Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 271646
Area Forecast Discussion
National Weather Service Portland OR
945 AM PDT Sat Aug 27 2016
.SYNOPSIS...Much cooler temperatures today and through the
upcoming week as onshore flow becomes dominant. A broad trough
centered along the B.C. coast this weekend brings westerly flow
aloft to the Pacific Northwest. The trough will probably dig south
near or just off the coast next week, however models continue to
waver on its strength and rain impacts.
.SHORT TERM...(Today through Monday Night)...Latest visible satellite
imagery reveals widespread marine stratus along the coast. A weak
front advancing from the northwest with bring some additional clouds
and perhaps a bit of light drizzle through the day along the coast,
particularly north. South of about Lincoln City, expect that the
clouds may push back to along the immediate coastline this afternoon.
With the general clearing inland, temperatures will still get up to
around or a few degrees above normal inland, warmest closer to
Eugene. Onshore flow continues tonight, deepening the marine layer
and enabling a more widespread inland intrusion of stratus. Little
change to the overall pattern through Sunday, as longwave trough
remains over the region with onshore flow.
Monday will bring a better stratus intrusion to the valley as the
surface onshore flow gets better support from winds aloft from the
upper trough digging offshore. However, models have been slowing the
progression and strength of the low with any precipitation threat
remaining offshore aside from continued periods of coastal drizzle.
.LONG TERM...(Tuesday through Friday)...Models continue to indicate
the upper trough will expand eastward to cover the PacNW by Tuesday
morning, although they have notable backed off on the strength since
24 hours ago. The 00z Runs of the GFS and the ECMWF now maintain a
lower amplitude trough aside from subtle east-west trough axis
position differences. The short wave previously progged to bring a
thunder threat to the Cascades has weakened and pushed any minimal
threat to the eastside so have removed the shower threat, let alone
thunderstorms from the grids. The next significant feature will be a
baroclinic zone associated with a slow moving frontal boundary also
now running slower than before with the eastern outskirts not
reaching the coast until late Tuesday afternoon at best with
potential rains holding on through Wednesday if the GFS pans out.
Again, both medium range models are trending weaker and would not be
surprised if further delays in timing and positional pushing further
north occurs over the next 48 hours of runs.
Evolution of the upper low has dramatically changed from 12-24 hours
ago. Now neither model is digging the upper low nor forming a closed
system off the California coast. Additionally, they are keeping a
drier westerly flow over the region for the bulk of late next week.
However, they are independently swinging seemingly random shortwaves
across the PacNW at time Wednesday through Friday so decided to keep
PoPs as is to indicate a general rain threat for the later part
of the extended. Temperatures continue to appear below normal,
however, did increase Wednesday night through Friday a fair amount
since the colder air mass will remain much further north. /JBonk
.AVIATION...Marine stratus pushed inland to the central Coast
Range this morning and has already begun its retreat to the coast.
Stratus will push back to near the beaches this afternoon.
Currently think both KAST and KONP will stay cloudy this
afternoon, but there is a chance skies will scatter out at KAST.
Stratus will build back inland tonight and will infiltrate at
least the northern half of the Willamette Valley.
KPDX AND APPROACHES...VFR conditions through Saturday. MVFR
ceilings move in between 10Z and 12Z Sunday and will remain
through mid day. /Bentley
.MARINE...Weaker high pres over the northeast Pacific will bring a
period of benign marine weather that will continue through the
weekend. Later Monday and Tuesday may see wind gusts around 20
knots ahead of a few fronts which may cross the waters. In
addition, seas will remain 6 feet or less this weekend and drop
below 5 feet early next week. /mh
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This discussion is for Northwest Oregon and Southwest Washington from
the Cascade crest to 60 nautical miles offshore. The area is commonly
referred to as the forecast area.