Gale Watch 68, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PST Sat Nov 26 2016

.SYNOPSIS...A front slow moving front over Western Washington will
bring rain and mountain snow today. As the associated surface low
weakens, precipitation will begin taper off to showers late this
afternoon and evening. Generally dry conditions are expected tonight
and Sunday morning before a front will bring showers and possibly
heavy mountain snow Sunday night into early Monday. The active
weather pattern will then continue through much of next week.

&&

.SHORT TERM...WV satellite imagery shows a large scale split upper
trough over the Western U.S. with the northern stream affecting
Western Washington. There was some surface low development overnight
near the north Washington coast which has now tracked northeast and
filled to around 1000 mb near the WA/B.C border this morning. Warmer
air aloft ahead of this low allowed snow levels to rise, now at
around 4000 feet in the north cascades, and around 5000 feet
Olympics and Cascades from Stevens pass southward. Hurricane ridge
and Mount Baker have received roughly a foot of snow. A winter storm
warning is in effect until noon for the north Cascades and Olympics.
accumulations of 4 to 8 inches from Snohomish county south have
mostly been limited to above 5000 to 5500 ft with a snow advisory in
effect. Colder air will be filtering into the area this afternoon,
dropping snow levels to near 3000 feet by evening. A few inches of
snow possible still in higher passes like Stevens but likely sub-
advisory amounts. May consider extending the warning for the north
Cascades where models hold onto heavier QPF longer today, but will
decide on this after looking over all the new 12z guidance.

For the lowlands...just rain today, moderate to locally heavy along
the front. Rain should ease this afternoon and become showery by
evening. A Puget Sound convergence zone could become active
affecting mainly western Snohomish county tonight with additional
rain. A lull or break in precipitation is expected across most the
area later tonight.

The next frontal system approaching from the northwest will arrive
by Sunday afternoon. Rain will spread across the area and snow will
pick up again in the mountains, especially by evening and continue
into Monday. Snow levels will be lower with this system and the
strong northwest flow aloft is favorable for orographic enhancement
along the west slopes of the Cascades and Hurricane ridge in the
Olympics. There will be at least advisory amounts of snow, and may
need to consider a warning for the Cascades/Olympics as snow amounts
could be near a foot with this next storm. Snow levels will fall to
2500 to 3000 feet. will make a decision prior to the afternoon
forecast issuance. Mercer

.LONG TERM...Previous discussion...What follows must be conveyed in
hushed tones...lest the feature be frightened off or jinxed...but
models do suggest an upper level ridge over the area starting late
Monday. The length of its stay is up for dispute...as always...with
the GFS being optimistic and keeping things dry from Monday night
through Wednesday morning. Will not discuss the ECMWF until it
embraces a more positive attitude toward life. Like all good things
though...an upper level trough will bring an end to this dryness
with more rain by Wednesday afternoon before activity tapers off
late Thursday morning or early Thursday afternoon...with the
possibility of another prolonged break in precip starting Thursday
night. SMR

&&

.HYDROLOGY...Renewed heavy rain over the Olympics--three to five
inches over the southern slopes--caused the Skokomish River to rise
again today. It is now forecast to remain above flood stage through
Monday morning. Rainfall amounts have been less impressive
elsewhere. The Chehalis at Grand Mound crested last night and is
heading down. The Chehalis at Porter is forecast to crest just below
flood stage tonight. A flood watch remains in effect for Thurston
and Grays Harbor Counties, mainly for the flood crest to work its
way downriver on the Chehalis.

Landslides remain a concern. USGS landslide indices show a continued
threat from the recent heavy rain. A special weather statement
addresses this.

The systems Sunday night and Wednesday appear to be fast moving.
Though more rain will fall, flooding amounts are unlikely. A cooler
and drier period is forecast later in the week. After the current
flooding episode has ended, more flooding is improbable over the
next seven days. Burke

.AVIATION...A front will slowly move through the area today, then an
upper trough will move through tonight. At the surface, moderate
southeasterly flow will become moderately onshore today. The air
mass is moist and stable. Ceilings are mostly VFR 5-6K ft though
there are lower ceilings on the coast. Conditions will deteriorate
to MVFR this afternoon and evening.

KSEA...Discussion above applies. Southerly wind 8-12 KT. CHB

&&

.MARINE...Southerly wind increased more than expected over the
interior this morning, bringing gale force winds to the east
entrance and small craft advisory strength winds elsewhere. Wind
will turn onshore later this morning, with small craft advisory
westerlies in the strait.

Winds will ease tonight and Sunday as high pressure briefly builds
over the area. Then another front will arrive Sunday night, with
gales forecast over the coast and possible over some inland waters.
Another front will arrive around Wednesday. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Various flood watch/warnings in affect for portions of Western
     Washington. See latest flood statements for details.

     Winter Weather Advisory until noon PST Saturday for Cascades of
     Pierce county-Lewis county-Snohomish and King Counties.

     Winter Storm Warning until noon PST Saturday for Cascades of
     Whatcom and Skagit Counties-and Olympics.

PZ...Gale warning east entrance.
     Small craft advisory all other waters.
     Small craft advisory for rough Grays Harbor bar.

&&

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