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National Weather Service Area Forecast Discussion for Seaside, OR USA

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Area Forecast Discussion
National Weather Service Portland, OR or
912 am pst fri nov 17 2017

.synopsis...showers will slowly diminish today and tonight as an
upper level trough crosses the region this morning, then a ridge of
high pres builds in. the ridge lingers into the weekend, bringing
dry weather for most of the region sat and sat night. another front
crosses sunday and sunday night bringing moderate rain with moderate
cascade snow, mainly above the passes. next week looks to be wet
overall, but forecast confidence in specifics declines tuesday
onward.

&&

.short term...today through sunday...main changes for the update
package were to ease off the showers faster this afternoon through
tomorrow. the upper ridge is building a little faster and stronger
than models indicated this time yesterday. if nothing else, looks
like there will be far more sun today than the last few days, though
still not enough to trigger deep convection as strong mid-level
warming begins in earnest this afternoon.

it now appears that the entire cwa will remain dry tomorrow with the
far northwest corner of the area being the last to dry out around
daybreak. also saw a bit of fog near sweet home and hillsboro so
added patchy fog those areas ending around 10 am.

looking ahead a bit, we may eventually need to issue a winter
weather advisory for snow above 5500 feet covering the cascades for
sunday. forecast updates will be available shortly. /jbonk

previous discussion follows: water vapor pictures showed the upper
trough approaching the coast early this morning. radar showed
widespread showers still over the forecast area, and likely to
continue that way for a few hours, but as the trough moves across
this morning showers will decrease. satellite showed scattered
showers behind the trough offshore, so cool air aloft will keep
enough instability this afternoon and evening for chances of
showers, but as the onshore flow also weakens later today and
tonight even the orographic showers will tend to diminish. a few
inches of snow accumulations can still be expected over the cascades
today, with the orographics favoring the oregon part of the cascades
in the westerly flow. recent trends in snow accumulations coupled
with the passage of the upper trough and weakening orographics
however suggest advisories snow not likely to be needed today, so
will go ahead and drop those.

as a surface high moves across southern oregon today and tonight,
the low level flow ultimately turns offshore for sat. under the
upper ridging, this will bring an end to showers for most of the
area, and allow some sun breaks to return. over the nw corner of the
forecast area a ssw onshore flow may make it difficult to completely
end the threat of showers sat and sat night. on sun the progressive
pattern continues, with another trough of low pres approaching
offshore. a sw flow will bring increasing moisture for clouds, and
for the nw part of the forecast area low level moist isentropic
flow, seen along the 290k isentrope, brings an increasing chance for
rain.

.long term...sunday night through thanksgiving day. no changes.
previous discussion follows. a rather moist frontal system will
continue to move across the region sunday night through early
monday. rain rates may be rather impressive, but models are in
decent agreement and suggest that the front should push through the
region fairly quickly as opposed to stalling over the area. weak
shortwave ridging may provide a very brief dry period later monday,
but the next frontal system will then quickly approach by tuesday.

while the forecast models have begun to cluster a bit more for the
first half of the week, differences do emerge around midweek.
however, the key difference remains with the timing and placement of
the next front as it approaches as soon as tuesday or perhaps later
into wednesday. there is a fair amount of moisture with this system,
so there could be implications on those who are planning travel in
advance of the holiday next week. for now, have focused the heaviest
rain tuesday night through early wednesday with this front, but snow
levels look to remain high (at least 5,000 feet) until at least
thursday morning.   cullen

&&

.aviation...showery weather continues through this afternoon.
flight conditions will be mostly vfr with occasional mvfr cigs
with the heavier showers. the showers will be mainly confined to
the south wa and north or coast and the cascades this evening and
tonight. patchy fog is possible for the interior valleys late
tonight into saturday morning. keug and kcvo are the most likely
airports to see fog since they have a better chance for clearing
skies late tonight.

kpdx and approaches...there will be little change in flight
conditions today. showers will continue through this afternoon
with mostly vfr conditions. there will be occasional mvfr
ceilings with the heavier showers, but the chance for this
decreases throughout the day. showers should end this evening.
bowen/tj

&&

.marine...low pressure has exited the waters this morning and
winds are decreasing behind it as high pressure begins to build.
seas in the waters off the northern oregon coast remain steep to
square, so will keep small craft advisory for hazardous seas.
however, even seas should improve this afternoon with decreased
winds. winds become southerly saturday as the next front
approaches. this front will likely generate gales or possibly
storm force winds on sunday with seas approaching 20 ft. a low
moving along the front will slow its progress and the strong
winds and seas may remain across the waters south of cascade head
through monday morning. another front is expected tuesday with
another chance for gales. bowen/tj

&&

.pqr watches/warnings/advisories...
or...none.
wa...none.
pz...small craft advisory for hazardous seas until 3 pm pst this
     afternoon for coastal waters from cape shoalwater wa to
     cascade head or out 60 nm.

&&

$$

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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 cwa, or forecast area.

Information courtesy of the National Weather Service - NWS Portland...
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