Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
155 AM EST Mon Nov 29 2021
Valid 12Z Thu Dec 2 2021 - 12Z Mon Dec 6 2021
...Overview...
The overall pattern will be featured by an upper-level ridge from
the eastern Pacific to the Intermountain West, and quasi-zonal
flow to low amplitude troughing from the Plains to the East Coast
with multiple shortwave perturbations pivoting around a synoptic
scale trough over southeast Canada. This will include an
organized low pressure system tracking across southeastern Canada
through Friday with a trailing front followed by a weaker low this
weekend, leading to rounds of mainly light precipitation across
the Great Lakes and northeastern U.S. By the weekend, showers and
some storms are likely to develop ahead of a developing storm
system from the Gulf Coast to the Ohio Valley.
...Guidance/Predictability Assessment...
The 00Z guidance suite is in decent synoptic scale agreement
through Friday with an upper ridge over the southwestern U.S. and
a broad trough over the Northeast U.S. As the flow pattern
becomes more quasi-zonal by Friday night and into the weekend,
model spread increases with respect to the next shortwave trough
entering the northwestern U.S., as well as a weak southern stream
upper low developing over northern Mexico, and overall forecast
confidence is below average for this time period owing to
significant run-to-run model variations. Some of the greatest
model differences are with the past few runs of the CMC that have
been out of phase along the West Coast for the end of the forecast
period and qualify as outlier solutions compared to ensemble
spaghetti plots for days 6 and 7. Therefore, the WPC forecast was
primarily based on a GFS/ECMWF/UKMET blend through Friday, and
then mainly ensemble means with some operational GFS/ECMWF and
some previous WPC continuity.
...Weather/Hazards Highlights...
Weather should be relatively quiet in terms of rainfall or snow
related hazards with not much in the way of heavy precipitation
currently expected through Saturday. Rounds of relatively light
precipitation are forecast for the Midwest, Great Lakes, and
Northeast with a progressive storm track over the area. Amounts
should be most substantial in favored lake effect and possibly
upslope areas, particularly downwind of Lake Ontario into the
Adirondacks. Then moisture is set to increase across the Southern
Plains and Lower Mississippi Valley late Friday and into the
weekend, which could lead to showers and thunderstorms becoming
more widespread there and then extending northeastward as the
surface low over the Great Lakes becomes better organized.
Temperatures are expected to be above normal for this time of year
across most of the nation through next weekend, with the greatest
positive anomalies expected for the central and northern Plains
where both highs and lows are forecast to be 15 to 25+ degrees
above average for late in the week. A few locations may approach
daily maximum and minimum temperature records with these mild
temperatures. By the weekend, temperatures are currently forecast
to moderate somewhat, but this is subject to change given forecast
uncertainties. No 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