/Great Lakes
midweek then ejects over the Northwest, with less amplified but
still active troughing/cyclonic flow for much of the rest of week
over the lower 48. This occurs as an early week North American
West Coast ridge is rounded by eastern Pacific shortwaves that cut
inland from the Northwest to the north-central Great
Basin/Rockies. This is also as an active southern stream provides
multiple trough energy passages underneath over the U.S. southern
tier to further focus inflow/activity, culminating with potential
QPF/wintry enhancement later week from the Southeast to up the
Eastern Seaboard. 

The WPC medium range product suite was primarily derived from a
composite blend of the GFS/ECMWF and GEFS/ECMWF ensemble means,
lending increasing weighting in this blend from the deterministic
models to ensembles through the period amid growing forecast
spread over time.


...Weather Highlights/Threats...

Great Lakes effect snows will persist midweek. Temperature
anomalies of 15 to 30 degrees below normal will be over the Upper
Midwest and Upper Lakes with reinforcing Arctic shots.

Out West, the pattern should become quite wet across the Pacific
Northwest by mid-later this week as Pacific shortwave energies
punch through rounding West Coast flow. Dynamic support for
activity will extend inland with impulse progressions, with
moderate to heavier snows to continue on favored terrain of the
north central Great Basin/Rockies into next weekend.

Models continue to show support for the development of a
significant waves of low pressure along a surface front in the
Gulf of Mexico by late next week, tracking northeastward across
the Southeast U.S. and up the Eastern Seaboard into next weekend.
Spread is considerable with the timing and the eventual tracks of
these features, but the pattern seems to favor organized
cyclogenesis to focus moisture and lift to produce a risk of heavy
rainfall across the Southeast. Subsequent system deepening,
northward lifting, and interaction with a slow to recede lower
atmospheric cold air damming over the East may offer an emerging
snow/ice threat to monitor.

Schichtel

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