Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 252104
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
204 PM PDT Tue Jul 25 2017

.SYNOPSIS...An upper low over north California will slowly move
inland tonight and Wednesday providing a slight chance of
thunderstorms for the Oregon Cascades. Marine clouds will
fill in along the coast this evening and move into the north
Willamette Valley Wednesday morning. The marine clouds will move
further inland Wednesday and Thursday nights. Inland clearing is
expected each afternoon.

&&

.SHORT TERM...Tonight through Friday...An upper low over Northern
California will slowly move across NW California through Wednesday
providing a slight chance for thunderstorms for the Lane County
Cascades. Some cumulus clouds are starting to sprout over the
Cascades south and east of Lane County early this afternoon. Chances
are not high that thunderstorms will grow over Lane County this
afternoon, but the chance is too high to not continue a mention in
the forecast.

Satellite imagery shows extensive marine clouds well offshore that
should move into the Portland coastal waters this evening for clouds
returning to the coast tonight. Light northwest winds should move
these clouds through the lower Columbia River Valley into parts of
the north Willamette Valley (including the Portland metro area)
Wednesday morning. Additionally, stratus may form over the SW
Washington and NW Oregon Cascade foothills, and expand into parts of
the Central Willamette Valley.

The inland clouds will clear early Wednesday afternoon, but the
reduction in hours of solar heating due to the morning clouds will
result in slightly cooler afternoon temperatures. Inland temperatures
will peak in the mid 80s Wednesday afternoon with areas along the
lower Columbia River including Scapoose and Kelso peaking in the
upper 70s to low 80s.

Models have slowed the progress of the upper low moving inland, such
that the steering flow continues to be more southerly Wednesday
afternoon. This means the slight chance for thunderstorms continues
for the Oregon Cascades, and was added to the Oregon Cascade forecast
for Wednesday afternoon. The biggest threat, which is not very big,
is along the Cascade crest. With the upper low slightly further north
than it will be this afternoon have extended the slight chance of
thunderstorms north of Lane County to include the  Mount Jefferson
area.

The upper low moves east of the Cascades Wednesday night, and an
upper level shortwave trough approaches from the northwest. This will
increase onshore winds and favor marine clouds returning to the coast
Wednesday evening. A deeper marine layer will allow the marine clouds
to move further inland early Thursday morning. Expect the entire
Willamette Valley and Cascade foothills to have cloudy or mostly
cloudy skies Thursday morning. The clouds will clear in the
afternoon, but not as quick as the previous day, and the afternoon
temperatures will be noticeably cooler (low 80s).

The upper level heights rise slightly Thursday night and Friday. Some
marine clouds will move inland Friday morning, but do not expect as
deep of a push as Thursday morning. This will allow for earlier
clearing Friday afternoon and slightly warmer temperatures. ~TJ

.LONG TERM...Friday night through Monday...An upper level ridge
builds over the Pacific NW Friday night through Monday as a couple of
upper lows deepen over the NE Pacific. THe first low moves north of
canada Sunday with the second low moving into the Pacific on Monday.
This will result in continued onshore flow, minimal inland stratus,
and slightly warmer temperatures. ~TJ

&&

.AVIATION...VFR across the region early this afternoon. IFR to
low-end MVFR cigs return to the central Oregon coast by 03Z Wed
and to the North Oregon and South Washington coast between 03Z
and 06Z Wed. Models suggest a deeper inland stratus penetration
late tonight and Wed morning, likely pushing up the Columbia
River to KKLS around 09Z and portions of the Willamette Valley
east of Interstate 5 near sunrise.

There is also the potential for TCU or CB to drift over Lane
County Cascades through the evening hours. Cannot rule out a
threat of thunderstorms for the Central Cascades, but the
probability is low.

KPDX AND APPROACHES...VFR with thin cirrus through at least 12Z
Wed. Latest model guidance shows stratus reaching the terminal
and vicinity around 12Z. Weishaar

&&

.MARINE...Wind over the outer waters, generally beyond 30 nm,
will continue to gust to about 25 kt through late tonight.
Current small craft advisories look good. Wind over PZZ250 slow
to develop, but will maintain the small craft advisory for wind.
Several models, including those of higher resolution, show
decreased wind speeds Wed. The GFS has 20-25 kt boundary layer
wind speeds over the far south end of PZZ255 and PZZ275 Wed
afternoon. Several other models are not as strong so will keep
the forecast below small craft wind advisory thresholds. The low-
level flow becomes more NWly the latter half of the week, which
will keep wind speeds below 20 kt. Thermal low pres along the
South Oregon coast strengthens over the weekend, for potential
small craft advisory wind speeds over the south waters.

Highest wave heights, pre-dominantly wind-wave driven, will
be through tonight. Overall wave heights ease Wed with a
reduction in the wind- wave component. Seas 5 ft or less can be
expected Fri and Sat, then come up a bit Sun as wind speeds
strengthen. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until midnight PDT tonight for Waters from
     Cape Shoalwater WA to Cascade Head OR from 10 to 60 nm.

     Small Craft Advisory for winds until midnight PDT tonight for
     Coastal waters from Cape Shoalwater WA to Cascade Head OR
     out 10 nm.

     Small Craft Advisory until 2 AM PDT Wednesday for Waters from
     Cascade Head to Florence OR from 10 to 60 nm.

     Small Craft Advisory until 2 AM PDT Wednesday for Coastal
     waters from Cascade Head to Florence OR out 10 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.


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