Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 250353

Area Forecast Discussion
National Weather Service Portland OR
853 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...Strong high pressure over
the NE Pac is nosing toward the Pac NW and will eventually build into
the region later this week. However, a weak frontal system will ride
over the top of the ridge and move onshore tonight and Mon. Satellite
imagery shows broken high clouds now streaming onshore. The fcst
models indicate a weak warm front and associated isentropic lift will
move onto the coast overnight, bringing a very slight chance of rain
or drizzle to the north coast and coastal mountains. Expect clouds
will increase through the day, with the remnants of the cold front
coming onshore in the afternoon and evening. The fcst models suggest
a little better chance of rain with the cold front, with the
potential for some light precipitation spreading east of the coast
range. But the best chances will again be for the coast and Coast
Range. Areas that do see rainfall will see very light totals, on the
order of a trace to a few hundredths of an inch at most.

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. Pyle/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 continue through the evening.
Expect a return of IFR cigs/vsbys along the coast around 08z, but
across interior the coverage of IFR stratus likely similar to
this morning - that is, a few local areas, but lower confidence
in impacts to any particular terminal past KKLS/KHIO. Expect any
restrictions inland to return to VFR by 19z Monday, while IFR
conditions likely linger a little longer along the coastal areas.

KPDX AND APPROACHES...VFR conditions this evening and likely
through overnight period. Only 15 percent chance for lower MVFR
to IFR cigs developing around 12-17z Monday.   Cullen


.MARINE...Light north winds continue tonight over the waters. A
front will cross the waters on Monday with a shift to south winds
but speeds generally 15 knots or less. High pressure then
develops over the waters Tuesday, with a return of north winds
and the potential for advisory strength winds returning by
Tuesday night south of Cascade Head and spreading north early
Wednesday. Seas remain generally 4 to 6 feet through the first
half of the week. Cullen





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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.