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afdsew

Area forecast discussion
National Weather Service Seattle Washington
827 PM PDT Mon Apr 24 2017

Synopsis...high pressure will bring mostly dry weather tonight
and the first part of Tuesday. Rain will return Tuesday afternoon
into Wednesday morning as a couple of weather systems move onshore.
Showers will linger later Wednesday then decrease Thursday and
Friday and end entirely by Saturday as high pressure builds
overhead. Another weak front will reach the area on Sunday.

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Short term...isolated showers will essentially come to an end
this evening as an upper ridge moves over the area. A few showers
will linger over the Cascades but The Lowlands should be dry by
midnight. Tuesday will start out dry then a front will spread
rain onto the coast around midday and into the interior during the
afternoon or early evening. The front is weakening as it moves
onshore and it looks like the bulk of the quantitative precipitation forecast will be over the
south interior in the late afternoon and early evening.

An upper short wave in the westerly flow aloft will spread another
shot of rain into the area early Wednesday morning. As has been
the case recently, this system is mainly moving inland to the
south with more of a glancing blow for western Washington. Best
chances for rain will again be over the south part of the forecast
area. Rain will taper and change to showers by Wednesday afternoon,
probably becoming focused on the coast and mountains and possibly
in a Puget Sound convergence zone north of Seattle.

Showers will linger into Thursday with moist onshore flow but the
trend will be for gradual drying as high pressure aloft starts to
build in from the west. High temperatures will continue to run a
few degrees below normal Tuesday through Thursday. Schneider

Long term...previous discussion...an upper level ridge will
begin building over the Pacific northwest on Friday, leading to
dry weather on Saturday. However, temps anticipated to remain
below normal due to increased cloud cover during the day ahead of
the next low pressure system.

Beyond Saturday, confidence in the forecast was low. The medium
range solutions differed on the timing of the synoptic scale
features. Therefore, chose to compromise between the various
solutions for Sunday and Monday. In other words, expect a
continuation of below normal temperatures and above normal
precipitation.

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Aviation...an upper level trough over the Pacific northwest will continue
inland through Tuesday morning as an offshore upper ridge moves
onshore. Northwest flow aloft to become more westerly Tuesday. At the
surface...low level flow becoming southerly overnight into Tuesday
morning. Air mass stable and drying though areas of lower level
moisture for clouds in the MVFR category likely Tuesday morning.

The next Pacific frontal system is expected to weaken as it
approaches the coast Tuesday with increasing mid/high level moisture
during the day. Precipitation to reach the coast during the
afternoon and inland Tuesday evening.

Ksea...big short-term news is the wind shift to southerly winds
occurred around 02z and should continue through Tuesday morning for
your air traffic flow pattern planning. Clearing skies this evening.
Then the likelihood of MVFR ceilings in developing lower clouds
Tuesday morning. Those clouds should break out by midday Tuesday
with increasing mid/high level clouds through the day ahead of the
next frontal system. Buehner

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Marine...a ridge of higher pressure to build onshore overnight
tonight with low level surface winds turning southerly. Small Craft
Advisory winds in the coastal waters and Strait should ease
overnight.

The next Pacific frontal system about 700 nm off the coast is
expected to weaken as it approaches the coast Tuesday. Yet coastal
water winds should pick up to Small Craft Advisory levels ahead of
this system. Post-frontal onshore flow quite healthy with a good
potential of gale force winds in the Strait Wed as higher pressure
builds behind this system. The strong onshore flow should ease by
Fri as higher surface pressure builds onshore. Buehner

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory until 10 PM PDT this evening for coastal
waters from Cape Flattery to James Island 10 to 60 nm-
coastal waters from Cape Flattery to James Island out 10 nm-
coastal waters from James Island to Point Grenville 10 to
60 nm-coastal waters from James Island to Point Grenville
out 10 nm-coastal waters from Point Grenville to Cape
Shoalwater 10 to 60 nm-coastal waters from Point Grenville
to Cape Shoalwater out 10 nm-West Entrance U.S. Waters
Strait of Juan de Fuca.

Small Craft Advisory until 3 am PDT Tuesday for central U.S.
Waters Strait of Juan de Fuca-East Entrance U.S. Waters
Strait of Juan de Fuca.

&&

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