High Surf in Oregon, November 25, 2020

Area Forecast Discussion...Updated
National Weather Service Portland OR
856 PM PST Tue Nov 24 2020

.UPDATE...Rain was indeed heavy right along the cold front as it
moved across the forecast area. The front held together long enough
to bring locally gusty winds to inland areas: PDX Airport reported a
gust to 38 mph as the front moved through, TTD had a gust to 32 mph,
and we received a couple spotter reports of gusts 30-35 mph. This
prompted us to issue a few Significant Weather Advisories as the
front moved east of I-5 and into the Cascades. Conditions
deteriorated rapidly above roughly 4000 feet as the front moved into
the higher Cascades, with wet roads quickly freezing over and snow
accumulating on roads just behind the front. We initially handled
this with a Sig Wx Advisory for the US 20 corridor between Tombstone
Summit and Santiam Pass, but have since began the Winter Weather
Advisory early to accommodate these conditions. Willamette Pass is
also presently seeing snow, according to ODOT webcams.

There will likely be a brief lull in precipitation tonight due to
subsidence behind the cold front, and this appears to be happening
based on latest radar and satellite imagery. Modestly unstable
onshore flow will cause showers to increase again Wednesday morning,
persisting much of the day. These showers will fall as snow in the
Cascades above 3500-4000', where we are still expecting 6-12 inches
of snow.  Weagle

&&

.SYNOPSIS...A strong cold front moved across the forecast area this
afternoon and evening, bringing a brief period of heavy precipitation
and gusty winds. This front is now crossing the Cascades, with snow
levels falling behind the front. After somewhat of a lull later
tonight, showers will increase again Wednesday, continuing into early
Wednesday evening. High pressure returns Thursday through at least
Sunday with valley inversions and light winds potentially leading to
stagnant air. A fast moving front brushes the northern areas Friday
night but seems unlikely to significantly mix the lower atmosphere.

&&

.SHORT TERM...Tonight through Friday...A cold upper trough with an
embedded decaying closed low is dropping south along the BC coast.
The initial cold front brought wind gusts 35-40 mph to the South
Washington coast and adjoining waters. It has also brought 1/2 inch
of rainfall inside of a couple hours to the north and central
Washington coast. The winds are likely weakening as it slowly moves
onshore but the moderate rainfall will likely hold together as the
trough has tapped into a subtropical moisture plume located near
30N, 155W.

Snow levels will briefly rise across the Cascades this evening as
the front approaches. Rain there will quickly turn to snow behind
the front as snow levels drop to around 3500 feet with several
inches of snow possible at the higher passes and at least a few
inches at Government Camp. Do have confidence in the colder air,
modest instability, and trailing moisture per the "popcorn" cumulus
very evident on visible satellite imagery. Flow will also become
more west to northwest to produce continued up slope flow on the
Coast Range and Cascades as the low moves overhead and the deeper
instability arrives. Have added a slight chance of thunderstorms to
the immediate coast and waters where sea surface temperatures will
act as the convective trigger. Don't expect a large number of storms
but more of "one-hit" wonders at best.

There will be somewhat of a break late tonight through early
tomorrow with lower accumulation rates and more significant rain
shadowing for the I-5 corridor. Moisture then increases again as the
back side of the trough arrives Wednesday afternoon. Expect another
several inches of snow accumulation along with brief periods of
convective enhancement through Wednesday evening. Rain and snow will
taper off after midnight Wednesday night into Thursday with a couple
inches of snow accumulation, on the high end, remaining possible
Thursday morning.

Also for tomorrow, ocean conditions will support high surf along the
coastline. Please refer to the Marine section for more specific
details on the High Surf Advisory currently in effect.

High pressure then builds Thursday and Friday with valley inversions
developing. This could also lead to valley fog that would be slow to
burn off and also to increasing likelihood for stagnant air
conditions developing. -JBonk

.LONG TERM...Friday night through Tuesday...The upper level ridge
flattens Friday evening as an upper trough moves inland just north
of Washington state. At the surface, a weak cold front will fall
apart as it moves inland across Washington and NW Oregon for
occasional light rain and Cascade snow Friday night into Saturday
morning. Do not currently expect that this front will disrupt the
subsidence inversion very much, and expect fog will develop in the
interior valleys across the central and south Willamette valley
overnight Friday into Saturday.

