High Wind in Oregon, October 18, 2017

Area Forecast Discussion
National Weather Service Portland OR
955 AM PDT Wed Oct 18 2017

.SYNOPSIS...A series of storm systems will bring periods of valley
rain, mountain snow and coastal winds through the weekend.
&&

.SHORT TERM...Today through Saturday...Water vapor satellite imagery
reveals a large shortwave trough and low pressure system moving
eastward towards the Pacific Northwest. A warm front is currently
lifting northeastward into the region this morning and resulting in
light rain falling along and north of a line extending between
Pacific City and Hood River. Not expecting this band of rain to shift
much farther south so PoPs were updated to reflect this trend. In
fact, the warm front should shift far enough north this afternoon
that rain may become restricted to extreme northwest Oregon and
southwest Washington for a period this afternoon. Meanwhile, as our
CWA enters the warm sector this afternoon, strong southerly winds
should push onto our coast. It still appears this will be a fairly
routine, albeit the first one of the season, high wind event for the
coast with wind gusts of 50-65 mph across our most exposed beaches
and headlands. The south Washington coast and Willapa Hills will be
in the best position to receive the higher wind gusts while areas
farther south along the coast may struggle to see 60 mph gusts.

A trailing cold front will then drop southeastward across the area
overnight and early Thursday. Expect winds to drop abruptly upon
frontal passage, which should happen along the north Oregon coast
late this evening and late tonight or Thursday morning along the
central Oregon coast.

Between IVT values approaching 750-1000 kg/ms and strong
frontogenesis initially, a band of heavier rain capable of producing
a 0.25-0.35" of rain per hour will likely impact the south
Washington coast and north Oregon coast as the cold front drops
southeastward. However, the main upper level trough will likely be
diving far enough south of the region that this front will weaken as
it pushes towards the central Oregon coast and Willamette Valley. As
a result, rainfall rates appear more likely to peak in the 0.10-
0.25" per hour range as the front moves through these areas. There
may be some tendency for it hang up and slow down a little as it
moves towards Lane County so steady rain may persist across the
southeastern half of the CWA well into Thursday afternoon.

Meanwhile, a colder and more unstable airmass will spread into the
region late Thursday afternoon and evening. This should turn
precipitation more showery with a few thunderstorms possible over the
coastal waters. Models are hinting that a weak surface low could
develop over the far northeast Pacific Thursday night, which could
create a wind field a little more conducive to rotating storms than
current model output suggests so this will be worth monitoring. In
addition, snow levels will lower to near the Cascade passes by late
Thursday night and Friday as colder air aloft filters into the
region. Moderately strong onshore flow coupled with the unstable
atmosphere should produce some decent snow accumulations in the
Cascades above 4-5kft with travel impacts certainly possible at
Santiam and Willamette Passes.

Any break in the wet weather will be short lived Friday night as the
next warm front spreads light to moderate rain into northwest Oregon
and southwest Washington late Friday night and Saturday. Subtle, but
significant model differences in where and exactly how long an
atmospheric river will then take aim at the region exist. /Neuman

.LONG TERM...Friday night through Tuesday....Snow levels will start
to rise as Saturday approaches. Another frontal system will start to
pass through the region on Saturday, which has the potential to bring
another round of significant rain into the area, possibly even more
than we see with the system on Thursday. Expect gusty winds
especially along the coast and in the higher terrain of the Coast
Range through Saturday.  Models show this system stalling over our
area, with the trailing cold front not moving through until as late
as Sunday evening or early Monday. Timing and location of heaviest
rainfall may continue to change with future updates, so those with
plans this weekend should keep an eye on the forecast as this weekend
approaches.

Going into early next week, upper-level ridging starts to build into
the Pacific Northwest, bringing us an extended break in the rain.
Temperatures climb back up into the 60s as well, but because of all
the recent rain temperatures will be hampered by morning fog. Expect
dry weather to continue through Tuesday. -McCoy
&&

.AVIATION...Generally VFR conditions over the southern half of
the Willamette Valley today, eventually giving way to MVFR
conditions late tonight after about 09z. Elsewhere, a mix of
conditions ranging from VFR to IFR this morning. Expect VFR
conditions to temporarily become more widespread this afternoon by
21z over the north interior sites, although there is a chance for
MVFR conditions to persist. MVFR conditons expected to return to the
north interior tonight, most likely after 07z. Along the coast
expect a mix of MVFR to IFR conditions to dominate through tonight.

KPDX AND APPROACHES...VFR expected to remain the most likely flight
conditions today and tonight though 09z, although there is a good
chance for temporary MVFR conditions, mostl likely through 20z
today. Expect a return to prevailing MVFR conditions after 09z
tonight as a cold front moves in.

&&

.MARINE...Southerly gales today into tonight as a warm front lifts
north through the northern coastal waters today, then a cold front
moves se across the coastal waters tonight. winds will drop off
behind the cold front tonight. There is a likely to be a coastal jet
influence ahead of the front, where winds are expected to gust as
high as 45 kt within about 10 nm of the coast. Seas will build in
response to the local winds, peaking on the order of 15 ft tonight.

Winds will likely drop into low end Small Craft Advisory criteria
behind the front, which should allow seas to drop a few feet early
Thu morning, but a large long period swell will arive later in the
day Thu from the nw. The swell will peak between 20 and 23 ft Thu
evening, then slowly subside Fri and Fri night. High Surf Advisories
will ultimately likely be needed late Thu through at least Thu
night. An additional storm system capable of producing at least Gale
Force wind gusts appears on track for Friday night and Saturday.
&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning until 11 PM PDT this evening for Coast Range
     of Northwest Oregon-North Oregon Coast.

WA...High Wind Warning until 11 PM PDT this evening for South
     Washington Coast-Willapa Hills.

PZ...Gale Warning until 11 PM PDT this evening for Coastal waters
     from Cape Shoalwater WA to Cascade Head OR out 10 NM-
     Waters from Cape Shoalwater WA to Cascade Head OR from 10
     to 60 NM.

     Gale Warning from 2 PM this afternoon to 9 AM PDT Thursday for
     Coastal waters from Cascade Head to Florence OR out 10 NM-
     Waters from Cascade Head to Florence OR from 10 to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar from 2 PM
     this afternoon to 6 AM PDT Thursday.

&&

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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 forecast area.