Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 160934
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
234 AM PDT Mon Apr 16 2018

.SYNOPSIS...A deep upper low will bring cool showery weather to
Western Washington today. Weak upper features combined with moist
onshore low level flow will result in scattered showers and sun
breaks Tuesday and Wednesday. An upper ridge will bring dry
weather Thursday into Friday. The next frontal system will move
through quickly on Saturday, with dry weather likely on Sunday.

&&

.SHORT TERM...A 5330 meter upper low is centered near the mouth of
the Columbia River early this morning, with showers rotating
across Western Washington. The upper low will move slowly
northeast across Washington through this evening. Deep moisture,
weak instability, and increasing onshore flow will bring more
showers to the forecast area. It be breezy to windy as well, but
winds should remain below advisory levels. Highs today will be
around 50, and the snow level will be around 3000 ft.

The heaviest precipitation should fall over the Mount Rainier
area, and we will issue a winter weather advisory for the
Cascades of Pierce and Lewis Counties through midnight; Paradise
should receive 8 to 16 inches of snow during the next 24 hours.

The upper low will be over Alberta on Tuesday, and a weak upper
ridge will move into Western Washington from the southwest.
Moderately strong low level onshore flow will continue, with
breezy to windy areas again. However, moisture and instability
will be shallow, resulting is scattered showers and sun breaks for
the interior lowlands. There will be more persistent showers at
the coast and over the mountains, and the snow level will be
around 2500 ft.

The upper ridge will move east of the Cascades Tuesday night, and
a weak upper trough will move over the area Wednesday, extending
northeast into the region from an upper low off the coast of
northern California. The result of that should be just spotty
light showers, generally decreasing during the day and ending
Wednesday night. McDonnal

.LONG TERM...An upper ridge will tilt positively across the
Pacific Northwest on Thursday and Friday, extending northeast from
an upper high well to the west of California. Models differ
somewhat on how quickly the ridge will progress across the area.
They agree that Thursday should be a dry day for almost the entire
forecast area; the GFS brings the leading edge of the next frontal
system inland on Friday afternoon, while the ECMWF delays its
arrival till Friday evening. At any rate, the next frontal system
will move through rather quickly on Saturday, probably giving way
to a day of dry weather on Sunday. McDonnal

&&

.AVIATION...Upper level trough shifting onshore over SW Washington
early this morning with E-SE flow aloft north of the low and light
westerly flow to the south of it. Flow aloft becoming W-NW across
the entire area tonight as the trough axis shifts east of the
Cascades. The air mass is moist and will become somewhat unstable
today. Areas of MVFR ceilings in showers to continue across Western
Washington today as upper trough moves through the region. Near the
surface, increasing low level onshore flow will give gusty S-SW
winds from later this morning through tonight.

KSEA...Discussion above applies. Areas of showers will move through
the area throughout the day with MVFR ceilings at times. Some
improvement in ceilings possible late tonight as trough shifts east
of the Cascades. Strengthening southerly gradient through Puget
Sound, especially later this afternoon and tonight, will bring some
gusty S-SW winds. S-SW wind 6 to 12 knots this morning rising to 15
gusting 25 knots later this afternoon and continuing overnight
tonight. 27

&&

.MARINE...Onshore flow will prevail today with strong high
pressure over the NE Pacific. A stronger onshore push will develop
tonight. Gale Warnings are in effect for the Coastal Waters and
central/east Strait of Juan de Fuca. Small Craft Advisories are
in effect for all remaining waters. Winds will gradually ease on
Tuesday. High pressure will develop over the area Wednesday and
Thursday giving lighter winds. The next front will arrive on
Friday for increasing southerly flow over the waters. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until midnight PDT tonight for Cascades
     of Pierce and Lewis Counties.

PZ...Gale Warning until midnight PDT tonight for Coastal Waters From
     Cape Flattery To James Island 10 To 60 Nm-Coastal Waters
     From James Island To Point Grenville 10 To 60 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm.

     Small Craft Advisory for rough bar from 4 PM this afternoon to
     11 AM PDT Tuesday for Grays Harbor Bar.

     Small Craft Advisory until 6 PM PDT this evening for Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From James Island To Point Grenville Out 10 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater Out
     10 Nm.

     Gale Warning from 6 PM this evening to midnight PDT tonight for
     Central U.S. Waters Strait Of Juan De Fuca-Coastal Waters
     From Cape Flattery To James Island Out 10 Nm-Coastal Waters
     From James Island To Point Grenville Out 10 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 6 PM this evening to 11 AM PDT Tuesday
     for Northern Inland Waters Including The San Juan Islands-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 8 PM this evening to 11 AM PDT Tuesday
     for Admiralty Inlet.

     Small Craft Advisory until 11 AM PDT Tuesday for Puget Sound and
     Hood Canal.

&&

$$

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