Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland Oregon
925 am PDT Thu Oct 20 2016
Synopsis...a rather moist cold front will move through southwest
Washington and northwest Oregon for periods of rain and some coastal
breezes today. This front is expected to stall near the Lane County
Cascades this evening, then dissipate overnight as weak high pressure
builds over western Oregon. The next weak disturbance will bring a
chance for showers later Friday and Friday evening, with drier
weather possible Saturday. The next cold front will move onshore
Saturday night ahead of a broad, cool upper low from Alaska that will
set up off the Pacific northwest coast Sunday and continue off the coast
into the first half of next week.
Short term...today through Saturday...velocity data from the Langley
Hill radar clearly shows a cold front moving into the mouth of the
Columbia River region this morning. This front will slowly sag south
through the remainder of southwest Washington and northwest Oregon
today and tonight. There is one last main wave along the front that
will stall its southward movement some until later today and this
evening, so most of the forecast area will see a fair amount of rain
today. Precipitation behind the front will ease off as there are not
many showers immediately offshore to move onshore.
The front does stall some in eastern Lane and Linn counties tonight
and some precipitation may linger there tonight.
The main area of showers and instability associated with the parent
lowto our current front are currently located out around 50n/144w,
but this approaches our forecast area Friday, perhaps not spreading
onshore until Friday afternoon and Friday evening. The models
indicate some instability over the coastal waters and along the coast
and added some thunder there, but this is not indicated for inland
The next disturbance looks to spread a front toward the coast by late
Saturday and especially Saturday night, but inland areas are looking
relatively dry Saturday.
If clouds break up much tonight, we could see some local fog Friday
morning, but the best chance of fog is in the northwest part of our
forecast area. We may have a better chance of some fog Saturday
Long term...no changes. Previous discussion follows...
(saturday night through wednesday)...longwave pattern appears to
change little Saturday night through Wednesday, with the broad Gulf
of Alaska low forecasted by most models to reconfigure off the pac northwest
coast by Sunday night then linger there well into next week. Model
consensus shows the main baroclinic zone stretched north-south across
Washington/or, which would lead to rain at times and generally unsettled
weather well into next week. As long as the baroclinic zone remains
draped across our forecast area, there will be the potential for one
or more shortwaves to develop into a stronger surface low for
stronger coastal winds...but models vary greatly on the details of
any such system. Adjusted pops upward in general for the extended
forecast period, though future shifts may need to consider increasing
them more toward the midweek period. Temperatures should remain a few
degrees below normal well into next week. Weagle
Aviation...cold front now extends from Olympia to Astoria, then over
the coastal waters of north Oregon. This front will push farther
inland today. Widespread MVFR with rain across the region this am,
with areas of IFR along the coast. Will see an isolated thunderstorm
along the front as it moves onto the coast through 18z. Ceilings will
improve to VFR behind the front, though occasional MVFR visible in
rain/drizzle will continue a bit longer. Will maintain occasional
MVFR persisting against the Coast Range and lower slopes of the
Kpdx and approaches...MVFR with rain until 20z...with front moving
across the region 18z-19z. Will see ceilings gradually improve to VFR
through 22z, with most precipitation ending by that point. Clouds
breaking up after 02z...and may be enough clearing to allow fog or
low clouds to form after 09z. Rockey.
Marine...cold front now onshore in Washington, but still lies over
the Oregon coastal waters. This front will push onshore this am. Will
see southerly winds 20 to 25 kt ahead of the front, but will turn
westerly and subside rapidly behind the front. Seas running 8 to 10
ft...but will hold at 8 to 9 ft once winds ease.
Broad area of low pressure remains over the Gulf of Alaska into early
next week. This will maintain active storm track across the north pac
and into the pac northwest. End result is a series of fronts will continue
to move across the region. Next will be a weak front late Fri, with a
somewhat stronger front arriving late Sat and Sat night. Some
potential that that Sat system will bring a round of low-end
southerly gales, with seas back at 12 to 15 ft. Another front arrives
later sun. Rockey.
Pz...Small Craft Advisory for winds on all waters this am.
Small Craft Advisory for rough Columbia River bar until 11 am
today, and again this evening.
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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.