Small Craft Advisory 287, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
910 PM PST Mon Nov 28 2016

.SYNOPSIS...High pressure aloft will build into the area from the
west overnight. Expect high pressure aloft to prevail during the
day Tuesday for mostly dry conditions. A weak low pressure system
will bring rain and mountain snow to the area Tuesday night and
Wednesday. The remnants of a warm front has the potential to ruin
the current dry forecast for Thursday.

&&

.SHORT TERM...
A northwest to southeast oriented convergence zone has been
persistent over Island and Snohomish Counties. For this reason,
the forecast was updated to raise PoPs and QPF amounts in the area
impacted by the convergence zone. The convergence zone should
slowly drift south and dissipate overnight as an upper level ridge
of high pressure builds in from the west. Otherwise, expect a few
showers over mainly the coast and mountains.

Depending on if we get some partial clearing, lows Tuesday morning
could be rather chilly or in the 30s outside the urban areas.
Localized fog is likely in the fog prone river valleys and low
lying areas in the southwest interior in the morning. High
pressure aloft will shift across the area as the upper flow
remains progressive, keeping most the area dry Tuesday with
sunbreaks or partial afternoon sunshine possible. Highs should
reach near 50.

Models were in good agreement on the next system arriving Tuesday
night. Rain will overspread the area, producing generally less
than half inch amounts. The best orographic lift and cold air
advection will be over the north Cascades, where over six inches
of snow is possible. A Winter Weather Advisory will probably be
needed for the Cascades of Whatcom and Skagit counties Tuesday
night. Snow amounts from Snohomish county southward will be
less.

Snow levels may fluctuate between 3000 to 3500 feet from
Snoqualmie southward during part of the storm, so mixed
precipitation could hold down snow accumulation in Snoqualmie
Pass. Colder air should filter in behind the front with showers
expected on Wednesday. A few more inches of snow could occur
on the favored west or northwest facing slopes but the passes
will probably receive less. The flow pattern will also be
favorable for a convergence zone, similar to what we had today.
The convergence zone should dissipate by Wednesday evening.

It's beginning to look doubtful as to whether there will be dry
across the area Wednesday night through Thursday. There will
likely be some lingering showers over mainly the coast and
mountains Wednesday night. The remnants of a warm front or warm
air advection will probably result in a gloomy, drippy day across
much of the area on Thursday. Mercer/05

.LONG TERM from the previous discussion...
Models continue to advertise a large scale shift in the pattern
Friday and over the weekend. The upper ridge will flatten Thursday
night, allowing a cold upper low over Alaska to sag southward
setting up moist zonal flow into Western Washington Friday. The
first in a series of waves will arrive with rain spreading into
most of the area Friday with little break before the next low
arrives Saturday. A tight zonal baroclinic zone and strong
westerly jet will remain directed into the area. The mountains
should pick up appreciable snowfall in this pattern. Most models
show a deepening surface low tracking across or just north of the
area Sunday brining more rain and heavy mountain snow. Winds could
also be strong in some areas depending on the exact track.

Long range models are still showing colder air arriving behind the
Sunday system from the north with a large surface high setting up
over B.C. It is too soon to know if this cold air mass will make it
into Western Washington but with moisture in the vicinity and a
closed low over the region, snow levels could be quite low. The
forecast indicates snow levels down to 1000 feet by Monday/Tuesday.
Will need to monitor this possible shift to a colder pattern knowing
the longer range models have a well known cold bias due to complex
terrain.  Mercer

&&

.HYDROLOGY from the previous discussion...
Any additional rain through Saturday should not be enough to
produce flooding on any area river. A stronger storm may bring a
decent amount of rain this weekend with lowering snow levels. The
Skokomish river would probably be the only river with a slight
chance of flooding by late Saturday and Sunday. Mercer

&&

.AVIATION...An upper ridge over the northeast Pacific is
forecast to move east over Western Washington Tuesday morning.
Northwest flow aloft easing through the night. The ridge will
continue inland late Tuesday as an upper trough approaches the coast
Tuesday night with increasing southwest flow aloft. At the surface,
onshore flow will continue to ease overnight as a ridge of high
pressure off the coast shifts onshore into Western Washington. A
Pacific frontal system is expected to approach the coast Tuesday
night. The air mass is moist and stable.

Areas of low and mid clouds this evening. Low clouds should fill in
overnight, especially over the interior, as pressure gradients ease.
Low clouds should lift and break up a bit Tuesday afternoon.

KSEA...Scattered to broken mix of low and mid level clouds this
evening with lower MVFR low clouds solidifying after 07z. Light
winds less than 5 knots becoming south 5-10 knots 16z-20z
Tuesday. Buehner

&&

.MARINE...A ridge of high pressure offshore is expected to move
over western Washington Tuesday morning. Low level onshore flow
easing overnight.

West swell around 20 ft this evening to gradually subside through
Tuesday.

A Pacific frontal system 900 nm off the coast this evening is
forecast to move onshore Tuesday night. Gales possible for the
Coastal Waters. Small craft advisory winds likely for all inland
area waters with this system.

Higher pressure is expected to rebuild over the area Wednesday and
Thursday. The next Pacific frontal system is forecast to reach
western Washington Friday. Buehner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...High Surf Advisory in effect until 11 PM tonight for the
	 Central Coast.

PZ...Small Craft Advisories are in effect for the Coastal Waters
	 and west entrance to the Strait of Juan de Fuca.

 	 Gale Watch in effect from Tuesday afternoon through Tuesday
	 evening for the Coastal Waters.

 	 Small Craft Advisory for rough bar in effect until 7 AM
 	 Tuesday for the Grays Harbor Bar.

&&

www.weather.gov/seattle

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