Area Forecast Discussion
National Weather Service Portland OR
303 PM PDT Thu Oct 19 2017


.SYNOPSIS...A series of storm systems will bring periods of valley
rain that may be heavy at times, mountain snow and coastal winds
through the weekend.


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.SHORT TERM...Today through Sunday...A modest atmospheric river
remains pointed at northwest Oregon and southwest Washington this
afternoon. Precipitation should gradually turn more showery tonight
as a colder and more unstable airmass spreads into the region. The
best chance for thunderstorms will be over the waters tonight and
early Friday morning, but cannot rule out a clap or two of thunder
Friday afternoon, mainly east of Interstate 5 in the Willamette
Valley.

Snow levels will likely lower to between 4-5kft late tonight and
Friday as the cool and more unstable airmass spreads across the
region. There should be some decent snow accumulations above 5000
ft, but temperatures appear marginal enough below that elevation
that impacts and criteria probably won`t warrant a Winter Wx
Advisory for the Cascade passes. Still would not be surprised to see
between 5-10" of snow around 5000-6000 ft through Saturday morning.
Expect showers to decrease Friday evening with a tendency for some
brief clearing. Model soundings indicate this may result in snow
levels temporarily lowering closer to 2500-3000 ft across the
Cascades, particularly near the east side of Mt Hood and the lower
terrain surrounding Mt Adams. This could matter for a few hours
towards daybreak on Saturday as the next round of widespread
precipitation arrives. Any snow accumulations at these lower
elevations should be brief as a warm front results in snow levels
rising to at least 7000-8000 ft by late Saturday.

The strong westerly flow that will follow behind the warm front will
likely enhance orographic precipitation across the coast range and
Cascades late Saturday. A distinct rain shadow should then develop
Saturday afternoon and evening and result in rainfall rates dropping
off dramatically from the first half of the day in the Willamette
Valley. A decently strong south to north pressure gradient will also
set up across the area, and likely produce at least breezy winds
along the coast and even in the Willamette Valley. Whether or not,
it will be strong enough to produce 58 mph gusts along the coast
(warning criteria) and 45 mph gusts in the Willamette Valley
(advisory criteria) remains unclear at this point.

Models have and continue to suggest an atmospheric river will then
take aim at the Pacific Northwest. There is high confidence that a
period of more intense rainfall will drop southward across the area
Sunday. However, it remains unclear if it will sit over northwest
Oregon or western Washington in the 12-18+ hours preceding Sunday
afternoon. This has big implications for our river forecasts given it
has the potential to produce 1" of rain in the valleys and 2-3" of
rain across the higher terrain in a 6 hour period if it remains
stationary. So the potential of 12-18 hours of that type of rain
would undoubtedly produce hydrologic issues even with it still being
early in the wet season, particularly across our burn scars and
smaller streams. Given model trends, suspect far northwest Oregon and
southwest Washington stand the best chance for having the atmospheric
river sit over it for an extended period of time, but it`s far from
guaranteed. These intense bands of rain can easily deviate 50-100
miles north or south from current model forecasts given the time
between now and then. /Neuman


.LONG TERM...Sunday night through Thursday....Models are in
agreement, that shortwave ridging will develop over the Pacific NW
early next week. This will bring a period of dry weather that will
likely be followed, by a weak trough towards the middle of the week.
This will provide the chance for more rain Tuesday night into
Wednesday. Models are suggesting that another ridge of high pressure
could bring a dry period late next week. Bishop


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.AVIATION...The persistent band of rain over much of NW Oregon
and SW Washington Thu will shift east of the area tonight. MVFR and
locally IFR conditions will tend to improve overnight as the main
area of rain moves off, and a more showery air mass moves in.
Generally expect improvement to VFR conditions between 03z and 08z,
although there is likely to be a few areas of MVFR conditions
persisting past 08z and well into Fri associated with heavier
showers.

PORTLAND AND APPROACHES...MVFR conditions are likely to prevail through
about 03z this evening, as the persistent rain starts to move out
this evening. VFR conditions are expected to dominate after 03z as
the air mass turns more showery, but it is likely that there will be
occasional temporary MVFR conditions beyond 03z and into Fri
associated with showers.


&&


.MARINE...Winds to slowly veer around from the south to the
west tonight and Fri, with occasional gusts to around 25 kt. Will
allow the small craft advisory in the north to expire this evening
as winds there appear likely to temporarily weaken some before
increasing again Fri morning.

A frontal system moving into the waters late Fri night and Sat will
bring the threat of gale force winds back by Sat. Expectations are
for this system to track a little further south than originally
anticipated, meaning the strongest and more persistent gales are
likely across the southern waters Sat and Sat night.

A long period westerly swell around 20 ft arrived early Thu
afternoon, and will likely continue at that level through most of
tonight. The swell is expected to slowly decrease Fri into Fri
night, eventually dropping below 15 ft by around midnight Fri night.
As winds increase though late Fri night and Sat and then another
swell train arrives from the west Sat night, expect seas to remain
in the 14 to 19 ft range through Sat night.


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.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory until 10 AM PDT Friday for Central Oregon
     Coast-North Oregon Coast.

WA...High Surf Advisory until 10 AM PDT Friday for South Washington
     Coast.

PZ...Small Craft Advisory for winds until 6 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.

     Small Craft Advisory for hazardous seas until 11 PM PDT Friday
     for Coastal waters from Cape Shoalwater WA to Cascade Head
     OR out 10 NM-Coastal waters from Cascade Head to Florence
     OR out 10 NM-Waters from Cape Shoalwater WA to Cascade
     Head OR from 10 to 60 NM-Waters from Cascade Head to
     Florence OR from 10 to 60 NM.

     Small Craft Advisory for winds until 1 PM PDT Friday 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 until 6 PM
     PDT 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 forecast area.