Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 160436 CCA
AFDSEW

Area Forecast Discussion...CORRECTED
National Weather Service Seattle WA
919 PM PDT Mon May 15 2017

.SYNOPSIS...
A strong upper level low pressure system will drift across the
area overnight and tomorrow. This will bring rain to the low
elevations and mountain snow, along with unseasonably cool
temperatures and low snow levels. Rain will become more showery
during the day Tuesday with a few thunderstorms possible south of
the Sound as the low pushes across southwest Washington. A
significant change in the weather pattern will emerge by mid to
late week with a strong drying and warming trend as an upper level
high pressure builds into the area.

&&

.SHORT TERM...
Current radar imagery depicts rain continuing to overspread the
area. Satellite imagery lines up nicely with model expectations of
a well defined low at 500 mb as it pushes south along the coast
of Vancouver Island.

Current observations show that snow levels are dropping with
temps down to 30 degrees at Paradise and 34 at Snoqualmie Pass.
Winter weather advisories remain on track and several inches of
accumulating snow look to occur overnight with snow levels
dropping to nearly 3000 feet by early Tuesday morning. Continued
the idea of a general snow accumulation amounts of 4 to 12 inches,
with highest amounts expected at the ski resorts, Paradise and
perhaps some higher passes like Stevens and White pass. Over a
foot is probable above 6000 feet. The winter weather advisory
remains up for the Cascades through Tuesday afternoon. Those
planning travel or outdoor activities in the mountains and passes
should pay close attention to the forecast and latest conditions.

Another cool and damp day is expected on Tuesday. The upper low
will track over the Olympic Peninsula and across the Southwest
Interior. A cold pool will accompany the low and assist in
producing Lifted Index of -1 to -3 C south of the Sound Tuesday.
This warrants inclusion of thunder in the forecast, and have
included a mention of small hail as freezing levels will be low,
as well as low convective temps in the lower 50s. Highs will
again top out well below normal but this should be the end of the
cool weather this week. The low will steadily exit the region and
heights will rise significantly through the mid-week. Wednesday
will be a transitional day with a few spotty showers but warming
temps with highs climbing around or just above the 60 degree mark
again. Then by Thursday, a warmer day is on tap. A strong mid
level ridge builds in, sending 500 mb heights climbing, and
surface temperatures doing the same. Will look for Thursday to
reach well through the mid 60s to near 70 with little shower
activity. Johnson

.LONG TERM...
The weather will stay on its warming and drying trend into the
latter portions of this week. A strong ridge of mid level high
pressure will produce heights up near and above 570dm and surface
temps will reflect highs around 70 for most areas by Friday. A
quick moving and weak trough of low pressure will slide across
northern Washington Saturday and this will introduce a little more
cloudiness, some spotty showers, and temps only near or slightly
cooler than Friday. However, in its wake an even stronger ridge of
high pressure will send heights along the west coast up to levels
not yet seen this year. With heights approaching 582dm+ by late
Sunday, this bodes well for well above normal temperatures. Sunday
will bring highs well through the middle 70s, then Monday - if
current forecast holds, would have a solid shot at 80s across the
southern portions of the area. Johnson

.AVIATION...An upper level low will move southeast across Western
Washington tonight and Tuesday. Southwesterly flow aloft tonight
will become northerly behind the low. Southerly flow at the surface
this evening will become onshore late tonight. The air mass is
stable and moist with increasing moisture all levels. Light rain
will fall through the evening, then turn to showers after midnight.
There is a slight chance of thunderstorms Tuesday afternoon over
southern sections.

A mix of VFR to MVFR conditions out there this evening as rain
continues over the area. Cigs everywhere will become MVFR 2-3k ft as
the evening progresses and will remain MVFR through the night. Some
improvement is likely Tuesday afternoon, especially north and west.

 KSEA...Discussion above applies. Southerly wind 6-12 tonight for
most of the TAF forecast period...turning more westerly late
Tuesday afternoon. CHB/SMR

&&

.MARINE...A passing front will bring an increase to winds over the
coastal waters as well as in the strait and adjacent waters.
Inherited small craft advisories look good for the most part as obs
show winds either solidly in SCA range or gusts/peak winds in that
range. Exception seems to be central strait...but with surrounding
waters inching up near or over criteria...suspect it is only a
matter of time before winds there meet similar speeds...especially
once direction shifts from southerly tow westerly in the wake of the
front overnight.

Onshore flow will continue the rest of the week. Daily small craft
advisory westerly winds are likely in the strait. A gale or two
can`t be ruled out in the strait but no particular day stands out as
having obviously stronger winds. Occasionally the coast will reach
small craft advisory level northwest winds in the afternoons and
evenings. CHB/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 5 PM PDT Tuesday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties.

PZ...Small Craft Advisory until 5 AM PDT Wednesday 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 until 5 PM PDT Tuesday 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-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 5 AM PDT Wednesday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 11 AM PDT Tuesday for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 11 PM this evening to 5 AM PDT
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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