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Area forecast discussion
National Weather Service Portland or
236 PM PDT sun Jul 23 2017 upper level low offshore of northern California will
increase mid-level moisture and instability over northwest Oregon for
a threat of thunderstorms over the Lane County Cascades this
afternoon through Wednesday. Less clouds tonight will lead to
slightly warmer temperatures Monday. There will be little change in
the weather Tuesday as the upper low slowly moves inland across
northwest California and central Oregon. Weak troughing will increase
onshore flow Wednesday through Saturday.


Short term...tonight through Wednesday...the Portland forecast area
is sandwiched between two upper level lows, one offshore of
north.California and the other over west.Canada. The northern low will move
east tonight and have little influence on the local weather. The
southern low will slowly move inland across northwest California Monday and
Tuesday before moving across east.Oregon on Wednesday. The models have
sped up the inland progression of this low around 24 hours than the
previous runs. This low will increase mid-level moisture and
instability mainly south of the area through Tuesday, with the
possibility for thunderstorms over the Lane County Cascades.

At the surface, weak high pressure is over the NE Pacific with low
pressure over north. California and weaker low pressure east of the
Cascades for north-northwest low-level winds. Expect less stratus inland
tonight, with a clear start to Monday supporting temperatures warming
into the upper 80s to low 90s.

Thunderstorms may drift west of the Cascades into the Cascade
foothills of Lane County Monday afternoon and evening, but should
otherwise remain confined to near the Cascade crest. There is
potential for elevated convection with these storms which would allow
them to go all night into Tuesday morning.

There will be little change to the weather Tuesday with the threat of
convection continuing for the central Oregon Cascades. The low-level
winds become more onshore Tuesday night into Wednesday morning, and
expect more marine clouds to move inland Wednesday morning, mainly
impacting the north Willamette Valley. These clouds will clear in the
afternoon for another warm and mostly sunny afternoon.The
thunderstorm threat becomes more confined to the Cascade crest
Wednesday as the upper low moves further east and the flow aloft
becomes more westerly. ~Tj

Long term...Wednesday night through Sunday...onshore flow from a
series of weak shortwave troughs will continue through next week.
This will maintain near normal temperatures. No chance of
precipitation during this time. -Bishop


Aviation...conditions across the forecast area are predominately
VFR this afternoon under mostly clear skies. There is still some
localized MVFR stratus in the Cascade foothill near kttd and
along the Columbia River near kast, but this stratus is expected
to clear later this afternoon. Gusty north-northwest winds will continue
into this evening, with gusts 25-35 kt along the coast and gusts
20-30 kt inland. Onshore flow will bring the return of low
stratus to the coast after 04z, with MVFR cigs around 1500 ft
likely. Another round of localized IFR fog is possible along the
coast early Monday morning, especially in sheltered valleys.
Otherwise, conditions inland will remain VFR under mostly clear
skies the next 24 hours.

Kpdx and approaches...predominately VFR the next 24 hours under
mostly clear skies. Gusty north-northwest will continue through around 05z,
with gusts 20-25 kt possible. /64


Marine...small craft advisories remain posted for winds and very
steep, hazardous seas across all our waters as north winds continue
to increase. Models continue to hint at the possibility of a few
local low-end gale force gusts over our southern waters this
evening, but they are not widespread enough to warrant a Gale
Warning. Winds will be slightly lighter in our northern waters,
but still strong enough to produce solid Small Craft Advisory winds. The gusty north
winds continue to produce steep seas this afternoon, with buoys
reporting 6 to 7 ft seas at 7 seconds. Seas are expected to
continue to build this evening as north winds persist, with guidance
suggesting seas building towards 9 to 11 ft in our southernmost
waters near Florence. These seas will remain predominately wind-
driven, with dominant periods around 8 seconds.

Models continue to delay the arrival of the upper trough which
would start the easing trend in winds. Winds on Monday remains
nearly as strong as today, especially for our southern waters.
Decided to extend the current Small Craft Advisory for winds/seas across all our
waters through Monday night. Gusty northerly winds look to
continue into Tuesday across our outer waters, though winds over
the northern waters will be slightly weaker due shortwave
activity near the Canadian border. High pressure over the NE
Pacific looks to weaken later this week as the upper trough moves
closer. This should bring a gradual decrease in winds/seas later
this week. /64


Fire weather...first critical fire weather pattern for the
forecast area in the short term. Water vapor imagery shows an upper
low centered a little west of kacv early this afternoon. Not a lot
of moisture within the low vicinity but plenty of mid and high-level
moisture spreading into southern Oregon. Models show the upper low
generally stationary tonight and Mon with 700 mb southeast to south
flow spreading into the south part of the forecast area Mon. There
will be a definitive boundary between the unstable and moist south
flow aloft over the southern areas and the stable west flow to the
north. NAM and GFS show convective differences for Mon afternoon
through Mon night. The GFS is a little more robust with the cape,
pushing up to 1500 j/kg over zone 608 00z Tue. Pattern recognition
would suggest Cascade areas south of Mt. Jefferson showing the best
convective potential. Have opted to go with a Fire Weather Watch for
nearly all of zone 608 valid 21z Mon through 07z Tue.

Convective threat continues Tue afternoon through Tue night, but
latest model run has shunted the best convective instability more to
the south. Think the threat could get to Mt. Jefferson. However, if
the latest NAM is right showing the upper low tracking across
northern California then there would be less of a threat for the
southern inland zones. Another watch may be needed for Tue
afternoon, but will let next shift or two mull that decision. The
low opens up and crosses the Cascades late Tue night or around 12z
Wed, which maintains some convective threat for the Oregon Cascades
through Wed morning. Weishaar


Pqr watches/warnings/advisories...
or...Fire Weather Watch from Monday afternoon through Monday evening
for Willamette National Forest.


Pz...Small Craft Advisory for winds and hazardous seas until
3 am PDT Tuesday for coastal waters from Cape Shoalwater Washington
to Florence or out 60 nm.


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