Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
329 PM EST Tue Nov 30 2021
Valid 12Z Fri Dec 03 2021 - 12Z Tue Dec 07 2021
...Overview...
During the Friday-Tuesday period expect an eastern Pacific mean
ridge aloft to support varying degrees of cyclonic flow across
southern Canada and portions of the lower 48, with the overall
mean trough axis tending to extend from central Canada into the
Great Lakes/Northeast. Upper troughing should be fairly shallow
late this week and then become somewhat more amplified after
Saturday. This pattern will carry along a series of surface waves
and frontal systems, most of which should produce light to
moderate precipitation. A front reaching the West early next week
may support a period of heavier activity from the Pacific
Northwest into the Rockies. Meanwhile one or more pieces of
energy over the eastern Pacific/far southwest corner of the U.S.
may consolidate into an upper low that settles near Baja
California. This feature should not produce much significant
weather during the period.
...Guidance/Predictability Assessment...
The most prominent uncertainties over the lower 48 seem to arise
from question marks with a couple streams of flow within a general
trough to the south of Alaska. A leading shortwave reaches the
Pacific Northwest by early Saturday and there are already
amplitude differences at that time with more significant spread
for the corresponding surface pattern off the Pacific Northwest
coast. A preferred intermediate solution would have a moderate
wave not far from the 00Z ECMWF mean position but somewhat weaker
than it or the 00Z ECMWF/12Z CMC. The latest GFS/UKMET are quite
weak and suppressed while the 12Z ECMWF is weaker/south versus the
prior run.
Early differences with this shortwave and then differences in how
streams interact within the trough south of Alaska lead to rapidly
increasing spread for low pressure tracking from the Plains into
eastern Canada/Northeast U.S. by Sunday-Monday. Latest GFS/GEFS
runs and the 12Z UKMET dig the incoming shortwave energy rather
strongly into the east-central U.S., leading to strong low
pressure over the Great Lakes/southeastern Canada by early Monday.
In contrast the 00Z ECMWF and latest CMC runs are more
progressive and less amplified, leading to weaker and farther
east/southeast low pressure. Highlighting the uncertainty,
individual ensemble members are very diverse and GFS/ECMWF runs
have occasionally switched places in recent days. For example a
number of recent ECMWF runs had some similarity to the GFS but the
00Z ECMWF happened to be similar to yesterday's 00Z GFS that was
on the weaker/suppressed side versus most GFS runs. The 12Z ECMWF
trended much stronger and a little northwestward with intermediate
timing. Prefer to maintain a compromise approach that is fairly
close to continuity.
Additional energy should drop into the western half of the country
by next Tuesday but with considerable differences for specifics.
The new 12Z GFS is on the deep side of guidance by early Tuesday.
Also note that 06Z/12Z GFS runs are on the eastern side of the
full guidance spread for the upper low near Baja California by
Sunday.
A blend of the 00Z/06Z GFS runs and the past two ECMWF runs
through the 00Z cycle, along with modest 00Z CMC/UKMET input,
provided a good representation of forecast preferences during the
first half of the period. The forecast transitioned to a mix of
the aforementioned GFS/ECMWF/CMC runs and 06Z GEFS/00Z ECMWF means
thereafter.
...Weather/Hazards Highlights...
A wavy front progressing from the West/Plains through the East
along with another wavy front farther north will bring relatively
light precipitation to the Midwest, Great Lakes, and Northeast
late this week into the weekend. Moisture increasing ahead of the
cold front extending into the southern Plains should produce
scattered showers and a few thunderstorms from the Deep South to
the Ohio Valley over the weekend, but amounts are not likely to be
excessive. A shortwave and frontal system may brush the extreme
northern Pacific Northwest/Rockies with some precipitation during
the weekend. This system could then produce a band of
precipitation from the northern Plains through the Northeast into
early next week. However confidence is low for exact
timing/track/depth of the surface low, so it will take additional
time to resolve precipitation type and amounts across this broad
area. A shortwave and cold front reaching the Pacific Northwest
by early next week should bring an episode of moderate to locally
heavy rain and mountain snows to western Washington and Oregon.
Expect some of this moisture to reach farther inland through the
West, with best potential for heavy snow over the northern
Rockies--including the higher elevations of northern Idaho,
western Montana, and western Wyoming.
Temperatures will be above normal for early December across most
of the nation through early next week. The greatest positive
anomalies should be over the central and southern Plains on Friday
with highs forecast to be 15-25F above average. Another episode
of well above normal temperatures is likely for parts of the
Plains next Monday-Tuesday ahead of the next cold front with a
broad area of highs and morning lows at least 10-20F above normal.
A few locations may approach daily record highs with these mild
temperatures. The Northeast U.S. will be an exception to the
generally warm theme as mean troughing aloft will keep
temperatures fairly cold with highs running up to 10 degrees below
normal through the weekend. However, no major arctic airmass
intrusions are expected during this forecast period.
Rausch/Hamrick
Hazards:
- Heavy precipitation across portions of northwestern California,
the Pacific Northwest including the Cascades, as well as parts of
the northern Rockies, Sun-Mon, Dec 5-Dec 6.
- Flooding occurring or imminent across portions of northwestern
Washington state.
- Flooding likely across portions of northwestern Washington state.
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