Models are in good agreement that the upper level ridge will amplify
this weekend reinforcing subsidence inversions across the region and
supporting light north to northeast winds. This will promote
radiation cooling overnight for chilly mornings inland with
temperatures in the 30s. Drying at the lower levels should hinder
widespread valley fog, but expect shallow radiation fog to form near
rivers and bodies of waters overnight that may expand in areal
coverage in the early morning areas. The strong subsidence
inversions will limit mixing at the lower levels for stagnant air
conditions that could lead to reductions in air quality.

An upper trough will move the ridge eastward Sunday night. There are
model differences in the expected strength of the upper trough and
the influence it will have on the atmospheric pressure closer to the
surface. These differences challenge the forecast confidence of a
potential front on Monday. Have followed the NBM guidance for Pops
and sky cover during this questionable period. ~TJ

&&

.AVIATION...The front has moved east of the Cascades and
widespread rain is turning to intermittent showers. Overnight
hours will be a mix of low-end VFR and MVFR with showers. This
general trend will continue through roughly 12Z Wednesday. High
pressure will build in behind this system and bring VFR
conditions by the end of the TAF period.

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

KPDX AND APPROACHES...Low-end VFR ceilings will continue through
the night with intermittent periods of MVFR ceilings and showers.
-BPhillips

&&

.MARINE...Satellite imagery earlier this afternoon showed a cold
front pushing eastward into PZZ270 and PZZ275. Ahead of this
front, gusty south to south-southwest winds are occurring.
Expect the gusty winds to continue through Tuesday afternoon as
the front traverses the waters, with winds decreasing rather
quickly behind the front early this evening. Until then, wind
gusts to 35 kt are expected across PZZ270 with a Gale Warning
currently in effect. A Small Craft Advisory is in effect for
PZZ275, where gusts around 30 kt are expected. A few isolated
gusts to 35 kt will be possible over northern portions of PZZ275,
but not enough to warrant a Gale Warning.

Although winds will be on the decrease this evening, seas will be
on the increase. This is due to an incoming northwest swell
stemming from a surface low in the Gulf of Alaska. Buoy 5 showed
wave heights of 16 ft at 16 seconds earlier this afternoon. Using
a simple but reliable swell decay tool, expect northwest swell
to rise to around 14 ft across PZZ270 by 1 am PST Wednesday. The
latest guidance suggests swell will build to around 20 ft by
late tonight and hold between 20-23 ft on Wednesday. With
periods between 16 and 18 seconds, this will create rather
chaotic and hazardous surf conditions for tonight through
Wednesday evening. Therefore, will maintain a High Surf Advisory
during that time.

A weak low will shift across the northern coastal waters late
Wednesday, with gusty westerly winds anticipated over the waters
Wednesday afternoon and evening. Gusts should range between 20
and 25 kt, with a few gusts possibly approaching 30 kt over the
outer waters. Winds and seas will ease later Wednesday night as
high pressure begins building into the region. High pressure
should linger over the waters Thursday and Friday, bringing
relatively benign winds and subsiding seas. The latest model
guidance is in agreement for another front moving across the
waters Friday night, before an offshore flow pattern develops
this weekend. -TK

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory until 4 AM PST Thursday for Central Oregon
     Coast-North Oregon Coast.

     Winter Weather Advisory until 7 PM PST Wednesday for Cascades
     in Lane County-Northern Oregon Cascades.

WA...High Surf Advisory until 4 AM PST Thursday for South Washington
     Coast.

     Winter Weather Advisory until 7 PM PST Wednesday for South
     Washington Cascades.

PZ...Small Craft Advisory until 10 AM PST Thursday for coastal
     waters from Cape Shoalwater WA to Florence OR out 10
     NM-Columbia River Bar-Coastal waters from Cape Shoalwater
     WA to Florence OR from 10 to 60 NM.

&&

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