Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 162200

Area Forecast Discussion
National Weather Service Portland OR
300 PM PDT Wed Oct 16 2019

.SYNOPSIS...Expect rain in the lowlands each day for the next
week. Snow levels will flirt with Cascade pass elevations late
Wednesday night through the weekend, but the bulk of snow
accumulation will occur above the passes. The stormy weather
pattern will drive high surf along the coast Thursday.


.SHORT TERM...An initial cold front has mostly moved through the
area this afternoon, with associated rain currently concentrated
in the Cascades based on latest radar imagery. Rain associated
with a second, stronger cold front is approaching the coast,
bringing additional rain. As with the first front, the coast and
Coast Range will likely get the brunt of rain with the second
front with some areas measuring more than an inch of rain in the
calendar day by the time it`s over. Behind the second front, rain
will turn to showers. Models have backed off some on overnight
instability, so tapered back the slight chance of thunderstorms so
that it stays over the waters tonight. However, instability
increases Thursday over land as the upper cold pool settles in, so
kept and even expanded the slight chance of thunderstorms during
the daytime hours.

The southerly component of the surface pressure gradient
increases Thursday, leading to mild and breezy conditions. It will
also mean that wind gusts are likely to be particularly noticeable
with showers Thursday since winds will already be starting out a
little higher than usual.

A reinforcing shortwave drops across the region Thursday night,
which will likely bring another round of steady rain overnight
Thursday into early Friday. The threat for thunderstorms will
return Friday night behind this front and ahead of the next upper
trough on Saturday, with the associated cold pool aloft enhancing
instability and reasonably strong shear again in place. This
general pattern has been known to produce some strong showers and
storms over the coastal water and near the coast, so will need to
continue to keep a close eye on trends over the coming days.
Models have begun to spin up a surface low late Friday night near
the Washington coast, so will also keep an eye on its potential
development. There is a scenario where it would produce strong
wind gusts across the entire forecast area, but impacts are very
track and strength dependent, so confidence is low in any given
solution at this point. Bowen

.LONG TERM...Saturday night through Wednesday...A persistent low in
the Gulf of Alaska will continue to spin off upper level
disturbances towards the Pacific Northwest and will continue through
Wednesday. The most notable event of the extended forecast will
occur through Sunday morning with the GFS and ECMWF showing 12-hr
liquid precip accumulations of over a half inch in parts of the
Cascades. Models suggest a brief break from precip on Sunday, but
the turnover for the next event is a quick one. Moisture originating
from the tropics will move though our area Sunday night into Monday.
Although the 12z GFS and ECMWF model runs precipitation totals have
decreased from previous runs, they are still showing over a inch
of precip for the coast and northern Cascades through Tuesday
night. Temperatures will remain steady throughout the extended
with highs in the 60s and lows in the mid to upper 40s. -Thaler


.AVIATION...A slow cold front is moving over Southwest
Washington/Northwest Oregon today. This front is advecting
stratus and precipitation over the area. The rain will be
stratiform in nature and quite widespread with periods of heavier
rainfall possible. With the heavier rain, visibility may be
reduced to MVFR or even IFR levels. Satellite imagery shows dense
MVFR stratus over the area. These MVFR ceilings will dominate
through around 04Z on Thursday. A weak ridge will form in the
post-frontal airmass which will allow ceilings to rise and the
rain to become more showery. However, this break is short lived
as by 15Z on Thursday, the leading edge of the next system will
be advancing towards the coast. This means that clouds will begin
to fill in again. One challenge with this forecast is the slight
chance of thunderstorms developing around 06Z on Thursday along
the coast. Confidence remains low on their development overnight,
so they were left out of TAFs. The thunderstorm possibility will
spread inland after 18Z on Thursday.

KPDX AND APPROACHES...Rain will persist through 18Z on Thursday with
periods of stronger showers. With the heavy rain, visibility may be
reduced at times. Deep MVFR stratus will persist through 05Z
Thursday when skies are expected to improve to VFR with the
frontal passage and unstable airmass. A sky reduction to MVFR is
possible again after 14Z Thursday as the next cold front


.MARINE...A very stormy pattern will persist through the
beginning of next week with a series of fronts passing over the
waters through Monday. The cold front that brought gusty
conditions to the waters has moved inland and the winds are
starting to weaken to 15 to 20 kt. Winds are expected to remain
15 to 20 kt with gusts to 25 kt Thursday evening through the
weekend. There is a chance of some higher winds starting Friday
night and is discussed further below.

At this time, seas are beginning to rise. Overnight, seas will
build from 14 ft to 20 ft by Thursday morning in the outer and
northern waters. Central waters will still see sea rises, but
slightly lower to 18 ft. These are high energy waves with a
period of 16 seconds. With these high seas near the shore, there
is a chance for high surf Thursday afternoon. After the sea
height peak Thursday afternoon, seas will begin to ease leveling
off around 14 to 16 ft Thursday night with a period of 15
seconds. Heights will remain steady at this level through
Saturday, then they will ease further to around 10 ft.

One of the main challenges with this forecast is that models are
beginning to show a mesoscale low developing along the northern
Oregon/southern Washington coast Friday night. As of this
writing, the low is showing a motion inland over the Columbia
which would cause winds to increase (potentially to gale force
speeds) in the central Oregon waters. However, confidence remains
low as there is still a lack of model continuity between runs
and model types. If the low shifts northward, it could bring
gusty winds to all the waters; if it shifts south, it could bring
little to no increase to the winds to the waters. -Muessle


OR...High Surf Advisory from 11 AM to 11 PM PDT Thursday for Central
     Oregon Coast-North Oregon Coast.

WA...High Surf Advisory from 11 AM to 11 PM PDT Thursday for South
     Washington Coast.

PZ...Gale Warning until 5 PM PDT this afternoon for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until 5 PM PDT Thursday for Columbia River


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