Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 182105

Area Forecast Discussion
National Weather Service Portland OR
205 PM PDT Fri Aug 18 2017

.SYNOPSIS...Onshore winds continue 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
early next week. The clouds should clear in the afternoons with
temperatures remaining near or above the seasonal normals. An upper
level ridge is expected early next week for less clouds and slightly
warmer temperatures. An upper level trough will bring a return slight
cooling, and possibly some showers late in the week.


.SHORT TERM...Tonight through Monday...The tail end of a weak cold
front will clip extreme SW Washington and NW Oregon this evening,
maintaining cloudy conditions for the coast and possibly producing
patchy drizzle or fog along the immediate coast. An upper level
trough over the area will result in a relatively deep marine layer
that will allow the marine clouds to seep inland through the coastal
gaps late tonight filling in parts of the Willamette Valley Saturday
morning. Clearing Saturday afternoon will result in near normal
afternoon temperatures (inland low 80s).

There will be little change Saturday night and Sunday as another weak
upper level trough moves over the area.  One change will be a lack of
a surface front Saturday night near the Washington and Oregon coast.
There is a warm front, but it is far north of the Portland forecast
area. So one difference in the forecast is there is no threat for
coastal drizzle or fog Saturday nigh and Sunday morning.

The more pressing concern for the short term forecast is the sky
forecast for Monday morning near the time of the solar eclipse
totality. The clouds are as tricky to forecast this day as any, with
the biggest challenges revolving around determining how widespread
the coastal clouds will be, and how far inland they will reach.

An upper level ridge builds Sunday night and Monday morning indicating
that the marine layer will be shallow, and limit the cloud ceiling
bases, and how far they can move inland.  However, the 12z models
show a weak upper level shortwave sneaking in under the ridge near
the time of the eclipse. They also show a surface front over the NE
Pacific. The models vary on the position and strength of this upper
wave, but seem to agree that any precipitation with the surface front
will remain well offshore.

However, clouds ahead of the front will likely move in along the
coast Sunday night and Monday morning, and some of the models are
forecasting light rain or drizzle associated with these clouds along
the coast. I doubt that any precipitation will materialize from these
clouds, but this indicates that there will be an increase of
low-level moisture.

An increase of low-level moisture may allow more clouds to develop
inland early Monday morning than we previously thought, mainly along
the SW Washington and north Oregon Cascade foothills. There is a
chance that these clouds will expand with north winds steering them
south into parts of the Central Willamette Valley. These
possibilities are not fully in the forecast yet, but it is a
possibility, and has decreased the forecast confidence for clear
skies in the Willamette Valley during the time of the eclipse.

The main reason this possibility is not in the current forecast, is
that the weather pattern expected for Sunday night andMonday is
almost identical to what we had last night (Thursday night) and this
morning (Friday). The clouds were persistent on the north coast this
morning, but the central Oregon coast was clear. The clouds moved
inland up the lower Columbia River briefly impacting Kelso and
Portland mid morning. Since the current weather pattern is so similar
to Monday`s, the clouds Monday morning will likely match what
happened this morning.

The best option for clear skies Monday morning continues to be east
of the Cascades pending areas near fires where smoke may be a
nuisance. ~TJ

.LONG TERM...Monday night through Thursday...An upper level trough
approaches Monday night and Tuesday from the NW enhancing the upper
ridge over the region. Meanwhile, an upper low settles over central
California. This will result in above normal afternoon temperatures
with possibly some Cascade showers in the afternoons. The trough will
move through Wednesday and Thursday and possibly bring a surface cold
front with it. Cooler temperatures with increasing chances for rain
is therefore expected late next week. ~TJ


.AVIATION...MVFR cigs will tend to persist near KAST this
afternoon, then spread southward with patchy drizzle along the
coast tonight and Saturday morning, possibly lowering to higher
end IFR in spots. Expect conditions to lift to VFR Saturday
afternoon on the central coast but may be slow to improve near
KAST until late. Inland will be VFR through this evening but then
become MVFR after midnight tonight except perhaps lowering to
MVFR prior to midnight near KKLS. Conditions in the inland
valleys will be mostly MVFR Saturday morning before becoming VFR
in the afternoon.

KPDX AND APPROACHES...VFR the remainder of this afternoon and
this evening. MVFR cigs may return not too far after 08z-10z
tonight and continue through the morning before becoming VFR by
the afternoon. pt

.MARINE...Expect a rather persistent pattern through early next
week with high pressure over the coastal waters and thermal low
pressure over NW California and SW Oregon. This will maintain
northerly winds, with gustier winds in afternoon/evenings,
primarily to south of Newport. A SCA for winds is now in effect
from this afternoon through Sat night for the central Oregon
coastal waters. The winds may ease a bit Sunday and Sunday night
then increase again Monday afternoon.

Seas will generally be around 5 to 6 ft through the period, but
may build to around 7 or 8 ft at times to south of Newport with
the stronger north winds. pt

PZ...Small Craft Advisory for winds until 5 AM PDT Sunday for
     Coastal Waters from Cascade Head OR to Florence OR out 60

     Small Craft Advisory for Rough Columbia River Bar from 2 AM to
     5 AM PDT Saturday.


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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.