Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 242141

Area Forecast Discussion
National Weather Service Portland OR
242 pM PDT Tue May 24 2016

.SYNOPSIS...A weak ridge of high pres aloft off the coast Tue
afternoon will give way to a nw flow aloft by Wed. Moist onshore
flow will increase again Wed and Thu returning a deep marine layer
that will result in some night and morning drizzle or light rain in
some areas, and isolated showers in the afternoons. Another low
pressure system is expected to drop down the coast from Alaska later
in the week, bringing showers and cooler temperatures to the region
for Memorial Day weekend.


.SHORT TERM...(Today through Thursday)...Radar showed scattered
showers mainly over the mountains Tue afternoon, with coverage
considerably reduced from the previous few days. The axis of the
upper trough that brought the showery weather stretched from the
northern Plains back across northern CA. The sw end of the trough is
expected to rotate further to the se, resulting an a nw flow aloft
strengthening over the next two days. A couple of weak shortwaves
Wed morning and Thu morning will tend to strengthen the low level
onshore flow as well, with a weak cold front moving in from the nw
Thu morning. This will result in replenishment of the marine air
mass, with temps progressively cooling by a few degrees each day.
With better low level moisture and the strengthened onshore flow,
expect to see some light rain or drizzle develop for the morning
hours each day, esp over orographically favored terrain in the n.
Shallow instability will also return a threat for afternoon and
evening showers.

thu night and fri gfs and ec in close agreement in bringing down a
closed low aloft as far s as wa. With a little better dynamics
associated with the low and a little deeper low level moisture
chances for precipitation will increase across the forecast area by
fri, but with best chances remaining in the orographically favored
parts of the n. With a thicker cloud cover and better chances for
rain, daytime temps will be on the cool side.

.LONG TERM...Friday night through Tuesday. By late Friday night, the
upper low over southern B.C. continues to advance south into eastern
Washington. Latest models suggest a slightly more inland (easterly)
track, but with a slowly evolving upper trough remaining over the
Pacific Northwest through the holiday weekend. Plenty of spread in
ensemble members as well as a lack of run-to-run consistency over the
past 24 hours has maintained lower than typical confidence in the
particular details of the forecast. However, the general trend does
support the building of a weak ridge over the eastern Pacific and
into the region on Sunday and Monday. So while a shower cannot be
ruled out through Monday for most locations, confidence is increased
in the trend of tapering moisture in the increasingly northwest flow
aloft. As such, edited the PoPs to reflect a chance of showers
Saturday transitioning to slight chance south/chance north Sunday,
then only a slight chance north for Monday. Also expect temperatures
to gradually trend warmer through the extended period - from a few
degrees below normal Saturday to near or a few degrees above normal
on Monday.  Cullen


.AVIATION...MVFR stratus was slow to develop and slow to clear
today. Still have MVFR cigs at KHIO, but all other terminals in
our area are now under VFR conditions. Onshore flow continues, so
we will see cigs lower to MVFR along the Coast this evening. With
a deeper marine layer, will probably see MVFR cigs along the Coast
overnight through Wednesday morning. Inland, cigs expected to
lower to MVFR at all sites late tonight or early Wednesday
morning. Should see cigs start to improve inland by 18Z. Along the
Coast right now the models show KAST may continue to see MVFR cigs
all day, while KONP has a better chance of improving to VFR by
around 20Z.

PDX AND APPROACHES...VFR conditions this afternoon through this
evening. Good chance of cigs lowering to MVFR with bases around
2000-2500 ft by 11-12Z Wed morning. Cigs should lift to VFR by
around 18Z                                                -McCoy


.MARINE...Currently seeing fairly benign conditions across the
waters this afternoon with seas around 5 to 6 feet. High pressure
expected to remain in place over the Northeast Pacific through
Friday. This means northerly winds over the waters, increasing to
15 to 20 knots during the afternoons and evenings through Friday.
Out west 20 nm offshore may see a few gusts to 25 knots. Fresh
swell from a decent fetch of northwesterly winds will allow seas
to build to near 7 feet tomorrow afternoon. This will also shorten
dominant wave periods to around 8 seconds. This is fairly steep
and approaching criteria for a small craft advisory for hazardous
seas. There is a chance seas in the northern waters out west of 10
nm sees seas get up around 8 ft, especially farther out closer to
Buoy 89. This would meet the square seas criteria for an
advisory, but holding off on the advisory for now due to some
uncertainty. Will see a chance for square seas in the northern
outer waters again Thursday afternoon and evening. Fetch starts to
break down on Friday so may not see seas get quite as high.

Over the weekend the forecast is more uncertain. There is a system
moving through which, depending on the track, will either disrupt
this pattern of northerly winds or just keep northerly winds a bit
weaker over the weekend. In regards to seas, combined seas will
probably stay below 10 ft through the weekend.            -McCoy




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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. is the U.S. government's official web portal to all federal, state and local government web resources and services.