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fxus66 ksew 300414 
afdsew

Area forecast discussion
National Weather Service Seattle Washington
914 PM PDT Wed Mar 29 2017

Synopsis...an upper level shortwave trough will move across western
Washington tonight which will keep showers and a Puget Sound
convergence zone going. Onshore flow will gradually weaken allowing
breezy winds to diminish overnight. An upper level ridge will begin
building over the region on Thursday with the chance of showers
decreasing through the day and a few sun breaks developing in the
afternoon. The upper level ridge will bring partly sunny skies and
warmer temperatures on Friday. A weak warm front may produce some
light rain Friday night and Saturday morning followed by a trough
with some showers on Sunday. Another front could arrive early next
week.

&&

Short term...830 PM radar shows showers over the interior have
diminished for the time being with the most action in a small
convergence zone stretching from S of Port Townsend across the sound
to around Marysville in Snohomish County. Showers are still active
along the coast as westerly flow aloft continues pushing them
ashore. Infrared imagery shows one more weak cloud band offshore along
roughly 128w at 03z/8 PM associated with an upper level trough axis.
The models seem a little fast with the trough axis which they have a
little ways ahead of the cloud band. It may not matter too much
since the portion of the band that should reach the Washington coast is
pretty weak and will probably only provide slight organization to
the showers. It doesn't look like there is very much shower activity
farther offshore behind the band so showers should begin tapering
off late tonight.

Last issue is that the models have rough consensus that the
convergence zone will slide south over mainly inland King County
after midnight as low level winds veer from west to northwest. Should
dissipate by Thursday morning.

An upper level ridge offshore with modest amplitude will approach
the area on Thursday then move overhead on Friday. Dry northerly
flow aloft will increase over West Washington on Thursday allowing the chance
of showers to diminish through the day. There is still lots of low
level moisture around and will be further supplemented by continuing
low level onshore flow. Low level westerly flow aloft will keep
clouds banked up against the Cascades. So it looks like skies will
remain cloudy through Thursday morning with possibly a few sun
breaks developing during the afternoon. Low level moisture should
diminish from the west so the coast and north interior have a better chance
at sunshine than interior areas like Puget Sound and the SW interior.

The upper level ridge axis will move over West Washington on Friday along with
the surface ridge. Unfortunately, westerly flow aloft offshore
upstream of the ridge will be pushing high level moisture up to the
ridge axis. This will diminish the sunshine over the area on Friday
with the cirrus filtering or blocking the solar insolation much of
the time. Still, compared to the soggy last couple of months any
sunshine is welcome.

The GFS and European model (ecmwf) agree that a weak warm front will move across West Washington
Friday night and Saturday morning. This front doesn't look very
exciting, probably just some light rain and possibly some local
breezy winds. Kam

Long term...previous discussion from the 330 PM afd...the GFS/ECMWF
solutions are a bit closer together today and show a fairly typical
Spring like pattern Sunday through the middle of next week. A trough
will brush the area Sunday with a chance of light rain or showers.
Again, this will not be a big rain producer but should bring some
cooler temperatures and clouds. There is a good chance for a break
in the rain sometime Sunday night through Monday night but there is
still enough uncertainty on timing of any brief ridging to keep some
chance pops in the forecast. Another system arrives around Tuesday
and Wednesday. Neither of these systems look very wet or cold, just
typical systems this time of year with some clouds, light spotty
rain, and near average temperatures. Mercer

&&

Aviation...southwesterly flow aloft this evening will become
northwesterly late tonight as an upper level trough passes, then
northerly on Thursday. At the surface, strong onshore flow behind an
exiting front will gradually ease tonight but remain moderate on
Thursday. The air mass is moist and slightly unstable with scattered
showers.

Cigs this evening a mix of VFR or MVFR over West Washington...with pae proving
to be a bit of an outlier thanks to convergence zone that has set up
there as cigs there sit on the MVFR/IFR division. Not helping that
rainfall at times is reducing vis to around 2 sm. For locations
outside of said convergence zone...showers over the olympic
peninsula will result in MVFR conditions for locations under such
showers. That activity is expected to move eastward...spreading over
the area as the upper trough makes its way through...making the
remainder of the area susceptible to cigs falling to MVFR during the
overnight hours. Chb/smr

Ksea...current obs compared to current radar suggest VFR to high end
MVFR conditions to remain in place for the evening and into the
overnight hours. Should see cigs fall down into low end MVFR by 12z
early Thu morning. These low cigs along with threat of showers will
persist at least into mid to late morning before conditions dry and
cigs lift for Thu afternoon. SW sustained winds 12-17 kts with gusts
to 25 kts still possible this evening and overnight...subsiding to
around 10 kts early Thu afternoon. Smr

&&

Marine...moderating onshore flow tonight in wake of exiting front.
Inherited headlines look good to leave up until their expiration.
While obs currently falling below thresholds for said headlines...it
still remains very borderline and would not surprise at all if obs
were to nudge upward into criteria. Gale Warning for central and
east Strait will be the first one to fall at 11 PM PDT...and will be
issuing follow-up Small Craft Advisory with the evening forecast package. Onshore
flow will persist on Thursday, with Small Craft Advisory strength
winds over most waters. Gradients will weaken Thursday night, and a
ridge of high pres will give lighter winds on Friday. Another front
will arrive Saturday. Chb/smr

&&

Hydrology...after the recent heavy rain over the Olympics the
Skokomish River remains a little above flood stage. It will continue
to recede slowly tonight and Thursday morning. Latest estimate is
that it will drop below flood stage around 9 am Thursday morning. No
other really strong and wet systems are expected for the next week
so flooding is not expected on area rivers during the next 7 days.

The heavy rain did increase the threat of landslides across West Washington
again. However, the light showers expected tonight and on Thursday
will not have much effect on soil saturation. The current statement
highlighting the landslide potential will be allowed to expire late
tonight. Kam

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory until 5 PM PDT Thursday for Admiralty
Inlet- coastal waters from Cape Flattery to James Island 10 to
60 nm-coastal waters from Cape Flattery to James Island out 10
nm-coastal waters from James Island to Point Grenville 10 to 60
nm-coastal waters from James Island to Point Grenville out 10
nm-coastal waters from Point Grenville to Cape Shoalwater 10 to
60 nm-coastal waters from Point Grenville to Cape Shoalwater
out 10 nm-northern inland waters including the San Juan Islands-
West Entrance U.S. Waters Strait of Juan de Fuca.

Small Craft Advisory for rough bar until 11 am PDT Thursday for
Grays Harbor bar.

Small Craft Advisory until 5 PM PDT Thursday for central U.S.
Waters Strait of Juan de Fuca-East Entrance U.S. Waters
Strait of Juan de Fuca.

Gale Warning until 11 PM PDT this evening for central U.S.
Waters Strait of Juan de Fuca-East Entrance U.S. Waters
Strait of Juan de Fuca.

Small Craft Advisory until 5 am PDT Thursday for Puget Sound and
Hood Canal.




&&

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