High Wind in Washington, January 17, 2018

Area Forecast Discussion
National Weather Service Portland OR
907 AM PST Wed Jan 17 2018

.SYNOPSIS...A low pressure system will approach the coast today
then stall offshore the British Columbia Coast Thursday and Friday.
This low will bring a mainly dry warm front to the area today
followed by a wet cold front tonight and Thursday. While winds over
the coastal waters will be strong today along and ahead of the cold
front (see marine discussion below), the offshore component
to the wind suggests gust potential will likely be rather limited

 Hazardous surf may result in
coastal flooding during the high tides on Thursday. Steady showers
are expected Thursday and Friday with snow levels below the Cascade
passes. Showers continue this weekend as another low approaches from
the Gulf of Alaska. Another moist cold front may provide heavier
snow for the weekend.

&&

.SHORT TERM...Today through Saturday...Radar shows warm frontal rain
has for the most part lifted north of the region, along with any
moist isentropic lift. A cold front approaches the coast late today,
which will likely spread rain back to the coast during the
afternoon. Elsewhere should be another mild and mainly dry day.

Remainder of short term discussion unchanged...A cold front will
then spread heavier rain from the coast inland this afternoon and
evening, stalling over the Cascades tonight. One inch of rain is
expected for the coast and the Willamette Valley, with up to 1.5
inches for the Cascades. The coast could see a brief burst of strong
winds as the front moves across it this afternoon. Gusts along the
beaches and headlands are expected to be around 55 mph this
afternoon, which is just under the High Wind Warning criteria of 58
mph.

Snow levels will lower to around 4500 feet late tonight into Thursday
morning as the cold front moves further inland. The low settles just
offshore the B.C. coast early Thursday and will remain relatively
stationary there through Friday night. Instability from this low
combined with ample moisture will support persistent showers Thursday
and Friday. An additional 0.5 to 1 inch of rain is expected between
Thursday and Friday afternoon. The south Washington and north Oregon
Cascades could gain 8 to 12 inches of new snow Thursday and Friday
with 4 to 8 inches for the Cascades of lane County. Snow levels will
lower to around 2500 feet Thursday evening.

Fetch generated from this low will build a very large ocean swell
late tonight and Thursday. This swell will result in hazardous surf
along the coast where very large breakers may result in coastal
erosion, and carry large debris like trees far up the beaches. The
large surf will cause water to rise further up the beach than normal,
especially during the high tides around midnight and noon on
Thursday. During these high tides, the ocean water may flood low
lying areas adjacent to the beaches including parts of Highway 101.
A coastal Flood Warning is therefore in effect for the south
Washington and north Oregon coasts late tonight through Thursday
night.

Another low approaches from the Gulf of Alaska Friday night as the
previous low weakens continuing showers over the area with snow
levels remaining around 2500 feet. The two lows will merge just
offshore the B.C. coast Saturday morning and remain through the
weekend. Another moist cold front will bring bring another round of
heavier rain to the coast Saturday afternoon or evening as snow
levels remain around 2500 feet. ~TJ

.LONG TERM...No Changes. Previous discussion follows...Saturday night
through Tuesday night. Active weather pattern continues through the
weekend. Wave after wave of energy will push into the Washington and
Oregon coast from a broad trough of low pressure over the Gulf of
Alaska. With this moisture, snow levels will be down 2000 to 3000 ft,
so this will be a great pattern to build the snowpack in the
Cascades. This will also, however, mean those planning travel through
the Cascades this weekend will need to
be prepared for snow, which could be heavy at times. Rain on the
coast and in the Willamette Valley will mean rises on rivers across
the area as well, though we are starting out with fairly low river
levels for this time of year.

There's some uncertainty in the pattern early next week, but general
consensus is that we may get a break in the precip late Monday into
Tuesday, with either broad or amplified ridging over the West Coast.
Left a slight chance for PoPs because with a more broad ridge, can't
rule out precip completely. -McCoy

&&

.AVIATION...VFR conditions across much of the forecast area
this morning. The main exception is IFR stratus in the Central
Columbia River Gorge. VFR to continue along the coast through
about 00Z Thu, then conditions will become more MVFR with passage
of a cold front. The coast will see a mix of MVFR and VFR after
about 04Z Thu. Inland areas to remain VFR through about 02Z Thu.
The cold front reaches the Willamette Valley and SW Washington
interior lowlands between 02Z and 04Z, with the potential for
MVFR. Post-frontal pre-dominant VFR develops in the interior
valleys between 07Z and 10Z. Higher terrain becomes frequently
obscured after 00Z Thu.

PDX AND APPROACHES...VFR to be the dominant flight category at
the terminal through at least early evening. Cold front moves
across the terminal around 04Z Thu, which will result in MVFR
conditions. Expect more VFR after 08Z in the post-frontal air
mass. East wind 20-30 kt with gusts to 40 kt will prevail at the
west end of the Gorge into the afternoon, then decrease. Wind
will become SW-W with the cold frontal passage. Weishaar

&&

.MARINE...Little change in the first couple days as model forecast
solutions have remained consistent over the last 24 hours. A strong
surface low center around 962 mb was located near 45N 140W at
15Z. The low moves northeast today with a warm front lifting
north through the waters. A strong trailing cold front is forecast
to move through the waters in the afternoon. Gusts 40-45 kt have
been noted at buoy 46089, which matches well with the 17Z HRRR
forecast 80m wind. Regional and Global models continue showing
potential for a few hour period of 50-55 kt gusts this afternoon
and early evening, primarily over the outer waters. Models
continue to show the strongest gusts over PZZ275 from about 21Z
through 00Z Thu. The coastal jet signature shown earlier by the
NAM has since disappeared on recent runs with the bulk of the
higher winds associated with the cold front itself.

Post frontal gales to then continue for the outer waters later
this evening through much of Thu. Models continue to trend
weaker with the late Sat/Sun system and are also showing slower
timing. The 00Z ECMWF has the cold front moving through the
waters Sun morning, compared to late Sat night in its previous
couple of runs.

Seas will continue in the lower teens this morning then build to
the upper teens by mid-day. The highest seas are expected to
move inside 60 nm by daybreak Thursday. Little change in modeled
wave guidance with seas 29-33 feet. The 12Z spectral guidance
shows 32-33 ft peak wave heights from 12Z-18Z Thu. This is
slightly higher than the Tue runs. The ECMWF continues to be 2-4
ft lower.

Areas of the shoreline will be highly impacted by the expected
large waves beginning late tonight. The expected 30-33 ft west
swell with 15-18 second periods will contain an enormous amount
of energy. Waves will generally break within a mile or so
offshore, with potential 40 ft or greater wave heights. These
waves will contain enough momentum to run water much further up
the beaches. It appears this event will be more significant than
the December 2014 impacts but less so than February 2006 where
significant damage occurred across low lying beach and harbor
areas. Seas diminish Fri, but will remain above 20 ft through Fri
night, then ease to the upper teens Sat. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Coastal Flood Warning from midnight tonight to midnight PST
     Thursday night for Central Oregon Coast-North Oregon Coast.

WA...Coastal Flood Warning from midnight tonight to midnight PST
     Thursday night for South Washington Coast.

PZ...Storm Warning until 7 PM PST this evening for Coastal Waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Gale Warning until 10 AM PST this morning for Waters from Cape
     Shoalwater WA to Florence OR from 10 to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 4 AM
     PST Friday.

     Small Craft Advisory until 10 AM PST this morning for Coastal
     Waters from Cape Shoalwater WA to Florence OR out 10 NM.

&&

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland

This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the CWA or forecast area.