High Wind in Oregon, November 17, 2020

Area Forecast Discussion
National Weather Service Portland OR
252 AM PST Tue Nov 17 2020

.SYNOPSIS...Strong low pressure offshore will push a cold front east
across the region today bringing rain and gusty winds. A cooler
unstable air mass will follow bringing showers Wednesday and
Thursday, along with a slight chance for thunderstorms to parts of
the region. Then, drying out for later Fri into Sat. The latter part
of the week sees a reduced chance for showers as surface high
pressure moves in, but then more rain likely returns late in the
weekend.

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.SHORT TERM...Today through Thursday...An active weather pattern is
expected to continue through the middle part of the week. Satellite
images early Tuesday morning showed a strong shortwave lifting north-
northeast off the north California/south Oregon coast. Ahead of the
shortwave, a large deep surface low was centered off the north
Washington coast near 129W. Models show this low continuing to
deepen as it lifts north this morning. A cold front trailing the low
is expected to slowly move east today, eventually moving east of the
Cascades this afternoon. The deep surface low has brought strong
southerly winds to the coastal waters early this morning, but an
easterly offshore component to the winds has limited strong winds
showing up on the coast so far this morning to a few of the
headlands. While gradients ease some later today as the low lifts
north, the cold front moving ashore late this morning will provide a
better opportunity for strong winds to materialize over more of the
coastal areas. Sufficient moisture coupled with good dynamics should
allow for a period of rain to accompany the front today as it moves
inland, then air mass becomes more unstable in the cooler air mass
behind the front during the afternoon. Model forecast soundings
continue to hint at sufficiently deep instability to keep a slight
chance of thunderstorms in at coastal areas this afternoon and
evening.

Models show the upper low sitting off the southeast Alaska coast
continuing to send periodic shortwaves rotating around the low,
across the forecast area Tuesday night through Thursday morning. As
the mid and upper level flow become more westerly with time, model
time height cross sections show moisture become deeper, especially
on Wednesday. The moisture coupled with shortwave energy and
favorable orographics for west facing terrain, will keep chances for
showers elevated to likely category or better into Thursday. Model
soundings show better, more widespread instability developing as
coldest air aloft moves in and couples with daytime heating Wednesday
afternoon and evening. This will result in an expanded area with a
slight chance of thunderstorms. Snow levels become progressively
lower into Thursday, dropping below Cascade pass levels. While this
should be a good period for picking up some snowfall in the
Cascades, the strength of flow and dynamics seem to suggest
accumulation rates will be modest, so no winter weather headlines
anticipated for the Cascades at this point in time.

.LONG TERM...Thursday night through Sunday...Models continue to
show a transitory ridge over the Pacific NW Friday into Saturday.
Good chance that Saturday will be dry with some variable high clouds.
Deterministic solutions are showing a front arriving along the coast
Saturday night into Sunday which will bring another round of rain and
winds into early next week.                      /BPhillips

&&

.AVIATION...06Z TAFs: A strong offshore low will continue to
move northeast towards the Canadian coastline tonight and into
tomorrow. For the most part winds across the area have shifted to
a southerly regime. Current radar shows the bulk of the
precipitation north of the Washington border and conditions
should be dry through the night. Most inland sites have improved
to VFR as ceilings have lifted and scattered out. Coastal sites
continue to see IFR/LIFR conditions but this is expected to
improve to VFR later this afternoon. Variable high clouds will
continue to move northeast through the night ahead of the cold
front. There is potential for some local areas of fog in
sheltered areas, but winds will likely be strong enough to mix
out any fog trying to form in most places.

The cold front is still expected to move onshore between 15Z to
18Z Tuesday. Highest impacts will be along the coast with strong
southerly wind gusts throughout the entirety of the TAF period.
With 70 to 80 kt winds aloft just offshore, southerly wind gusts
up to 65 kt at the surface is not out of the question for KONP.
Southerly wind gusts in the valley will likely be between 20-30
kt. Speed and directional wind shear will be a concern between
15Z to 20Z. Expect rain and low-end VFR ceilings to return by
15Z Tuesday.

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

KPDX AND APPROACHES...VFR to continue through the night with
variable high clouds. Highest impacts will be after 15Z Tuesday
ahead of a cold front. Expect strong southerly winds with
potential for LLWS up to 50 kt.     -BPhillips

&&

.MARINE...Not much in the way of changes for today as none of the
models are really quite capturing the current surface pressure
reality. Many of the hires models have speeds too high for
current buoy obs while the coarser regional scale models have
the pressure fields in good shape, but are keeping speeds too
low. WInds across the waters are southerly across the board with
gusts around 30 kts. Still expect to see these speeds
dramatically ramp up as the surface cold front squeezes the
southerly flow against the coast range west slopes under a
somewhat stable airmass. This should still result in a coastal
jet with storm force gusts mainly inside of 20 nm offshore as a
result. There is still a fair amount of wiggle room in the range
of potential speeds, even at this short lead time, given the lack
of uniform agreement from the numerous modeled wind fields. Have
opted to keep the stronger end of the storm force gusts peaking
around 60 kts as an inversion estimated to be around 1500-2000
feet still appears like it will provide an additional vertical
compression and wind speeds acceleration to the jet. There is a
minimal chance (5-10%) speeds will be higher than this and about
a 25% chance gust speeds will end up being closer to high end
gales around 45 kt. Regardless, seas will largely be wind wave
driven and peaking at 22-26 feet, highest near shore under the
higher wind speeds.

Winds and seas will drop off fairly quickly behind the front with
perhaps an hour or two of gales before falling back into solid
small craft speeds below 34 kts. Seas will fall several feet for
their max heights and still likely remain in the mid teens for
tonight.

A much weaker surface low, still remaining not closed, looks to
move northwest across the waters Wednesday. Winds may flirt with
gale force gusts, but the main impact will be to maintain seas
around 10 feet until Thursday morning. Much more benign
conditions will prevail Thursday through most of Saturday.
ANother low pressure system will bring increasing southerly winds
Saturday afternoon with some models bringing a weakening closed
low center eastward across the Olympic Peninsula Saturday night.
/JBonk

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 7 AM this morning to 3 PM PST this
     afternoon for Central Oregon Coast-North Oregon Coast.

     Coastal Flood Advisory from noon today to 4 PM PST this
     afternoon for North Oregon Coast.

WA...High Wind Warning from 7 AM this morning to 3 PM PST this
     afternoon for South Washington Coast.

     Coastal Flood Advisory from noon today to 4 PM PST this
     afternoon for South Washington Coast.

PZ...Storm Warning until 1 PM PST this afternoon for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Gale Warning until 4 AM PST early this morning for coastal
     waters from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until 10 AM PST Wednesday for Columbia
     River Bar.

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