Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 242147

Area Forecast Discussion
National Weather Service Portland OR
247 PM PDT Sun Sep 24 2017

.SYNOPSIS...A weak front arrives Monday, with light rain
brushing southwest Washington and far northwest Oregon. High
pressure builds later Tuesday and remains over the region through
Thursday and possibly next weekend. Temperatures will peak in the
mid-80s under offshore flow.

.SHORT TERM...Tonight through Wednesday...High pressure is building
offshore with the axis a little west of 140W. A shortwave will ride
along the north end of the ridge then drop south over the region as
the ridge axis drifts to 130W tomorrow. There is a consensus of
models showing a few to several hundredths of rain along the far
north coastal area/ north coast range/Willapa hills between late
tomorrow afternoon and early tomorrow evening, so have pushed pops
up to at least the likely category. Still have significant
reservations about how far inland this system will bring rain as it
falls apart under the anti-cyclonic motion. Thus, have continued a
limited PoP mention for areas east of I-5. Main timing focus remains
during the early evening hours.

The upper ridge continues to build north Tuesday then continues to
drift east Wednesday. A thermal trough pushes north along the
Cascades Tuesday then drifts westward Tuesday night becoming
established over western Oregon Wednesday. This results in easterly
flow across the Cascades to the valley with a secondary node of the
thermal trough bringing weaker flow from the valley and across the
coast range to the coast. This will bring temperatures into the
upper 70s Tuesday and into the lower to mid 80s Wednesday.
Additionally, the coast will see temperatures pushing 80 Wednesday
along the north coast where the winds will be most enhanced by the
Mt. Hood/Mt. Adams gap followed by the lower Columbia gap. /JBonk

.LONG TERM...Wednesday night through Sunday...The upper ridge
continues getting pushed east by  a high amplitude upper trough
digging well offshore. The surface thermal trough axis will likely
weaken Wednesday night but could still exert enough influence to
hold onshore flow at bay Thursday.

Southwesterly flow aloft dominates the flow by Thursday night with
the ECMWF being the most aggressive at advancing the upper trough
over the area. Both the GFS and Euro swing a weak front toward the
area Thursday night and early Friday with the GFS showing the
frontal energy and associated precip dissipating before the front
hits land. Added in some low end PoPs for Thursday night and Friday
as a slight nod toward the non-zero rain threat, but potential QPF
remains meager at best.

A brief period of zonal flow is in play for Saturday and Sunday from
the combined lifting of the trough and a return of a building ridge
off the California coast. May get another weak shortwave or two
crossing during this time for very low end PoP chances, mainly
across the north. /JBonk &&

.AVIATION...VFR conditions through early evening then some IFR
CIG and/or vis developing especially along the coast. Expect IFR
coverage elsewhere to be similar to this morning, not quite making
it to PDX or most of the Central or southern Willamette Valley.
Conditions will again improve to VFR by 19Z inland and 21Z at the
coast, except possibly later on the north coast around KAST.

KPDX AND APPROACHES...VFR conditions through today. Small chance for
MVFR CIG after 12Z Monday. Bowen


.MARINE...Quiet marine weather continues into next week with
light winds less than 15 knots and seas less than 6 feet. North
winds will start to increase late in the day on Tuesday as the
thermal tough builds up the coast. Winds are likely to rise above 20
kt Tuesday afternoon beginning in the southern waters before
expanding northward. These gusty winds will likely be short lived as
the gradient relaxes and winds switch to offshore by late in the day
Wednesday. Bowen




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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.