Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Sat Nov 13 2021
Valid 12Z Tue Nov 16 2021 - 12Z Sat Nov 20 2021
...Overview...
Latest guidance continues to show the upper pattern settling into
broadly cyclonic mean flow, with a hint of a strengthening eastern
Pacific ridge by next weekend. The relatively low amplitude and
progressive nature of this regime should decrease predictability
for forecast details behind a vigorous shortwave that will move
eastward from western North America Tuesday onward and support a
strong southern Canada surface low and trailing cold front
crossing most of the lower 48. This system will produce a period
of strong/gusty winds over northern tier areas of the U.S. and
then some rain over the east-central U.S. Well above normal
temperatures over the Plains and southern half of the West at the
start of the period on Tuesday will moderate and move eastward
ahead of the advancing cold front. Somewhat below normal
temperatures behind the front will spread into the central/eastern
U.S. mid-late week. During the latter half of the week the
precipitation and surface pattern details farther west become
increasingly uncertain.
...Guidance/Predictability Assessment...
Models have continued to show some difficulty in resolving the
details of the strong upper trough (likely with an embedded closed
low tracking just north of the U.S.-Canadian border around
Wednesday) and thus with the forecast of the cold front crossing
the lower 48 and anchoring low pressure in Canada. 12Z/18Z GFS
runs had been on the slower/sharper side of the envelope with the
upper trough and thus slow with the cold front. The new 00Z GFS
made a considerable adjustment toward the 12Z ECMWF--as did the
00Z UKMET from a previously more suppressed surface evolution.
Meanwhile latest CMC runs and the 12Z CMC mean are on the fast
side. Finally, latest 00Z runs thus far appear to confirm 12Z
guidance favoring a moderately flat/progressive handling of the
upper trough once it reaches the East, in contrast to the
slow/amplified depiction in yesterday's 00Z ECMWF. An initial
blend of 12Z/18Z guidance available at the time of forecast
preparation ultimately led to an evolution closest to the 12Z
ECMWF.
Upstream shortwave details still appear to be a ways from being
resolved. There is a leading feature around 145-150W longitude as
of early Tuesday and even latest 00Z runs range between a diffuse
shearing out (most models in some fashion) and a stronger one
(CMC), leading to dramatic differences in western U.S.
precipitation coverage/intensity. Whether left over from the
initial shortwave or originating farther upstream, additional
Pacific energy comes into the West while northern stream flow from
the Northeast Pacific and vicinity may also reach Canada and the
northern lower 48 late in the period. Thus far the GFS/CMC runs
have been leaning toward the progressive side for the general area
of shortwave energy expected to reach the West by Thursday and
continue along thereafter while the ECMWF/ECMWF mean have been
slower. Latest GEFS means have been moderately progressive too but
with less of a surface reflection. The past 12 hours of GFS runs
have been trending less amplified with the overall trough, thus
leading to a more suppressed surface system over/near the
Southeast by late in the period--a nod to the ensemble means that
have generally tended to keep fairly strong high pressure over
much of the East. The progressive nature of the pattern could
suggest the 12Z ECMWF/ECMWF mean (with a wave back over the Plains
early Saturday) could be somewhat slow but at least they have been
more consistent thus far. The latter half of the manual forecast
trended toward a majority weight of the 18Z GEFS/12Z ECMWF means
due to the uncertainty, with the rest composed of a little more
12Z ECMWF weight versus the 18Z GFS.
...Weather/Hazards Highlights...
Expect mostly light to moderate rain and higher elevation snow
over the northern Rockies through Tuesday in association with the
upper trough/leading cold front pushing through the region. The
potent surface low tracking across southern Canada Tuesday onward
will bring a period of strong/gusty winds to northern tier areas,
especially the Northern Plains on Tuesday into Wednesday. Moisture
should increase ahead of the low's trailing cold front from
midweek onward, leading to mostly light/moderate rain over
portions of the Mississippi, Ohio, and Tennessee Valleys along
with some light precipitation in the Great Lakes region. The
Florida Peninsula may see increasing rainfall for a time mid-late
week as moisture interacts with a stalled front to the south, with
fairly low confidence in coverage/intensity details at this time.
Likewise confidence is below average for any precipitation that
could reach the West around mid-late week, with the full range of
possibilities ranging between only light/scattered activity and an
area of heavier/more organized precipitation. The significant
uncertainty extends into the central and eastern U.S. by
Friday-Saturday with minimal confidence in any specific solution
for precipitation coverage/intensity.
Well above normal temperatures will continue into Tuesday over the
Plains and southern half of the West. The warmest readings versus
normal should be over the Plains where plus 15-25F anomalies will
be common and some daily records for highs/warm lows will be
possible. Currently the best potential for highs to approach/reach
record values is over the western half of Texas. As the cold front
emerging from the Northeast continues onward, warmth will extend
into the South and East Wednesday-Thursday but in modified form
with highs/lows generally only up to 10-15F above normal. Cooler
air behind the front will spread mostly 5-10F below normal
temperatures across the central and eastern U.S. from Wednesday
through the rest of the week.
Rausch/Tate
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