Damaging Winds in Oregon, November 6, 2021

Area Forecast Discussion
National Weather Service Portland OR
1102 AM PDT Sat Nov 6 2021

.SYNOPSIS...A showery weather pattern will bring Cascades snow,
brief valley downpours, small hail and a chance for a short lived
thunderstorm through Sunday before additional storm systems bring
more rain to the region this coming week.

&&

.SHORT TERM...Today through Monday...Water vapor satellite imagery
this morning reveals a broad shortwave trough over the northeast
Pacific sliding eastward into the Pacific Northwest. This upper
level trough is spreading cold air aloft (-28C to -32C at 500mb)
across the region under deep WSW-SW flow aloft. This has resulted in
plenty of showers and isolated thunderstorms streaming off the
Pacific into southwest Washington and northwest Oregon. Expect brief
downpours with small hail with the strongest showers. Between
temperatures moderating a couple degrees aloft overnight and the
flow turning slightly more southwesterly, we should see showers
decreasing across our southeastern zones, specifically the Lane and
Linn County Cascades overnight. Nonetheless, another embedded
shortwave trough swinging through the parent trough will likely
bring another surge in showers and isolated thunderstorms Sunday.

All of these showers are bringing snow to the Cascades above 2500-
3500 feet this morning. Accumulations in the 2500-4000 feet
elevation range will generally remain sub-advisory level, but will
be pushing, if not exceeding advisory levels above 4000 feet where
storm total accumulations of 6-12" appear likely to fall in the OR
Cascades and 12-18" across the south WA Cascades. Given there will
be travel impacts at times below 4000 feet and storm total
accumulations in areas above 4000 feet could result in people
camping (elk season) in the Cascades becoming stranded, opted to
issue a Winter Weather Advisory for our north Oregon and south
Washington Cascade zones.

Finally, satellite imagery and KLGX do show that a weak low pressure
is moving northeastward towards the Olympic Peninsula. This will
produce some gusty winds out of the SW-WSW along the coast over the
next few hours, particularly across Pacific and Clatsop Counties,
but wind gusts should generally top out in the 35 to 50 mph range
and remain below high wind criteria.

Otherwise, building 500mb heights over the region Monday appear
likely to be enough that most of the region should experience a 12-
18 hour window of drier weather. Rain still appears likely to move
into the region Monday night into Tuesday, though. /Neuman

&&

.LONG TERM...Tuesday through Friday...No changes...Previous
discussion from Friday night follows...Cluster analysis shows
agreement among model ensembles in the upper pattern at the
beginning of the long term: Amplified with ridging to our east and a
trough to our northwest. However the depiction of a surface low to
develop somewhere in the eastern Pacific is vastly different between
ensemble members of the CMC, GFS, and ECMWF. Fortunately, most
members of the EPS place the low west of 130W, which assuages some
concerns for impactful weather in our area from this system. Many
members of the GEFS, on the other hand, develop some respectably
intense surface lows around (40, -130), but suggest any low that
forms would weaken as it lifts northeastward into our area. The GFS
has, on several occasions already this season, overly developed
intense surface lows too far poleward of the main upper trough in
the medium to long term, but that doesn't mean this system will not
require careful attention over the coming days. If the GFS is
correct in developing an intense surface low south of the main upper
trough, and this feature winds up staying strong as it approaches
our area with a favorable track and rightly-oriented pressure field,
then gusty winds and/or heavy precipitation could result for
one/more parts of NW Oregon and SW Washington. However, the most
likely scenario would just be another round of garden variety rain
showers for the area, with flooding concerns remaining low at this
time.

Cluster analysis shows agreement in the long term models that ridging
will expand northwestward into the Pacific Northwest by mid to late
week, but the western extent of the ridge is uncertain. Some members
take the jet stream far enough west to warm and dry our area by
midweek, but many are suggesting that we'll remain close enough to
the baroclinic zone to be brushed by multiple shortwaves which could
bring additional chances for rain through the end of the long term.
-Bumgardner

&&

.AVIATION...Somewhat unstable air mass over the region through
tonight. Expect mostly VFR conditions but will have brief
periods of MVFR as more organized showers and isolated
thunderstorms develop through the afternoon. Showers gradually
ease in the evening but HRRR, HRW hires models indicating another
round of more organized showers in the 10-16Z Sun time frame.
This may bring increasing MVFR conditions, but forecast confidence
is low. Winds will also be gusty through the afternoon, then
ease some tonight.

For detailed regional Pac NW aviation weather information, go
online to: https://weather.gov/zse

KPDX AND APPROACHES...Showers with a slight chance for
a few thunderstorms through the afternoon. Already had one
thunderstorm between KPDX and KTTD. Small hail also reported with
this storm. Generally expect VFR with brief MVFR through the
early evening. Winds stay mostly in the 10 to 15 kt range, but
gusty at times through the afternoon.

&&

.MARINE...Active weather continues, with another fast moving
system passing to the north and northwest of the region later
today. This will tighten pressure gradients this am, enough to
support south to southwest gusts 35 kt on the northern waters
through the afternoon. Seas mostly 13 to 15 ft. Will maintain
current Gale Warning on the northern waters (from Cape Shoalwater
to Cascade Head), though the strongest winds and highest seas
will generally be well offshore. To the south, winds will stay
sub-gale with seas 12 to 15 ft, so will hold current Small Craft
Advisory on those waters (Cascade to Florence).

Seas and winds will subside a bit tonight, holding at 15 to 25
kt with seas 10 to 12 ft for later tonight through Sunday.

But, much more active weather returns Monday, as next series of
strong fronts push across the coastal waters. Likely to be solid
gales on the waters, with seas building back into the 13 to 16 ft
range. Longer range showers active weather through at least Wed.

.BEACH HAZARDS...The Saturday and Sunday afternoon high tide
poses a risk for minor tidal overflow again. The Saturday high
tide will be the highest of the two, and may result in similar
impacts as the Thursday tidal overflow event. During tidal
overflow, low lying areas near bays, sloughs, and the lower
reaches of the coastal rivers may be briefly flooded.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Coastal Flood Advisory until 5 PM PDT this afternoon for North
     Oregon Coast.

     Winter Weather Advisory until 10 PM PDT this evening for
     Northern Oregon Cascades.

WA...Coastal Flood Advisory until 5 PM PDT this afternoon for South
     Washington Coast.

     Winter Weather Advisory until 5 PM PST Sunday for South
     Washington Cascades.

PZ...Gale Warning until 7 PM PDT this evening for coastal waters
     from Cape Shoalwater WA to Cascade Head OR out 60 nm.

     Small Craft Advisory until 4 AM PST Sunday for coastal waters
     from Cascade Head OR to Florence OR out 60 nm.

     Small Craft Advisory until 11 AM PST Sunday for Columbia River
     Bar.

&&

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