Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
150 AM EST Thu Dec 2 2021
Valid 12Z Sun Dec 5 2021 - 12Z Thu Dec 9 2021
...Overview...
The overall upper level pattern is forecast to transition from
quasi-zonal to more amplified, with the western U.S. upper ridge
becoming established farther west over the eastern Pacific and
multiple troughs passing over the Plains and Midwest. There will
likely be two organized storm systems to contend with through the
middle of next week, with the first tracking from the Midwest to
the Northeast through late Monday, and the second one tracking
from the central Rockies to the Ohio Valley and Mid-South by
midweek. An Alberta clipper-type system may affect the Northern
Plains by next Thursday.
...Guidance/Predictability Assessment...
There has been an overall improvement in the consolidation of the
00Z model guidance compared to recent days as the overall pattern
becomes more amplified, with forecast confidence now average
compared to below average in recent days. With the first storm
system crossing the Great Lakes/Northeast through Monday, the GFS
was faster and farther north than the model consensus, including
its own ensemble mean, so it was not favored through early
Tuesday.
The character of the next shortwave nearing the Northwest early
next week will depend on how much if any interaction there may be
between northern stream energy just south of Alaska and a separate
upper low farther south, with no clear consensus evident yet for
exactly what will happen. This leads to noteworthy model
differences, with the GFS initially quite strong with the
shortwave and faster, and the CMC is considerably slower and
farther west, and a good deal of spread among individual ensemble
members. The CMC is also farther east than the consensus with the
upper low near northwest Mexico, but this does not appear to have
a significant impact on sensible weather at this time.
The first half of the forecast incorporated the ECMWF/CMC/UKMET
and some of the ECENS, with the GFS excluded due to the reasons
previously mentioned. However, the GFS was slightly more in line
with the overall pattern by late in the period, so it was
substituted for the CMC along with more of the ensemble means.
...Weather/Hazards Highlights...
There is the potential for a band of light to moderate snow from
northern Michigan to northern New England through Monday as an
organized storm system passes through the region. Precipitation
type and intensity over the eastern Great lakes and New England
will depend on the exact low track. Moisture increasing ahead of
the cold front is expected to result in scattered showers and a
few thunderstorms from the Deep South to the Ohio Valley through
Monday morning. There could be some locally moderate to heavy
rain embedded within this area but with fairly low confidence
regarding exact placement.
The shortwave and cold front forecast to reach the Pacific
Northwest by Sunday night and through Monday should produce a
period of moderate to locally heavy rain and mountain snows over
western Washington and Oregon, with some of this moisture reaching
the northern Rockies by Tuesday. Heavy snow will be possible for
the higher elevations of northern Idaho, western Montana, and
western Wyoming. In addition, some moisture will likely extend
farther south over favored terrain in the Great Basin and central
Rockies. By the Wednesday-Thursday time period, more rainfall is
likely for the east-central U.S. ahead of this same storm system,
and wintry precipitation farther north across the Great Lakes and
New England.
Temperatures are expected to be near to above normal for early
December across most of the nation through the early to middle of
next week, with the greatest positive anomalies expected for
western portions of Texas and Oklahoma on Tuesday ahead of the
next cold front. On the other end of the spectrum, cold weather
is forecast to continue across the Northeast U.S. with highs
running up to 10 degrees below normal through the weekend with a
cold upper level trough overhead, and another round of below
average readings on Tuesday along the East Coast which should
moderate by Wednesday. However, no major arctic airmass
intrusions are expected during this forecast period.
Hamrick
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
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