Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 170402

Area Forecast Discussion
National Weather Service Portland OR
901 PM PDT Wed Aug 16 2017

.SYNOPSIS...Persistent surface high pressure over the NE Pacific
combined with low pressure east of the Cascades will maintain onshore
winds the next several days. Slight variations of the upper level
pressure pattern will result in slight variations in how expansive
nighttime/morning clouds will be through the weekend. The clouds
should clear in the afternoons with temperatures remaining near the
seasonal normals. An upper level ridge is expected early next week
for less clouds and slightly warmer temperatures.


.SHORT TERM...Tonight through Saturday...Satellite imagery shows
extensive clouds over the NE Pacific this evening beginning to move
over the SW Washington and NW Oregon coast and coast range. These
clouds seem to be remnants of an old front traveling across the
Pacific. Onshore winds will continue to transport these clouds
towards the Washington and Oregon coast tonight filling in these
areas with low clouds. A passing weak upper level shortwave trough
north of the area has deepened the marine layer enough for some light
drizzle or rain to reach the coast.

Some of these clouds to move through the coast range gaps towards the
Willamette Valley late tonight into Thursday morning. There is also a
possibility for clouds to form along the South Washington and North
Oregon Cascade foothills late tonight and expand south along the
foothills into parts of the north and central Willamette Valley
Thursday morning.   The best chance for Valley clouds Thursday morning
is for the north and central Willamette Valley with the least
confidence for clouds in the south Willamette Valley. The clouds
should clear in the afternoon with inland temperatures warming to
near the seasonal normals (low 80s north, and mid 80s south).

There will be little change in the weather Thursday night and Friday
as another upper level trough passes by to the north and low-level
onshore winds persist. The marine layer should deepen late Friday
night and early Saturday morning as an upper level trough develops
over NW Oregon for a better chance for more widespread clouds
Saturday morning.  Therefore the temperatures Saturday afternoon will
be slightly cooler. ~TJ

.LONG TERM...No Changes. Previous discussion follows...Saturday night
through Wednesday. Overall, rather benign pattern with varying
degrees of onshore flow for the weekend. Weak system will pass to
over region on Saturday, but seems system is splitting. This will
keep local mostly dry, with threat of any showers staying well north
over British Columbia. But this system will allow for increased
onshore flow, with some spotty drizzle along the the coast. Mainly,
expect more in way of clouds around region for Sat, and again later
Sat night into Sun.

Now, for the big question...what about Monday?At this time, looks
like will slip back into the pattern we have had the past few days.
This will keep threat of late night and morning clouds along the
coast, mainly to north of Newport. But, interior looks to remain
mostly clear on Mon through can not rule a few clouds along and to
north of the Columbia River prior to 10 am. Temperatures staying
near seasonable.

But changes appear in store for the region for Tue and beyond. Upper
low off Calif coast will start drifting northward, bringing a return
to southerly flow over the region. This will allow for increasing
threat of afternoon/evening thunderstorms over the Oregon Cascades.
Temperatures still near seasonable levels.   Rockey.

.AVIATION...A decaying frontal boundary will bring increased
clouds onshore later tonight into Thu morning. IFR conditions
with drizzle have already developed at KAST, and expect the
clouds to push onto the rest of the coast by around midnight. The
clouds also appear on satellite to be pushing fairly quickly up
the lower Columbia, so expect they will easily reach the northern
interior TAF sites. The latest model guidance suggests MVFR
conditions reach KPDX, KTTD, and KHIO by around 09Z, which seems
reasonable. Confidence is lower as to conditions from around
KSLE and areas southward. Think KSLE will see a few hours of MVFR
early Thu morning, but KEUG may remain VFR. The clouds may last
until around midday for the northern interior, and well into the
afternoon at the coast.

KPDX AND APPROACHES...Expect MVFR cigs to reach the terminal
by about 09Z, then linger through most of Thu morning. Clouds
should break up and give way to VFR in the afternoon and evening.


.MARINE...High pres will remain centered over the coastal waters
through the weekend, with thermal low pres over northern CA and
south west OR. This will maintain the summer-time northerlies,
with breezy winds developing during the afternoon/evening hours
over the next several days, especially south of Newport. Decided
to drop the small craft advisory for winds in effect this
evening, as the latest obs were not supporting it any more. Have
also issued a new advisory for tomorrow afternoon into early Fri
morning. T

Seas will generally be around 5 to 7 ft through the period, but
may build to around 8 ft at times as the winds increase. Seas may
also become steep at times during the afternoon/evening hours.


PZ...Small Craft Advisory for winds from 1 PM Thursday to 5 AM PDT
     Friday for Coastal Waters from Cascade Head OR to Florence
     OR out 60 nm.


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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.