Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 120345

Area Forecast Discussion
National Weather Service Seattle WA
845 PM PDT Mon Jun 11 2018

.SYNOPSIS...Dry weather is forecast Tuesday though clouds will
increase during the afternoon. A weak cold front will bring some
rain to Western Washington Tuesday night and Wednesday morning,
and an upper trough will keep relatively cool showery weather over
the region through Thursday. High pressure will build over the
area for dry and warmer weather late this week into the weekend.


.SHORT TERM...Northwest flow aloft prevails over Western
Washington ahead of the small upper ridge over the offshore waters
near 130W. There is a light northerly onshore pressure gradient
across the region. Moisture in the marine layer has been gradually
drying during the day and -- aside from diurnal cumulus buildups
over the Olympics and Cascades -- skies are mainly clear. The
offshore upper ridge will progress to the Pacific Northwest coast
overnight; the air mass should remain nearly dry with light flow,
and marine layer stratus should be minimal even along the coast.

The upper ridge will move quickly east of the Cascades by late
Tuesday morning, and a weak frontal system will approach the
coast from the offshore waters by late afternoon. Tuesday should
start off mostly sunny, but high clouds will increase during the
late morning and afternoon once the ridge axis has moved east.
Highs should be mainly in the 60s to lower 70s.

The front and its precipitation will move inland rather slowly
Tuesday night, with the upper trough following the front into
Western Washington Wednesday afternoon. Most of the forecast area
should receive measurable precipitation mainly late Tuesday night
and Wednesday morning, but rainfall amounts for the interior
lowlands should be mostly less than a quarter inch. By Wednesday
afternoon, the post- frontal showers will become confined mainly
to the west slopes of the mountains and the Puget Sound
convergence zone. Wednesday will be a little cooler with highs
mostly in the lower to mid 60s.

The upper trough axis will shift slowly east of the Cascades
Wednesday night, but the trough will deepen and cyclonic flow
aloft should keep showers going Wednesday night. The Puget Sound
convergence zone should also continue through Wednesday night.
Showers will taper off on Thursday, and we should see some sun
breaks. Thursday will be another relatively cool day, though, with
highs in the lower to mid 60s and the snow level around 5500 ft.

.LONG TERM...Here is the long term section from the afternoon
forecast discussion -- Friday through Sunday, the upper level low
remains close enough, over Montana mainly, that some return flow
showers are possible over the Cascades each afternoon and evening.
Models show some instability associated with this moisture, so
have included a slight chance of thunderstorms over the Cascades
these days. It will be dry over the lowlands, with few clouds, so
as heights slowly rise the temperatures will also rise--from the
low 70s Friday to near 80 Sunday. On Monday the upper level low
will depart and heights will rise into the 580s. Have put some mid
80s in the forecast for Monday and conceivably it could get even
warmer. Burke


.AVIATION...Northwest flow aloft over Western Washington this
evening will back to westerly early Tuesday and then back further
to southwest on Tuesday afternoon. The air mass will remain
mostly dry tonight with light surface flow. High level moisture
will increase Tuesday afternoon.

Aside from patchy morning stratus along the coast, ceilings there
should remain 6000 ft or higher through Tuesday. For the Western
Washington interior, there should be no significant cloud cover
below 12,000 ft through Tuesday.

KSEA...Northerly wind 4-8 kt, switching to southwest late Tuesday
afternoon or Tuesday evening. No significant cloud cover below
12,000 ft through Tuesday. McDonnal


.MARINE...Light onshore flow will continue through this evening,
with low-end small craft advisory westerlies in the central and
eastern Strait of Juan de Fuca. A cold front will cross the
waters early Wednesday, followed by moderate onshore flow from
Wednesday afternoon through Friday. Small craft advisory
conditions are likely over the coastal waters ahead of and with
the cold front Tuesday night and early Wednesday. McDonnal


PZ...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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