Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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783
FXUS66 KSEW 300358
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
858 PM PDT SUN MAY 29 2016

.SYNOPSIS...Skies will clear tonight as an upper level ridge
builds offshore. The upper level ridge will move slowly east across
western Washington Monday and Tuesday bringing sunny skies and a
short warming trend. A weak weather system will bring increasing
clouds Wednesday and a chance of showers Thursday and Friday.
Another upper level ridge will build next weekend for sunny and
warmer conditions.

&&

.SHORT TERM...RADAR shows showers ended over W WA late this
afternoon and visible satellite imagery shows clouds diminishing
with mostly clear skies.

Warm advection well offshore along 140W, caused by southerly flow,
will pump up the the downstream upper level ridge, which is just
offshore from W WA. Models still support sunny skies and a warming
trend as they bring the ridge axis just offshore Monday afternoon
then inland over W WA Tuesday afternoon. However, model solutions
have changed in the past 24 hours, starting with the Tuesday period.
They are now going with a slightly weaker upper level ridge and
staying with N-NE low level offshore flow, rather than developing
easterly flow. Onshore flow also redevelops more quickly Tuesday
afternoon. The evolving model solutions won`t affect Monday and
Tuesday very much, with sunny skies and Sea-Tac highs in the lower
70s Monday and near 80 Tuesday.

Onshore flow shifts back onshore Tuesday afternoon as the upper
level ridge axis moves over the Id panhandle and a weak cold front
offshore approaches the region.

Wednesday now looks quite a bit cooler than previous model
solutions, with highs dropping back into the mid 70s. Flow aloft
will be WSW with high clouds from the approaching weak cold front
spreading over the NW part of W WA.

GFS, ECMWF, and Canadian models all show a second cold front
catching up with the initial system, then spreading some rain over
the northwest part of W WA sometime Wednesday night or Thursday.
However, the models disagree on timing with the faster ECMWF
bringing rain Wednesday evening, the Canadian a few hours later, and
the GFS holding off until Thursday, The current forecast
compromises and has a chance of rain Wednesday night. Kam

.LONG TERM...previous discussion from the 3 PM AFD...the EURO and
GEM are remarkably wet with a weak weather system that drags through
the area on Thursday. The GFS is drier. As a compromise will bring a
chance of showers to most areas Wednesday night through Friday
morning. Clouds will cause a slight dip in high temperatures on
Thursday. Then models rebuild heights over the weekend for a return
to 80 degree temperatures. Burke

&&

.AVIATION...
High pressure at the surface and aloft will build over the northeast
Pacific tonight and Monday. The upper ridge will move into Western
Washington Monday night. Light northerly flow aloft will become
westerly Monday night. The air mass is stable and becoming dry.

KSEA...Northeast wind 4-6 kt after 06Z. Winds will shift to more
northerly about 19Z Monday. VFR conditions with few-sct clouds around
6k ft this evening...then mostly clear. DTM

&&

.MARINE...
Residual flow through the Strait will maintain Small Craft winds in
the Central and East portion through much of tonight. Winds over
the other waters will likely stay below 20 KT tonight through Monday.

High pressure will build over the offshore waters on Memorial
Day with northwesterly winds up to 20 kt in the afternoon and
evening. Winds will become northerly inland and northerlies will
dominate on Tuesday as a thermally induced trough of low pressure
develops northward along the Oregon and south Washington coasts.

Forecast models are now about a day quicker in shifting the
thermal trough eastward across the Cascades. Onshore flow is
now expected to develop Tuesday night and continue through
Wednesday night or Thursday. Winds are likely to reach Small Craft
Advisory criteria in the central and east portions of the Strait
of Juan de Fuca during the late afternoon and overnight hours
Tuesday and Wednesday.

High pressure over the offshore waters early Thursday will shift
east into the coastal waters later Thursday then inland on Friday.
DTM/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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