High Wind in Oregon, October 24 to 25, 2021

Area Forecast Discussion
National Weather Service Portland OR
443 AM PDT Sun Oct 24 2021

.SYNOPSIS...A strong fall storm will impact the region today through
Monday night with strong winds, high surf, locally heavy rain, and
enhanced thunderstorm potential. The worst conditions will be found
along the coast, with hazardous conditions also expected in the
adjacent coastal waters. The active pattern will continue through
the coming week, with temperatures staying near normal.

&&

.SHORT TERM...THROUGH TUESDAY NIGHT...Explosive cyclogenesis is
well underway across the NE Pacific this morning as an intense
surface low rivaling the deepest on record in this part of the
Pacific Basin nears peak intensity approximately 500 miles west
of the mouth of the Columbia River. The low continues to track
eastward towards the coast powered by a 150 kt jet streak
rounding the base of an amplified upper level trough digging
over the region. Open ocean buoys ahead of the system are
reporting widespread gale to storm force gusts and seas in the
low to mid 20s to as high as 37 feet, with abundant lighting
activity also being noted. These conditions will continue to
spread into region over the next few hours as an occluded front
arcing well out ahead of the low moves into the coastal waters
and then pushes inland. A band of rain is currently moving
across the area ahead of the front from southwest to northeast.

The bulk of the forecast details are relatively unchanged from
previous packages as the low tracks towards Vancouver Island and
gradually weakens to around 980 mb through Monday afternoon. A High
Wind Warning remains in effect for the coast from mid afternoon
today through Monday morning, with gusts as high as 65 mph expected
on the beaches and headlands. Likewise, the Wind Advisory is
unchanged for the Willamette Valley, with gusts in the 40 mph range
expected from late this morning through early Monday morning. The
setup remains favorable for strong to severe thunderstorms along the
coast and in the adjacent waters beginning later this morning.
Impressive vertical wind shear and 0-1 km helicity values,
increasing modest low level instability fueled by steepening lapse
rates, and strong dynamics will combine to support an enhanced
threat for low topped supercells and a few coastal tornadoes into
this evening. The thunderstorm threat will also spread inland to the
Willamette Valley and Cascade foothills during the afternoon, but
more meager instability should help to limit the scope of the threat
in these areas. Finally, expected rainfall amounts have changed very
little, with 2-3 inches in the Coast Range and Cascades and closer
to 1 inch in the valleys still looking like a good bet through
Tuesday morning. Hydro concerns therefore remain on the low side for
the next few days barring the impact of any convective cells today
and tonight.

The pattern will start to de-amplify and calm down at least somewhat
by Tuesday as we transition to moist zonal flow in the wake of
the storm, but this will still maintain high PoPs across most
of the area through Tuesday and potentially into the middle of
the week. /CB

&&

.LONG TERM...WEDNESDAY THROUGH SATURDAY...The pattern still
generally looks to stay on the wet side through much of the
coming week as mean upper level troughing continues to reside
over Alaska and the North Pacific. Flow will briefly come more
up from the southwest for the Wednesday system, looking like
another atmospheric river event setting up. By no means is it
very strong at this point and the exact track has been going
further north and may only end up affecting WA. Continued to
utilize an ensemble heavy approach and maintained chance PoPs
through much of the period given relatively large model spread.
One similarity that looks to be developing is ridging for next
weekend which may end up bringing a dry Halloween to the
forecast area. /CB/Kriederman

&&

.AVIATION...As of 09Z conditions were primarily VFR, but with
localized lower conditions along the coast and in the Cowlitz
River Valley of southwest Washington. Coastal areas are expected
to become predominant MVFR by 12Z, with IFR conditions possible
through 18Z. Improvement to VFR expected around 18Z, but
intermittent MVFR conditions will remain possible through 12Z
Mon. South wind gusts up to 45 kt are likely along the Oregon
coastal areas Sun afternoon through 12Z Mon. Southeast wind gusts
35 to 40 kt can be expected along the south Washington coast.
Model guidance suggests the immediate beaches and coastal strip
have a higher-than-normal chance of thunderstorms after 18Z.

Inland areas to remain mostly VFR through the 12Z forecast
period. However, the central Columbia River Gorge, Upper Hood
River Valley and east slopes around Mt. Adams likely to see more
MVFR as offshore low-level flow results in continued up-slope
conditions. South wind gusts 25-35 kt develop in the interior
valleys after 18Z Sun.

