Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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486
FXUS66 KSEW 291540
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
845 AM PDT SUN MAY 29 2016

.SYNOPSIS...Showers will diminish today as an upper level low over
British Columbia moves slowly east. A strong upper level ridge will
build over the region Monday, bringing sunny skies and warmer
conditions through the middle of next week.

&&

.SHORT TERM...Showers have been persistent in Skagit and Snohomish
counties this morning. Have updated the forecasts and grids to give
higher pops to those areas this morning. Models drag a convergence
zone south during the day today and dissipate it in the afternoon.
Some clearing in western areas is already apparent in satellite
imagery. This trend will allow high temperatures to rise into the
60s today. Only a few showers, mainly in the mountains, will
continue this evening and things should completely dry out after
midnight as an upper ridge builds offshore.

The upper ridge axis will be along the coast Monday and right over
western Washington Tuesday. At the surface the flow will turn
northerly and offshore as thermally induced low pressure develops
along the coast. Skies will clear and temperatures will warm each
day, reaching the low 70s Monday and near 80 Tuesday. Burke

.LONG TERM...Wednesday may not turn out to be the warmest day of
the week as flow seems solidly onshore by then, but highs still seem
likely to be around 80. A weak weather system will bring a chance of
showers to the coast on Thursday and perhaps a thunderstorm or two
to the Cascades. Clouds will cause a slight dip in high temperatures
on Thursday. Then models rebuild heights over the region Friday and
Saturday for a return to 80 degree temperatures. Burke

&&

.AVIATION...
An upper level trough over British Columbia extending south
across Washington will shift eastward through tonight. High
pressure aloft will build over the northeast Pacific this
afternoon through Monday. Light northwesterly flow aloft. The air
mass will be generally stable and moist in the lower levels with
some slight destabilization of the boundary level expected during
the afternoon hours today. The air mass will gradually dry
tonight and become mostly dry on Monday as low level northerly
flow develops.

Convergent flow around the Olympics is producing showers from the
northeastern tip of the Olympic peninsula northeastward to Skagit
and central Whatcom county. Another area of convergence is
developing from west of downtown Seattle northeastward through
Snohomish county. This second area of convergence is producing
showers, some fog, and areas of ceilings between 500 and 1000
feet. Latest mesoscale models and the HRRR show the areas of
convergence dropping southward through the central Puget sound
area between 18Z and 21Z and possibly making it south of Seatac
airport between 21Z and 00Z before it gradually dissipates after
03Z. This appears like a reasonable solution given the ridging
occurring offshore and the upper trough shifting off to the east.
Expect the IFR and low MVFR ceilings to gradually improve to
BKN020-025 OVC035-040 around 18Z then for the lower layer to
mostly scatter out 20Z-22Z as the boundary layer destabilizes
slightly with late May insolation.

VFR conditions are expected to dominate tonight into early Monday
with clearing after 20Z Monday as low level north to northeast
flow develops and upper level riding builds onto the coast.
Albrecht

KSEA...South to Southwest wind 3-6 KT will become variable around
18Z then shift to northerly 6-8 KT after 21Z. The wind shift will
occur as a convergence zone to the north of the terminal moves
southward during the afternoon hours. There will likely be a 2-3
hour period of opposing flow between KBFI and KSEA midday to early
afternoon. Cigs around 010-012 will lift to 020 around 18Z then
scatter out leaving a broken layer 035-050 after 21Z. Showers will
generally increase as the convergence zone sags southward today,
then will end early this evening as the convergence shifts off to
the southeast and weakens. Expect VFR conditions in northerly flow
6-8 KT tonight and Monday. Albrecht

&&

.MARINE...
Post-frontal onshore flow will prevail through tonight but
gradually ease. Small Craft Advisory westerly winds are expected
in the central and eastern portions of the Strait of Juan De Fuca
later this afternoon and tonight until about 4 AM PDT. Northern
portions of Admiralty Inlet may see a few hour period of Small
Craft Advisory conditions during the evening hours with the inflow
through the Strait of Juan De Fuca. Winds over the other waters
will have winds staying below 20 KT.

High pressure will build over the offshore waters on Memorial
Day. This will give the potential for northwesterly winds up to 25
KT over portions of the coastal waters Monday afternoon and
evening. Winds will become northerly inland and dominate on
Tuesday as a thermally induced trough of low pressure nudges
northward along the western Oregon and south Washington coasts.
The thermal trough will push inland Wednesday afternoon and
evening ushering in a marine push during the evening hours.
Albrecht

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory until 4 AM PDT Monday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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