Area Forecast Discussion
National Weather Service Portland OR
236 PM PDT Sun Aug 20 2017

Corrected typo in aviation section

.SYNOPSIS...Onshore flow will produce varying degrees of morning
clouds, afternoon sunshine. with inland temperatures in the 70s and
80s for much of the next week. Monday and Tuesday will likely be the
warmest days as a flat upper ridge moves through. A weak system will
bring significant cooling to the forecast area late Wednesday and
Thursday, as well as more clouds and a few light showers to the
forecast area Wednesday night and Thursday. Expect drying as
temperatures warm back to above seasonal normals levels next weekend.

&&

.SHORT TERM...Tonight through Wednesday...Other than some passing
higher clouds from a weak system passing by to our north, skies were
sunny over most of southwest Washington and northwest Oregon this
afternoon. Low clouds are consolidating along the south Washington
and north Oregon coasts this afternoon, largely due to the system
passing by to our north. Temperatures today are still on track to be
near or just above seasonal normals, with 80 to 85 common inland from
Vancouver southward.

As mentioned above, low clouds are solidifying off the coast this
afternoon and these will definitely push onto the south Washington
and north Oregon coasts tonight. The surface pressure pattern
suggests they may be less solid on the central coast around Newport
tonight, but viewing of the eclipse from Newport could be a little
iffy Monday morning. The low clouds will likely push inland along the
Columbia River later tonight and early on Monday to Kelso with some
clouds trying to reach the Vancouver and Portland areas. Believe the
clouds in the Vancouver and Portland area will mainly be scattered
before mostly clearing before the main part of the eclipse, though a
brief ceiling is possible near KTTD Monday morning. Farther southward
in the Willamette Valley conditions will stay mostly clear for the
eclipse on Monday morning. Some smoke from fires in the Oregon
Cascades could drift west on Monday but this will be slow to occur
and probably not affect eclipse viewing.

Otherwise Monday afternoon will be sunny as the morning clouds clear
to a little ways off the coast under a building upper ridge. The air
mass will be warming, and coastal temperatures could reach the lower
70s in areas, with upper 80s and lowers 90s common inland except mid
80s near Kelso.

The upper ridge starts to move off to the east Tuesday, but it is
still close enough for another warm day inland Tuesday with
temperatures in the mid 80s to around 90. We don`t expect much in the
way of lower clouds inland on Tuesday, though the coast will see
increasing lower clouds that may persist or hug the coastline much of
the day.

Southwest flow aloft will continue to increase Tuesday night and
Wednesday as the upper ridge continues moving eastward and the next
system approaches. This will mean persistent cloud cover at the
coast, perhaps even some spotty drizzle, and more morning low clouds
inland with resultant cooler temps. Temperatures inland will likely
be close to or a bit below seasonal normals Wednesday.

Some convection late Tuesday spreading north from a weak low over
California might clip the Willamette Pass area, but southwest flow
aloft will limit any coverage and how far north it can reach in our
Cascades as it mainly spreads into central and eastern Oregon. The
approach of the system Wednesday is making any chance of convection
in that area looking less likely as the southwest flow aloft and the
low level onshore flow increases. Tolleson

.LONG TERM...Wednesday night through Sunday...Models are in good
agreement a shortwave trough will slide eastward across British
Columbia and Washington Wednesday night and Thursday. This should
bring cooler temperatures, increased cloud cover and a chance of
light rain, particularly along the coast and across the Cascade
foothills, but even portions of the Willamette Valley could see some
light rain. Shortwave ridging should then build over the Pacific
Northwest and bring a return to more sunshine and above average
temperatures towards next weekend. /Neuman/Tolleson
&&

.AVIATION...VFR conditions remain at the coast and inland this
afternoon. Expect a layer of marine stratus to move into the north
coast this evening around 03-04z with MVFR cigs. Farther down the
coast, expect some stratus to move in early morning around 10-12z,
perhaps with broken IFR cigs for a few hours. Winds remain northerly
on the central coast tonight then becomes a little north-northeast
toward morning. Thus, confidence is not high in the amount of marine
stratus on central coast. Current forecast is for broken IFR cigs,
11z-16z. Also expect some marine stratus to move up the Columbia
River to around Kelso early Mon morning. There might get some clouds
near PORTLAND around sunrise, but likely just scattered for a couple of
hours, 12-16z time frame.

PORTLAND AND APPROACHES...VFR conditions tonight and Monday. Marine
stratus will push part way up the Columbia River tonight to around
KLS with MVFR or even IFR cigs. May get some low clouds near PORTLAND,
but likely just a scattered layer for a couple of hours, 12-16z time
frame.
&&

.MARINE...A persistent weather pattern will result in little change
in the marine weather the next several days. High pressure holds over
the NE Pacific while a thermal trough builds along the south Oregon
coast. This supports north winds across the south Washington and
north Oregon waters. The thermal trough strengthens each afternoon
such that the north winds will increase and become gusty each
afternoon. The winds south of Cascade Head will have frequent gusts
to around 25 kt which supports the current small craft advisory for
winds.

The seas will be locally generated from these winds and generally
remain 4 to 6 ft with periods at 9 to 10 seconds. The seas will be
choppier in the southern waters for the afternoons with dominant
periods possibly around 8 seconds. tw
&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory for winds until 11 PM PDT Monday for
     Coastal Waters from Cascade Head OR to Florence OR out 60
     nm.

     Small Craft Advisory for Rough Columbia River Bar from 3 AM to
     7 AM PDT Monday.
&&

$$



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.