Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
310 AM EDT Fri Oct 29 2021
Valid 12Z Mon Nov 01 2021 - 12Z Fri Nov 05 2021
...Overview...
A fairly stagnant upper-level pattern will be in place for much of
the medium range period, with a strong mean ridge over western
Canada, a more modest ridge over the western U.S., and a mean
trough extending from the eastern half of Canada into the central
and eastern U.S., before ridging shifts slowly eastward into the
Plains Thursday and Friday. At the surface, a chilly high pressure
system behind a cold front should cause below normal temperatures
across the central U.S., while the initially slow-moving cold
front enhances rainfall in the Southern Plains and Lower
Mississippi Valley. Rounds of precipitation are also possible in
the West as upper-level shortwaves and frontal systems approach.
...Guidance/Predictability Assessment...
Model guidance from the 12/18Z cycle is in reasonably good
agreement at the beginning of the period with the overall pattern.
Some minor differences in track and timing remain for shortwaves
coming into the Northwest as well as pivoting within the
central/eastern U.S. low amplitude trough, but do not appear to
cause notable differences in the pattern until around Wednesday
onward. At that point, shortwave energy within the eastern trough
appears to pinch off the most in the ECMWF compared to other
guidance, and then model differences arise with upstream energy in
the west side of the trough. By Friday, the 12Z ECMWF has a closed
low across the Ohio Valley separate from the Canadian trough,
which is within the realm of possibilities dependent on the
evolution, but at this point is the only deterministic model to do
so, and the new 00Z run shows a different structure. Additionally,
shortwave energy in a mean trough across the eastern Pacific
varies quite a bit with position and timing from midweek onward.
The ECMWF brings troughing onshore most quickly by early Thursday,
while GFS runs are slow and strong with the energy and
aggressively create a closed low across California by Friday. The
12Z CMC was in between these solutions. Despite the variations in
evolution, at this point there were not clear outliers, and
blending the different solutions together ended up creating a good
compromise forecast. The WPC forecast utilized a deterministic
model blend of the 12Z and 18Z GFS and the 12Z ECMWF/UKMET/CMC
early in the period, adding the GEFS and EC ensemble means into
the blend by day 6 and 7.
...Weather/Hazards Highlights...
The frontal systems and shortwaves coming into the West will cause
periods of enhanced precipitation--first on Monday and then
Wednesday into Friday. Northern California and far southwestern
Oregon should see the heaviest precipitation, with some spreading
into the Pacific Northwest (especially during the latter period)
and lesser totals eastward into the Intermountain West, with
normal terrain enhancements and higher elevation snow. Some snow
may extend into the central High Plains on Monday into Tuesday,
but amounts remain uncertain. Then as moisture flows into the
vicinity of a frontal system across the south-central U.S.,
rainfall chances will increase on Tuesday into Wednesday for the
Southern Plains into the Lower Mississippi Valley. Some
precipitation could spread east and northward for the latter part
of the week, with the higher elevations of the Central
Appalachians and the interior Northeast seeing some possibility
for snow, but confidence in specifics is low.
Behind the cold front and underneath high pressure, well below
normal temperatures are expected across the central and eastern
U.S., especially in terms of highs. Highs of 10-20F below average
are expected across central portions of the Plains into the Middle
Mississippi Valley and Ohio Valley. Chilly lows in the 20s as far
south as Kansas and Iowa are forecast, which is 5-10F below
average, and some areas may experience their first freeze around
next Tuesday or so. Above normal low temperatures are expected
under ridging in the West, though highs should be right around
average.
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