Small Craft Advisory 222, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
305 AM PDT SAT SEP 10 2016

.SYNOPSIS...A weak frontal system will move onshore today bringing
increased cloudiness and a threat of light showers mainly to the
coast and mountain areas into Sunday. Higher pressure aloft is
forecast to build over the region Monday and Tuesday for clearing
skies and warmer above average daytime temperatures. The dry weather
pattern should extend through the coming week.

&&

.SHORT TERM...Satellite imagery early this morning showed a weak
frontal system moving through western Canada with the tail end
dragging across NW Washington. Doppler weather radar showed very
little associated precipitation. The parent upper trough was still
in the NE Gulf of Alaska and is forecast to dig SE through Sunday in
response to the building upper ridge 150/160W ahead of a deep upper
low just east of the dateline this morning.

As the upper trough digs SE, 500 mb heights fall with cooling aloft.
Not a lot of associated moisture, yet enough to produce a few light
showers mainly along the north coast and in the mountains. The
surface and lower level flow pattern is also favorable for a weak
Puget Sound Convergence Zone to develop this evening, move south and
dissipate in the central Cascades Sunday morning. Have adjusted the
pops to reflect the above through still quite limited.

Low level offshore flow to turn onshore today with the passage of
the weak front and in response to the incoming upper trough. Onshore
gradients should peak late this afternoon all combining to produce
increased cloud cover today into Sunday morning. In the wake of the
passing upper trough Sunday, low level flow quickly turns offshore
for a clearing trend Sunday afternoon. Sunday temperatures should be
the coolest of the two weekend days for the interior.

Summer is not over...yet. Progs agree on the building offshore upper
ridge leaning over into British Columbia as an upper low develops
well south of Washington Monday. The combo of subsidence aloft and
low level offshore flow should warm daytime temperatures well above
average Monday after a cool morning. Buehner

.LONG TERM...The warming trend continues Tuesday with highs in the
70s to lower 80s for probably the warmest day of the week before the
low level flow turns light onshore Wednesday and Thursday. At this
point, the mid range guidance begins to offer different solutions.
The Canadian and ECMWF concur on kicking the upper low inland and
rebuilding the upper ridge over the Pacific NW later in the week.
The GFS has a flatter ridge with lower heights. In either case, the
dry weather continues. The question becomes whether or not low level
flow turns offshore with warming temperatures late in the week. Stay
tuned here. Buehner

&&

.AVIATION...Light westerly flow aloft over Western Washington
today becoming northwesterly tonight. A weak system will bring a
front through Puget Sound by this afternoon with moderately strong
onshore flow developing later today and this evening. Stable
conditions with moisture increasing at all levels.

Surface based moisture will also result in patchy IFR fog early
Saturday morning in prone valleys. Otherwise, VFR stratus develops
across the area this afternoon due to increasing onshore flow.

KSEA...Mid and high clouds increasing this morning, then VFR stratus
around 4k ft fills in this afternoon. MVFR cigs expected later
tonight and Sunday morning. North to northeast wind 3-6 kt, becoming
northwest 5-8 kt this afternoon. dtm

&&

.MARINE...a front will push across the inland waters this morning
with increasing onshore flow. Solid westerly small craft winds 15-30
kt are expected in the Central and eastern Strait, especially late
afternoon and evening. Winds could briefly reach to 35 kt for a
couple hours and a gale watch remains in effect. Small craft winds
to 25 kt could also develop across the southern part of the north
inland waters, north portion of Admiralty inlet, and the coastal
waters tonight. Northerly flow develops Sunday, then becomes more
offshore on Monday. Mostly light onshore flow Tuesday through mid
week. dtm

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale watch central and eastern strait of juan de fuca late
late this afternoon and tonight.

&&

www.weather.gov/seattle/

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html