Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 252139

Area Forecast Discussion
National Weather Service Portland OR
239 PM PDT Mon Sep 25 2017

.SYNOPSIS...A weak upper level disturbance will bring a few sprinkles
or areas of light rain to SW Washington and NW Oregon through this
evening. High pressure will build into eastern WA/OR Tuesday,
resulting in offshore flow and warmer temps through Thursday. The
next chance for rain comes around Friday, as a weakening cold front
moves into the Pac NW. Showers and cooler temperatures may linger
into the weekend.


.SHORT TERM...Tonight through Thursday...KRTX radar is presently down
due to a mechanical failure, but surface observations continue to
show some spotty light rain or sprinkles across the forecast area.
Visible satellite imagery shows weak low pressure in the process of
occluding off the Pac NW coast, but still providing enough lift for
those showers/sprinkles this afternoon. Expect the low and associated
isentropic lift to continue to weaken through this evening,
eventually bringing an end to the spotty precip by tomorrow morning.

High pressure at the surface and aloft will continue to strengthen
off the coast Tuesday, then nose eastward into Washington Tue night
and Wed. Meanwhile, thermal low pressure will build north along the
Oregon Coast by Wednesday. This will induce some gusty offshore flow,
which will result in a decent warmup for Wednesday and Thursday. With
most models forecasting 850 mb temps around +17 deg C, this should
push most inland valley locations well up into the 80s Wed/Thu. As is
often the case, the coast will be trickier for forecasting temps,
depending highly on how long the offshore flow can push back on the
seabreeze Wednesday. With decent alignment of offshore flow from the
surface up through 700 mb, coastal areas should approach 80 degrees
Wed. This is reflected well in MOS guidance and our forecast high for
Wed is pretty close as well.

Models show thermal low pressure lingering near the Oregon coast Wed
night/Thu, while easterly flow aloft turns more southerly. This
should help induce a southerly wind reversal along the coast, likely
bringing areas of fog and low clouds back to coastal areas. With this
wind reversal and an approaching Pacific cold front, expect the
marine layer to deepen a bit late Thu/Thu night...perhaps enough to
bring some low clouds into the inland valleys by Friday morning.

.LONG TERM...Thursday night through Monday...Southwesterly flow
across the region Thursday night and Friday as an upper trough
advances through the forecast area. Forecast models in general
agreement with a weakening front pushing across the area. Boosted
PoPs a little higher as models have begun to increase consistency of
solutions and cohesiveness of frontal band. A brief period of zonal
flow returns Saturday from the combined lifting of the upper trough
and a return of a building ridge off the California coast. However,
models then diverge a bit for the first half of next week, so have
maintained some low end PoP chances for the start of next week in a
nod to the ECMWF solution which provides for a deeper upper trough
with more moisture (compared to the drier GFS solution). Expect that
temperatures will trend back to slightly below seasonal normals for
the weekend through early next week.   Cullen

.AVIATION...VFR cigs continuing lowering and will start
obstructing coastal terrain late this afternoon. The higher
Cascades will become obscured starting 00-04z this evening and
last through much of the night. Expect cigs to begin lifting
around 15z as flow starts turning northeasterly and finally
offshore by this time tomorrow. Clouds will likely dissipate by
mid-day for VFR conditions across all areas in the afternoon.

KPDX AND APPROACHES...VFR conditions expected through the period,
however, visual approaches look to be impacted around 04z
continuing through about 15z Tuesday morning. There is an outside
chance clouds could clear before daybreak and bring some thin
ground fog to the field. In the very unlikely (<10% probability)
event clouds clear several hours earlier and a thicker surface
stratus/fog event happens, LIFR conditions could affect
arrivals due to ongoing construction on 10R/28L. /JBonk

.MARINE...A weakening cold front will cross the waters this
evening with prefrontal southerlies gusting 15-20 kt or so.
Northerlies increase late in the day on Tuesday as a thermal
trough builds up the coast. The overall strength of these
northerlies has weakened with today`s model run and it`s looking
more marginal than before with gusts maybe hitting 23 kt. Will
forgo issuing an advisory for the time being. Regardless, the
thermal trough will drift offshore and weaken later Wednesday.
Beyond Wednesday, winds will remain light for the remainder of
the work week. Seas will remain 4 to 6 feet through the period.
Saturday may bring a pacific low off the B.C. coast for
increasing westerlies heading into the weekend. /JBonk





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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.