Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 240356
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
855 PM PDT Sun Jul 23 2017

.SYNOPSIS...An upper level low near the northern California coast
will produce a chance of thunderstorms over the central Oregon
Cascades at times through Tuesday night into Wednesday morning.
Weaker onshore flow will lead to fewer low clouds tonight and Monday
morning and only a modest increase mainly at the coast Monday night
and Tuesday for continued moderate summer-time weather. Increasing
southwest flow aloft midweek as a low sets up in the NE Gulf of
Alaska will lead to increased onshore flow Wednesday into next
weekend for more morning low clouds especially in the north interior,
near seasonal temperatures, and a reduced threat of any thunder.
&&

.SHORT TERM...Tonight through Wednesday...Southwest Washington and
northwest Oregon are in between a system passing to the east through
southern Canada and another low near the northern California coast.
The system to the north will move eastward tonight with decreasing
onshore flow and just patchy low clouds or fog anywhere later tonight
and Monday morning, with the best chance at the coast.

The low near the northern California coast was producing
thunderstorms in Klamath and Lake Counties this evening, but nothing
farther west like the models having been suggesting. The movement of
the storms suggest that they will stay east of our forecast area but
will keep a small chance over the Lane County Cascade crest area,
though this is looking a bit doubtful right now.

The low to our south will remain along the northern California coast
Monday and move onshore Monday night and Tuesday, producing more
afternoon and evening thunderstorms that could reach our central
Oregon Cascades such as in Lane and possibly Linn Counties. The flow
aloft to our north remains light westerly, but the onshore flow will
remain a bit weak, with the best chance of low clouds or fog Monday
night and Tuesday morning being near the coast.

The low to our south will be moving eastward just south of the Oregon
border Tuesday night and Wednesday, and by Wednesday should be mostly
east enough to reduce the threat of thunder in our forecast area. The
models indicate increasing onshore flow and a possible marine push
that should lead to more low clouds inland Wednesday morning than on
previous days. This should knock inland temperatures down into the
mid 80s on Wednesday. Tolleson

.LONG TERM...No Changes. Previous discussion follows...
Wednesday night through Sunday...Onshore flow from a series of weak
shortwave troughs will continue through next week. This will maintain
near normal temperatures with low clouds at the coast as well as
inland in the nights and mornings. No chance of precipitation or
thunder during this time. -Bishop
&&

.AVIATION...VFR conditions across the area this evening. Will see
some areas of clouds develop along the coast later tonight, but will
likely not see widespread coastal clouds like we saw last night. With
that, reduced flight conditions at coastal terminals are less
certain. Still think that clouds could very impact the terminals
overnight tonight, bringing in IFR conditions. Clouds that develop
may drift in and out of the airports though overnight. Expect
conditions at coastal terminals to improve to VFR around 17-19Z.
Inland, expect VFR conditions to persist for the next 24 hours with
mostly clear skies.

KPDX AND APPROACHES...VFR for the next 24 hrs. Decreasing wind this
evening. N-NW winds ramping back up Monday afternoon with gusts 15-20
kt. -McCoy
&&

.MARINE...No changes. Previous discussion follows...
Small Craft Advisories remain posted for winds and very steep,
hazardous seas across all our waters as N 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 SCA winds. The gusty N 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 N
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 SCA 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...No Changes. Previous discussion follows...
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.

WA...None.

PZ...Small Craft Advisory for winds and hazardous seas until
     3 AM PDT Tuesday for Coastal Waters from Cape Shoalwater WA
     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.


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