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Area forecast discussion 
National Weather Service Seattle Washington
345 am PDT Tuesday Apr 21 2015

Synopsis...a strong cold front over the coastal waters this morning
will sweep inland this afternoon and this evening bringing an abrupt
change to the weather. Cool and locally windy conditions with a
chance of showers can be expected tonight into Wednesday. Snow
levels will fall tonight into Wednesday in the mountains. A weak
front will move across the area on Thursday bringing a chance of
rain. Moist southwest flow aloft will bring periods of rain next
weekend.

&&

Short term...the upper level ridge that gave sunny and warm weather
to western Washington over the past few days has moved east of the
Cascades overnight. A marine push is now underway early this morning
as onshore pressure gradients gradually ramp up. Satellite images
show a rather strong cold front sitting from central Vancouver
Island south-southwest into the Oregon and northern California offshore waters.
This front will move inland this afternoon and sweep east of the
Cascades this evening.

Stratus ahead of the incoming front has moved to the coastline and
will push inland during the morning hours. Between 21z and 00z...the
GFS increases the kuil-kbli pressure gradients to around 6 mb. 500 mb
heights fall from 560-564 dam early this morning to around 545 dam
late tonight and the freezing level will fall to around 3500 feet by
Wednesday morning. Given the strong convergence of the flow around the
Olympics and upslope flow into the west slopes of the
mountains...expect showers to be likely in the interior from about
Seattle northward and in the mountains this evening. The convergence
zone activity and upslope flow will keep showers going into
Wednesday as the upper low departs to the southeast of the area.
Temperatures will only top out in the 50s to around 60 today and
will fall into the 40s tonight. Temperatures will top out in the middle
and upper 50s on Wednesday...quite a bit cooler than what we have
been experiencing in the recent past.

Cold air advection and west-northwest flow aloft with and behind the front and
to the south of a developing upper low over Vancouver Island that
will track east-southeast will combine to give a good gale to the Strait of
Juan Delaware fuca and west to northwest winds 20-35 miles per hour with gusts to 45 miles per hour to the
Admiralty Inlet area. The strongest winds are likely to occur on the
west side of Whidbey Island.

A Wind Advisory has been issued from 2 PM this afternoon to 1 am PDT
Wednesday for the Admiralty Inlet area. Other areas around western
Washington will see breezy to locally windy conditions late this
afternoon and evening as well. A Special Weather Statement is also
being issued for the mountains for the expectation of sharply colder
weather there and a couple of inches of snow above about 3500 feet.

Another frontal system will move across the area Thursday with rain
likely on the coast and a chance of rain inland. Cool weather with a
chance of showers is expected on Friday. Albrecht

Long term...models are rather consistent in digging a deep 515 dam
500 mb low to around 50n 145w Saturday night and Sunday as cold air
dumping out of the Bering Sea moves southeast into the Gulf of
Alaska. Flow to the south of the sprawling upper low to our west
will tap into some tropical or subtropical moisture near Hawaii and
wrap it NE into our area this weekend. Over the weekend...3 to 5
inches of rain could fall over the Olympics and north Cascades with
a inch or two possible in The Lowlands. Expect temperatures to
moderate somewhat over the weekend...but heights remain low enough
so that snow levels only briefly rise to over 6000 feet. Model
consistency and continuity beyond Sunday is poor. No major changes
were made to the extended forecast. Albrecht

&&

Aviation...westerly flow aloft will become southwest this morning
as an upper level trough approaches from the west. Increasing low
level onshore flow today will help the air mass to become moist at
low levels this morning. Marine stratus has moved onto the north
coast as of 06z...expect it to spread onto the central
coast...Chehalis gap and south interior by day break. The forecast
for the central sound remains a bit tricky with this pattern...the
expectation remains for limited low clouds forming and remaining
short lived this morning.

A weak cold front remains on track to move through the forecast area
this afternoon...followed by an upper level trough Tuesday night.
This will result in a short period of light rain and showers
behind the front. The primary impact will be increasing low level
winds today.

Ksea...light north winds...becoming southerly to 8 knots late...then
southwest 8 to 12 knots by this afternoon. Thin marine stratus possible
at the terminal around 13z.

&&

Marine...strengthening onshore gradients this morning have already
produced gale conditions in the central and east Strait this
morning. Small Craft Advisory winds remain forecast for all
remaining area waters through tonight as a cold front works through
the coastal waters this afternoon and inland during the evening.
Expect increasing northwesterly winds over parts of the coastal
waters to reach gale levels behind the front...while gale westerlies
remain forecast for the central and eastern Strait tonight. Small
Craft Advisory winds will continue for all other marine zones
through much of tonight.

A second weak cold front will cross the forecast area Thursday and
Thursday night...with a stronger frontal system expected to reach
the area Saturday.



&&

Sew watches/warnings/advisories...
Washington...Wind Advisory Admiralty Inlet area 2 PM this afternoon until 1
am PDT Wednesday.
Pz...Gale Warning through tonight central and eastern Strait.
Gale Warning for tonight coastal waters zones 150 170 173.
Small Craft Advisory all other waters through tonight.

&&

$$

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

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