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Area forecast discussion 
National Weather Service Seattle Washington
336 PM PDT Tuesday Oct 13 2015 upper ridge will build and amplify over western
Washington through Thursday morning...leading to a drying of the
air mass. Lingering low-level moisture tonight will lead to
widespread fog over The Lowlands on Wednesday morning. Moderate
low-level offshore flow on Thursday morning will further dry the
air mass. The next rain-producing front should arrive
Friday night...with unsettled weather continuing until Monday.


Short term...visible satellite imagery this afternoon shows
gradual clearing is underway. The clearing should quicken around

Later tonight...there will be a classic setup for fairly
widespread lowland fog. The ingredients will be a lingering
surface-based moist layer of less than 1000 feet in
depth...mostly clear skies above...and light north-NE boundary layer
wind. So look for dense fog in spots on Wednesday morning.

As the upper ridge amplifies on Wednesday with its axis near the surface high pressure will spread from the interior
of b.C. Into eastern Washington on Wednesday night. This will initiate moderate
offshore pressure gradients and breezy east winds downwind of the
Cascade gaps. This will flush much of the lingering low-level
moisture out to sea and help to further dry the air mass.
Therefore...fog will be more confined to The Lowlands of the SW
interior and favored valleys (such as the Snohomish/Snoqualmie
valley) on Thursday morning.

On Friday...the upper ridge axis will move from western Washington to The
Rockies...with southerly flow aloft. Middle-high clouds will
gradually increase...though the low-level air mass will still be
mild and dry. By Friday evening...a weakening upper trough will approach
130w and start spreading rain onto the coast.Haner

Long models are in decent agreement with the upper
pattern all the way through day 7...or next forecast
confidence is better than usual at that time range. This week's
upper ridge will start to flatten and accelerate off to the east
on Saturday...opening the door to a negatively tilted but
weakening upper trough to approach from the west. This should
spread rain onto the coast on Friday night. The weakening of the trough
should mean that only light rain will spread into the interior on
Sat. The timing of the rain's arrival is about 6-12 hours sooner
than what was advertised yesterday.

The upper trough axis will pass across western Washington on Sunday
morning...with another trailing shortwave on Sunday night or
Monday morning. In association with the trailing shortwave...both
models take a low center of varying strength onshore between haida
gwaii and the northern tip of Vancouver Island...with a trailing
warm front spreading rain into western Washington around Sunday night. This
will keep the pattern wet and unsettled. The trailing shortwave
will be followed later Monday by northwest flow aloft...possibly with a
little cyclonic curvature in the flow. This will support lingering
showers on Monday.

Models quickly push a transient upper ridge axis across the area
next it is the most likely day of the extended forecast
to remain dry. Haner


Aviation...westerly flow aloft tonight and Wednesday with an
upper ridge slowly building over the eastern Pacific. At the
surface...high pressure over the coastal waters will shift into
British Columbia tonight and Wednesday. Air mass stable with areas
of low level moisture. Low clouds and fog should increase later
tonight and then burn off at least partially later Wednesday.

Ksea...stubborn low clouds are expected to break up late this
afternoon...then fog and low clouds are likely again Wednesday
morning...breaking up in the afternoon. Schneider


Marine...high pressure over the coastal waters will shift into
British Columbia tonight and Wednesday. Offshore flow will develop
Wednesday night and Thursday...then weaken on Friday. Small Craft
Advisory east winds are likely West Entrance Strait of Juan Delaware
fuca Thursday morning. A couple of fronts will pass through the
waters over the weekend...probably bringing Small Craft Advisory
southerly winds to the coastal waters at times. Schneider


Sew watches/warnings/advisories...




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