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Area forecast discussion 
National Weather Service Portland Oregon
236 PM PDT Thu Oct 27 2016

Synopsis...a cold front draped across the pac northwest is finally
beginning to accelerate eastward across the Cascades this afternoon,
as a weak upper level trough offshore moves toward the southern Washington
coast. Steadier rain is tapering off as a result, but additional
showers are possible this afternoon as the upper trough moves
onshore. Showers should decrease this evening, with a lull in
precipitation expected Friday. The next low is expected to set up
off the coast Friday night, with another round of unsettled weather
expected through the weekend. Split flow will likely keep any of the
more organized systems from impacting our area through early next
week, but there are some hints that the pattern could become more
active again toward the end of next week.


Short term...(tonight through sunday)...our stubborn frontal system
is finally beginning to gain some eastward momentum this afternoon as
a shortwave upper trough approaches the Long Beach peninsula from the
southwest. Lightning data overlaid on water vapor imagery shows a few
strikes occurring within the core of the upper level cold pool, so
still cannot rule out some thunder reaching our north coastal zones
through early evening. Some sunbreaks are also developing inland, but
the air mass aloft is likely a little too warm for deep convection in
the Willamette Valley. Showers should decrease overnight, though a
following disturbance could continue to clip the north coast with a
few showers after midnight.

Given that some sunbreaks are already occurring, some degree of
clearing is expected inland overnight despite rather light flow and
moist air at the low levels. Given the saturated ground, it probably
will not take long for valley fog to form where clearing occurs.
Morning fog may take some time to burn off Friday, which could hinder
afternoon temperatures from getting much above 60 degrees. Some
easterly flow may develop through the Columbia Gorge by the end of
the day, but at this point it looks like it will be a little too late
to push temperatures much further into the 60s. Generally expect a
partly sunny day Friday with near normal temperatures.

Low pressure and another frontal zone will be developing offshore
Fri/Fri night, as another broad upper low drops southeast from the Gulf of
Alaska. It does not appear that this frontal zone will have the same
moisture tap as its predecessor, so rainfall amounts should be fairly
light with the front as it moves across our County Warning Area Fri night into
Saturday. 12z deterministic models show reasonable agreement in
bringing the upper low onshore Sat night/sun, which should lead to a
cool and showery weekend overall...especially Sunday. Snow levels may
flirt with the Cascade passes Sunday as the upper trough, but any
potential accumulations appear light at this point. Weagle

Long term...(sunday night through thursday)...overall, a cooler
pattern looks to persist through much of next week, through a split
flow pattern will prevent any of the more organized systems from
tracking into the area. However, some precipitation generally
continues as a broad trough remains in place, with a more notable
break possible later Monday through Tuesday as models try to build
some weak, transient ridging across the Pacific northwest. However, some of
this will be dependent on the evolution of a disturbance that is
expected to move into California by early Tuesday. Therefore, have
made little adjustment to the forecast through the first part of
next week. A more organized frontal system will approach the region
around Wednesday, but with some timing differences between forecast
models, utilized a blend of the faster European model (ecmwf) and slower GFS to
depict increasing pops by Wednesday evening. Cullen


Aviation...cold front now progressing eastward and is about ready
to cross the Cascade crest at this hour. Convection is starting to
increase just off the coast and expect breaks in the inland cloud
cover should begin to trigger stronger shower activity there.
Based on current cloud cover, initial cell initiation should
occur along the I-5 corridor between roughly kuao south to keug.
Still cannot completely rule out a sporadic thunderstorm, but it
does appear any that do develop will likely produce less than a
handful of strikes. Showers and thunder threat will quickly
diminish close to sunset. Expect some lingering debris clouds for
a good portion of the night which then allows for a threat of fog
and/or low MVFR stratus. For now, only have 1/2 sm fog for the
most likely places of khio and keug, and sct 010-015 cigs inland
elsewhere. Exception is kttd where a broken deck at 015 seems most
likely. Feel conditions will improve back to MVFR around 17z for
keug and khio with the broader area returning to VFR around
19-20z. Coastal terminals, except perhaps ktmk, will likely see
enough wind overnight to prohibit much long-term low stratus/fog

Kpdx and approaches...VFR, however, could see MVFR cigs under a
few heavier showers with an outside chance of a thunderstorm
within 20 nm mainly 20z-03z. Increasing threat for a 010-020
stratus deck to develop later overnight. If it does fully develop,
then expect clearing around 19z. /Jbonk


Marine...broad upper low will be the dominant player through the
weekend with surface lows or fronts bringing Small Craft
Advisory level winds to the waters at some point each day. Nearly
square seas now will gradually ease tonight and tomorrow morning
before another 10-12 ft longer period westerly swell arrives early
Saturday. Models disagree with regard to a surface low approaching
the waters early next week but could see either about a 48 hour
break in winds or another couple days of gusts around 25 kt.
Stronger surface lows and seas back into the teens will likely
return for the Wednesday-Friday timeframe. /Jbonk


Pqr watches/warnings/advisories...
Pz...Small Craft Advisory for winds and hazardous seas until 1 PM
PDT Friday for coastal waters from Cape Shoalwater Washington to
Cascade Head or out 60 nm.

Small Craft Advisory winds and hazardous seas until 6 am PDT
Friday for coastal waters from Cascade Head or to Florence or
out 60 nm.

Small Craft Advisory for rough Columbia River bar until 5 PM
PDT this afternoon.



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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.

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