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

KPDX AND APPROACHES...VFR cigs expected to prevail at the
terminal and vicinity through 12Z Mon. There is a 30-50 percent
chance of MVFR in heavier rain through the morning hours. Rain
tapers to showers Sunday afternoon, but thunderstorms will be
possible as well. East wind gusts 25-30 kt at KTTD and 30-40 kt
in the most exposed areas at the west end of the Gorge through
mid-afternoon. Local LLWS guidance indicates a high likelihood at
KPDX and KTTD and will include for the 12Z TAFs. Weishaar

&&

.MARINE...Forecast remains on track and minimal changes were made
for the short term period. The 06Z NAM indicated a surface low
pres of around 950 mb out near 45N 136W. The NAM, GFS and ECMWF
deterministic runs are in general agreement showing a 944 mb
central pressure at 45N 133W at 12Z. The EPS ensemble members are
generally between 940 and 950 mb. Based on a climatology study
by WPC, the lowest pressure ever recorded in the area between
30N-50N and 120W-140W is 944 mb, ironically also on October 24,
but in 1977. So this is a historic storm, but should likely
remain far enough offshore to keep the strongest winds out of our
waters. The EPS ensembles valid 18Z show 45-55 kt wind gusts over
the waters. This is in line with the 4km WRF-GFS. The strongest
wind period looks to be late Sun afternoon through about 12Z Mon.
Current Storm Warnings look to be in good shape. The latest
model output shows the initial wind surge expected early this
morning a little weaker. Thus, storm force gusts may not develop
until late morning. The 15Z Mon end time for storm force gusts
looks reasonable.

Wind speeds gradually diminish 18Z Mon through 12Z Tue. Another
surface trough moves through the waters Tue morning, which may
result in a brief period of 30-35 kt gusts. Sub-20 kt wind speeds
are expected Tue night through the remainder of the extended
period.

Combined seas will become more wind wave/fresh swell driven as
winds increase through the day. By mid-evening Sunday, the dynamic
fetch generated higher swell will arrive to the waters. Then a
SW swell around 25 feet will arrive to result in 30-35 ft
combined seas. Nudged seas up another 1-2 ft during the peak
period to account for the below-average past performance of the
wave models. The 00Z spectral guidance for buoy 46050 shows the
peak seas occurring about 2-3 hours earlier than the the 23/00Z
run, or around 03Z Mon. The 00Z GEFS probabilistic wave height
guidance shows a 50-60 percent chance of seas exceeding 30 ft
over the waters and likely closer to 70 percent over the outer
zones. Seas to remain above 25 ft through Mon evening and fall
below 20 ft around 12Z Tue. Weishaar

&&

.BEACH HAZARDS...The powerful low and associated gale to storm force
winds expected over the next few days will also generate rough and
hazardous seas and produce significant coastal impacts through
Monday night. A Coastal Flood Warning remains in place as swell
heights will build to above 25 feet tonight into Monday, with
breakers as high as 35 feet in the surf zone. Exercise extreme
caution on area beaches and do not venture out onto outcroppings
or jetties. /CB

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 11 AM this morning to 5 AM PDT Monday for
     Central Willamette Valley-Greater Portland Metro Area-
     Lower Columbia-South Willamette Valley.

     High Wind Warning from 3 PM this afternoon to 9 AM PDT Monday
     for Central Oregon Coast-North Oregon Coast.

     Coastal Flood Warning from 8 PM this evening to 5 AM PDT
     Tuesday for Central Oregon Coast-North Oregon Coast.

WA...Wind Advisory from 11 AM this morning to 5 AM PDT Monday for
     Greater Vancouver Area-I-5 Corridor in Cowlitz County.

     High Wind Warning from 3 PM this afternoon to 9 AM PDT Monday
     for South Washington Coast.

     Coastal Flood Warning from 8 PM this evening to 5 AM PDT
     Tuesday for South Washington Coast.

PZ...Storm Warning until 8 AM PDT Monday for coastal waters from
	 Cape Shoalwater WA to Florence OR from 10 to 60 nm.

     Small Craft Advisory until 11 AM PDT Monday for Columbia River
     Bar.

 	 Small Craft Advisory until 7 AM PDT this morning for coastal
	 waters from Cape Shoalwater WA to Florence OR out 10 nm.

 	 Storm Warning until 8 AM PDT Monday for coastal waters from
	 Cape Shoalwater WA to Florence OR out 10 nm.

&&