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Area forecast discussion 
National Weather Service Seattle Washington
902 am PDT Monday Mar 30 2015

Synopsis...warm air advection will keep rain at times over the
coast and northern interior today. A cold front will spread rain
across the area tonight followed by an upper level trough Tuesday
for showers and cooler weather. Showers will diminish Thursday as
weak ridging builds over the area ahead of a frontal system that
will affect the area on Friday.

&&

Short term...a warm front over southern b.C. Is clipping western Washington
this morning with a few light showers along the coast and near the
Canadian border. The interior will remain dry and mostly cloudy. The
air mass is pretty mild and we will see temperatures in the low to middle 60s
today. The current forecast is on track.

The cold front will slide S/southeast tonight for a period of wetter
weather across western Washington. Rain will mainly affect the coast this
evening...with rain in the interior after midnight tonight. The
upper level trough and onshore flow will maintain cool and showery
weather through Tuesday. Snow levels will drop to around 3000 feet
with a few inches of snow in the mountains. A Puget Sound sound
convergence zone may enhance amounts. Models also show a negative
lifted index and cannot rule out isolated thunderstorms Tuesday
afternoon. Highs on Tuesday will be in the low to middle 50s - a few
degrees below normal.

The air mass will remain cool and unstable as we head into Wednesday
with a trough still overhead. Low level onshore flow will prevail
for scattered showers. 33

Long term...previous discussion...medium range models continue to
show upper level ridging on Thursday as a deep upper level low moves
into the Gulf of Alaska. The brief ridging on Thursday will likely
give the a break in the showers before a front arrives Friday. Both
the GFS and European model (ecmwf) differ on the track of the trailing upper level
low. The European brings it down the northwest coast before opening
up and moving into northern California Monday night. The GFS takes a
much more progressive tack...opening the low up and bringing it into
British Columbia Saturday afternoon. The current forecast continues
to reflect a cool and unsettled pattern through the weekend.

&&

Hydrology...flooding is unlikely over the next 7 days.

&&



Aviation...a broad low amplitude upper level ridge centered over Idaho
at 15z will move over west Montana by 00z. A cold front along 130w at 15z
will approach the coast this afternoon...reaching the north coast around
03z and crossing Puget Sound sound around 06z. SW flow aloft will increase
as the front approaches.

The air mass over West Washington is somewhat moist and stable with ceilings mainly
scattered-bkn050-070 with higher layers. Patchy MVFR/IFR ceilings mainly over
S Puget Sound sound and the SW interior should dissipate by 18z.

Ceilings are expected to lower to MVFR levels tonight as the cold front
moves across the area. Onshore flow behind the front will keep ceilings
MVFR with local IFR possible Tuesday morning. S winds through the
interior will become gusty by this afternoon as the front approaches.

Ksea...VFR conditions will continue through 03z with cloud layers
scattered-bkn060 and higher. Ceilings are expected to lower to MVFR as the
cold front approaches...then crosses Puget Sound sound around 06z. Surface
winds will be S-SW 6-12 knots. Kam

&&

Marine...a small surface ridge will remain over West Washington today...while
an offshore cold front approaches the coast. The cold front will
reach the north coast this evening around 8 PM then cross Puget Sound sound
before midnight. Small craft advisories are in effect for S winds
ahead of the front for both the coast and inland waters. The latest
mesoscale models have backed off slightly on the S winds through the
inland waters today with pockets of winds peaking close to 20 knots.

Strong westerly onshore flow will develop in the Strait immediately
behind the front tonight. A Gale Warning is in effect. There remains
a difference between the models regarding wind speeds. The gfs40
uil-bli pressure gradient reaches between 4 and 5 mb which would be
enough for gale force winds. However all the mesoscale models
including the 06z Canadian lam top the winds at 30 knots for a high end
Small Craft Advisory.

Weaker onshore flow will prevail Wednesday and Thursday as a ridge
builds offshore. There should be enough west pressure gradient through
the Strait for Small Craft Advisory winds in the evenings. Another cold front will
probably reach the area Friday.

There will be a westerly swell of 10 to 12 feet over the coastal
waters through Tuesday. Bar conditions will be rough as well.

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory coastal waters...entrances to the
Strait...Admiralty Inlet...northern inland waters...Puget Sound
sound and Hood Canal.
Gale Warning central Strait of Juan Delaware fuca and East Entrance
Strait this evening through Tuesday afternoon.
Small Craft Advisory Grays Harbor for rough bar.

&&

$$

You can see an illustrated version of this forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html

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