Coastal Flood and Sneakerwave in Oregon, January 17 to 18, 2018

Area Forecast Discussion
National Weather Service Portland OR
922 PM PST Wed Jan 17 2018

.SYNOPSIS...A cold front moving across the interior lowlands will
shift across the Cascades overnight. An upper level trough of low
pres off the coast will then send a series of disturbances inland
through the end of the week, keeping the region in a cooler showery
pattern. Another front will move inland Sunday, bringing a round of
heavier rainfall and some mountain snow.

&&

.SHORT TERM...Tonight through Saturday...Rain band associated with a
cold front now moving through the Willamette Valley and southwest
Washington. Rainfall amount have been modest about 0.3" to 0.6" over
6 hours inland, and 0.7" to 1" in the coast range. The front should
be in the Cascades around midnight with rain rates easing.  A cooler
air mass will in place Thu so temperatures will be closer to normal
after near record high temperatures in the 60s today in some parts of
the CWA.

Cross sections of moisture indicate deep enough moisture to continue
with near 100 pops as the front moves through overnight, and will
retain categorical pops through the night into Thu morning with the
next shortwave moving through. Moist onshore flow continues through
Fri and Fri night as a few more shortwaves move through keeping pops
in the likely to categorical range for showers. Model soundings
continue to indicate marginal instability, centered along and off the
coast Thu afternoon as the cold upper trough moves closer, so will
keep a slight chance for thunderstorms over the coastal areas then.
Have introduced a slight chance of thunderstorms over the outer
coastal waters Thu morning with instability moving in a little
earlier there.

Large seas associated with the deep surface low that is running up
towards the central BC coast tonight will arrive tonight and
continue through Thu before decreasing again Thu night. The system
bears a resemblance to a storm back on 10 Dec 2015, where seas
topped out in the range of 31 to 32 ft with periods around 17
seconds, that wound up producing some property damage around Depoe
Bay and Cannon Beach. Buoy observations from 46002 and 46005, about
300 to 350 miles offshore support seas of that magnitude reaching the
coast Thu.

Remainder of previous discussion unchanged...
On Sat weak ridging aloft initiates weak warm air advection. will
keep likely pops in for what should transition to be more stratiform
precipitation. For the next few days with snow levels coming down
behind the front tonight, will see snow adding up in the Cascades
again. At this time however, no system appears overly strong or
heavy, so accumulations through Sat should occur at rates under
advisory criteria.

.LONG TERM...Saturday night through Wednesday...Active weather
continues through the weekend, with coast/valley rain and mountain
snow. Will likely see a decent amount of snow in the Cascades on
Sunday, with snow levels down around 3000 feet (below the passes).
Rain for everyone else means rivers will continue to rise through
Sunday. Transient ridge builds over the Pacific Northwest late
Monday into Tuesday likely means a break in the rain, but energy
riding over the top of the ridge means I kept a slight chance of
rain and clouds in the forecast for Monday night into Tuesday.
Another trough will bring more rain and another stretch of active
weather starting late Tuesday through the end of next week. -McCoy
&&

.AVIATION...Mostly VFR this evening will last much of the night,
but will see an increased threat of MVFR at the coast again
around 12Z with additional rain. Inland areas will see generally
VFR with the same increased chance of MVFR but between about 12Z
and 15Z Thu.

PDX AND APPROACHES...VFR through much of tonight with increased
chances for another few hours of MVFR with additional rain
between about 12Z and 15Z Thursday. Bowen/Weishaar

&&

.MARINE...Post-frontal low-end gales continue. Although wind
gusts may temporarily drop below 35 kt, will likely want to keep
Gale Warning in place as winds are expected to pick up again
during the day Thursday. Seas have been slow to increase this
evening, with local buoys only ranging from 17 to 21 ft as of 9
pm. However, decaying the observed swell from both buoys 5 and 2
shows swell of 28-30 ft arriving at the coast Thursday morning,
so am still confident in very large waves. Upped forecast a bit
to account for these offshore observations - now expecting around
35 ft at buoy 89 and 30-32 ft at buoys 29 and 50 Thursday. See
the Coastal Flood Warning for details on expected coastal
impacts.

Winds and seas diminish Friday, but will remain above 20 ft
through Friday evening, then lowering to the lower teens by 12Z
Sun. 12Z model runs are in better agreement regarding the Sunday
system. Looks like gale force wind returns to PZZ255 and PZZ275
in the 06Z-12Z Sunday time frame and then move into the north
zones after 12Z. Indicated gusts around 45 kt over PZZ275 Sun
morning. Winds will likely push seas back above 20 ft. Wind
speeds and seas settle down again early next week. Bowen/Weishaar

&&

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

WA...Coastal Flood Warning until midnight PST Thursday night for
     South Washington Coast.

PZ...Gale Warning until 4 PM PST Thursday for Coastal Waters from
     Cape Shoalwater WA to Florence OR out 60 NM.

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

&&

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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.