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Area forecast discussion
National Weather Service Portland or
250 PM PST Wed Jan 17 2018

Synopsis...a cold front will move inland this evening, crossing
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...cold front was moving onto
the coast early this afternoon, ushering in a solid looking band of
rain along with some gusty winds. Models indicate the front itself
pushing into the Cascades this evening, with a baroclinic zone
lingering into early Sat morning. The initial shortwave that pushed
the front up to the coast was seen in water vapor pictures lifting
NE into Vancouver Island, but another shortwave lifting up from the
vicinity of 4n 132w will move across early Thu. 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.

Large seas associated with the deep surface low that is running up
towards the central British Columbia 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. Gww deep water wave heights over the coastal
waters expected to top 30 ft early Thu coupled with long periods
will pose a threat for coastal flooding late tonight and Thu, with
the peak threat occurring through the daytime high tide early Thu
afternoon.

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...leading edge of a cold front just offshore at 21z.
Coastal areas were VFR at 21z, but conditions expected to lower
into MVFR by late afternoon. Vis may slip into high-end IFR as
the cold front moves onshore. The coast will be mainly MVFR
00z Thu through 04z or so, followed by a mix of VFR and MVFR
thereafter. Latest model guidance suggests another surface trough
will impact the coast around 12z for a higher threat of MVFR.
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 with the front
and a few hours after passage. Post-frontal pre-dominant VFR
develops in the interior valleys between 07z and 10z, but
increasing chance of MVFR between 12z and 15z Thu. 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. Another surface trough is expected to move across the
terminal between 12z and 15z for an increasing threat of MVFR.
Weishaar

&&

Marine...general forecast trending as expected. A 960 mb to 964
mb low pres center was located near 48n 136w at 21z. Storm force
gusts to 58 kt have been noted at buoy 46005, with a 51 kt gusts
at 46050 at 21z. The forecast 80m wind field from the 19z hrrr
seems to be meshing well with buoy observations. Peak wind will
occur through about 01z Thu, with the strongest gusts over
pzz275. 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 Thu. 12z model runs are in better agreement
regarding the sun system. Looks like gale force wind returns to
pzz255 and pzz275 in the 06z-12z sun time frame and then move
into the north zones after 12z. Indicated gusts around 45 kt over
pzz275 Sun morning. Wind speeds settle down early next week.

Seas starting to build this afternoon, near 30 ft at buoy 46005
at 21z. Latest enp guidance indicates seas near 40 ft at 46005
valid 00z Thu. The highest seas are expected to move inside 60
nm by 12z Thu. Little change in modeled wave guidance with seas
29-33 feet. The 12z spectral guidance for buoy 46029 shows 32-33
ft peak wave heights from 12z-18z Thu. This is slightly higher
than the Tue runs. The European model (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 the potential for 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 evening. Seas will continue to subside Fri
night and Sat, easing to the lower teens by 12z sun. However,
forecast gale force wind sun will likely boost seas to near 20
ft. Weishaar

&&

Pqr watches/warnings/advisories...
or...coastal Flood Warning from midnight tonight to midnight PST
Thursday night for central Oregon coast-north Oregon coast.

Washington...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 Washington to Florence or out 60 nm.

Small Craft Advisory for rough Columbia River bar until 4 am
PST Friday.

&&
$$

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