Prognostic Meteorological Discussion
Issued by NWS

000
FXUS02 KWBC 201846
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
246 PM EDT Wed Oct 20 2021

Valid 12Z Sat Oct 23 2021 - 12Z Wed Oct 27 2021

...Multi-day Heavy Rainfall and Mountain Snow Threat over the
West...
...Heavy Rainfall Threat over the Midwest...


...Overview...

It remains highly likely that an active storm track over the
eastern Pacific in association with an anomalous upper level
trough will result in multiple rounds of impactful weather for the
West Coast and extending well inland across the Intermountain West
and Rockies. Lead shortwave impulses and weather focusing surface
lows/fronts will eject eastward across the Plains and then the
Midwest states and Northeast/Mid-Atlantic, with the main upper
level trough/wavy front reaching the Plains by mid next week.
Elsewhere across the eastern U.S., a surface high should be in
place after the cold front exits the coast on Friday, followed by
an upper ridge axis developing ahead of the central-western U.S.
trough by the end of the forecast period.

...Guidance Evaluation/Preferences...

Despite high likelihood of the impending heavy precipitation event
over the West, it has become increasingly evident that run to run
model continuity has become increasingly problematic with local
focus with the protracted series of dynamic Pacific systems to
slam into the West Coast and subsequently progress downstream over
the central and eastern states. The GEFS/ECMWF ensembles show the
forecast spread, but at least the means are in good agreement and
provide good WPC product continuity. The 12 UTC model suite has
overall trended toward these means, but in particular the
GFS/Canadian.

...Weather/Hazards Highlights...

Across the West Coast region, multiple rounds of enhanced rains
are expected from Washington through especially California that
will likely be heavy at times with moist onshore flow ahead of
multiple Pacific storm systems. This will especially hold true for
Sunday and into Monday as a deep and long fetch moisture plume
shifts southward from the Pacific Northwest across California as
part of an atmospheric river event. Some locations may get on the
order of 4 to 8+ inches of rain over the entire forecast period,
particularly across favored windward terrain which may result in
some instances of runoff issues and/or flooding. Burn scars will
also be especially vulnerable. Heavy snows are likely for the
higher elevations of the Cascades and Sierra, with over a foot of
accumulation possible. Snow levels are expected to lower Sunday
into Monday as the core of the upper trough moves inland. Strong
winds are also possible for the coast and along the mountain
ridges.

Some of this moisture is expected to reach the Intermountain West
and the central/northern Rockies with significant snowfall for
some of these areas, with a cold front reaching the Plains and MS
Valley Tuesday into Wednesday that may present an emerging and
expanding lead rainfall/convective focus. Across the central to
eastern U.S., a less certain but wavy lead front lifting northward
across portions of the Midwest/Mid-Atlantic this weekend and into
next week is expected to produce scattered to numerous showers
with the potential for a few convective complexes that offer
potential for locally heavy downpours/runoff issues, especially
over the Midwest.

In terms of temperature highlights, it will feel more like late
September across much of the central and southern Plains as the
upper level ridge remains in place across this region, with highs
running up to 15 degrees above normal ahead of the cold front.
Cooler than normal temperatures are forecast for the West Coast,
influenced by the upper trough and mainly overcast skies with
highs decreasing to 10-20 degrees below normal.

Hamrick/Schichtel


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

$$