Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 AM EDT Wed Oct 20 2021
Valid 12Z Sat Oct 23 2021 - 12Z Wed Oct 27 2021
...Multi-day heavy rainfall and mountain snow threat inland from
the Pacific Northwest and especially California...
...Overview...
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. Shortwave impulses are expected to
to eject eastward across the Plains and then the Midwest states,
with the main upper level trough reaching the Plains by the middle
of next week. 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...
The models and ensembles are in decent synoptic scale agreement on
Saturday with the deep trough over the West Coast region and the
closed low over southeast Canada. Changes become apparent by
Sunday with a lead shortwave ejecting eastward from the broader
scale trough to the west. The 12Z CMC was an outlier solution and
out of phase with both the timing and especially the magnitude of
the system crossing the Plains and Midwest by Monday, so none of
the 12Z CMC was used for this forecast period. The new 00Z CMC
was not as extreme with this feature, although still on the strong
side of the guidance across the eastern U.S. by Tuesday. For the
systems reaching the West Coast, there remains some model
disagreement regarding timing and placement of the surface low
centers, with the GFS indicating a more southern track towards
northern California. By next Wednesday, the 12Z ECMWF is faster
with the timing of the main trough axis across the Plains, and the
CMC/GFS favoring a slower solution with potential closed upper low
development. The WPC fronts/pressures forecast was primarily
derived from a GFS/ECMWF/UKMET blend through this weekend, and
then GFS/ECMWF and their respective ensemble means for Monday
through Wednesday.
...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. Heavy snows are
likely for the higher elevations of the Cascades and Sierra, with
over a foot of accumulation possible above pass level. Snow levels
are expected to be lower by late Sunday and 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 by
Tuesday. Across the eastern U.S., a lead warm front lifting
northward across portions of the Midwest this weekend and into
Monday is expected to produce scattered to numerous showers with
the potential for a few mesoscale convective complexes.
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
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