Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 210411 CCA

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

.SYNOPSIS...A weak upper level disturbance is departing the area,
with today`s mid and high clouds on the decrease. High pressure
builds in on Monday and Tuesday, for warming temperatures and clear
skies inland, and occasional low clouds in northerly flow along the
coast. Another 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...Satellite imagery shows a
relatively narrow band of stratus just offshore, extending from
southwest WA through central Oregon. Very little change in the wind
direction overnight along the coast, and based on the latest HRRR and
00Z NAM low-level cloud, ceiling, and RH forecasts, expect this
stratus band to persist along the beaches and just offshore. Right
now, skies scattered to mostly clear over coastal ASOS sites, and
little change to the northerly winds will keep the stratus near the
coastline, with a push into the lower Columbia River valley. This
evening`s HRRR trends indicate a slightly better than even chance to
view the eclipse along coastal sites by mid morning. For a high
probability though, inland locations are a much better bet. In the
Willamette Valley, the flow above the surface veers to
north-northeasterly overnight with some drying above 950 mb. There
could be some patchy thin stratus far northern Willamette Valley and
Cascade foothills before 16Z, but overall should be minimal and
short-lived with clear skies thereafter. The remainder of the
forecast discussion is unchanged from previous AFD. DRB

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...A sliver of low clouds have developed along the
immediate coast. These clouds have not made it to the coastal
ASOS sites this evening, but web cameras and a few observations
at KONP suggest the cloud layer is shallow with very low cigs
(200-500 ft AGL). Expect IFR conditions to impact KAST and KONP
tonight through Monday morning. Light east winds along the
central Oregon coast may clear the skies near KONP late in the
morning. The winds should remain N-NW north of KTMK keeping
cloudy skies with CIGS lifting to MVFR late in the morning. Low
clouds will likely develop in the lower Columbia River with MVFR
cigs for KSPB and KKLS for a few hours in the morning. Otherwise
VFR conditions for inland areas. Gusty north winds expected near
KONP, KCVO, and KEUG Monday afternoon.

KPDX 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 KPDX, but likely just a scattered layer for a couple of
hours, 12-16z time frame. ~TJ


.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. The thermal trough strengthens
each afternoon such that the north winds will increase and become
gusty each afternoon. The winds south of Cascade Head have gusted
to around 25 kt the past couple of afternoons and evenings and
will continue to do so through Monday, supporting the current
small craft advisory for winds. These winds have been most
noticeable within 10 nm, but can not rule out the possibility for
increased winds further out so will continue to have the advisory
for out to 60 nm south of Cascade Head.

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


PZ...Small Craft Advisory for winds until 11 PM PDT Monday for
     Coastal Waters from Cascade Head OR to Florence OR out 60

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



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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. is the U.S. government's official web portal to all federal, state and local government web resources and services.