Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
241 PM EST Tue Dec 14 2021
Valid 12Z Fri Dec 17 2021 - 12Z Tue Dec 21 2021
...Guidance/Predictability Assessment...
The latest guidance and the ensemble means are in fairly good
agreement initially and through most of the extended period, thus
leading to an above average confidence with this forecast. The WPC
medium range product suite was primarily comprised of a blend of
the 12Z CMC/ECWMF/GFS/UKMET and the NAEFS, GEFS and EC ensemble
means. The inclusion of the means increased during the middle and
later periods. The ensemble support was also strong and WPC
product continuity was well maintained. These models solutions
diverge more significantly early next week for Days 6/7 with the
timing/amplitude of embedded systems, but most notably with the
multi-stream flow over the East/Western Atlantic where the GFS
develops a major coastal low while the ECMWF suppresses
development.
...Weather/Hazards Highlights...
A strong surface low pressure system will be lifting north through
eastern Canada/Canadian Maritimes Thursday and Friday, with its
associated trailing frontal boundary draped across the
Mid-Atlantic to the Desert Southwest. Showers and thunderstorms
along this slow moving boundary is expected to be moderate to
heavy at times, which may lead to excessive rainfall and localized
flooding concerns from the Southern Plains through the
Mid-South/Ohio Valley. Well to the south, upper ridging should
support some record warm temperatures ahead of the attendant cold
front, with positive anomalies upwards to 15-25F shifting
southward over the East to the South/Southeast into Friday and
Saturday.
A developing frontal low will bring a swath of wintry
precipitation from the Great Lake states through northern New
England into the weekend as the trailing enhanced rainfall axis
drops southward across the Gulf Coast and Southeast as cooling
high pressure builds across the Plains and to the East. Some of
the guidance is suggesting that by early next week this feature
may spin up and track up the East Coast; potentially yielding
maritime threats, spreading rain near the Mid-Atlantic and
possibly snow for the Northeast.
Meanwhile, moisture surging onshore the West Coast is causing
moderate precipitation across the Northwest and into California
ahead of an approaching Pacific system; which is expected to
gradually weaken into Friday. Moisture from another Pacific system
may then impact the Pacific Northwest with moderate to possibly
locally heavy precipitation amounts into the weekend, with
additional energies digging southward over the eastern Pacific
early next week that would eventually affect California.
Campbell
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy precipitation across portions of the Pacific Northwest and
the Northern Great Basin, Sat,
Dec 18.
- Heavy rain across portions of the Lower Mississippi Valley, the
Central Appalachians, the
Tennessee Valley, the Middle Mississippi Valley, the Southern
Plains, and the Ohio Valley, Fri-Sat,
Dec 17-Dec 18.
- Heavy rain across portions of the Southern Plains, Sat-Sun, Dec
18-Dec 19.
- Heavy snow across portions of the Northern Rockies and the
Northern Great Basin, Sat-Sun, Dec
18-Dec 19.
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