Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 271601
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PDT FRI MAY 27 2016

.SYNOPSIS...Am upper level low will remain over British Columbia
through this weekend.  A series of small weak upper level shortwave
troughs embedded in the flow around the main low will bring
scattered showers at times to western Washington. Much warmer
weather is expected beginning Memorial day and continuing into the
middle of next week as an upper level ridge builds offshore then
moves over western Washington.

&&

.SHORT TERM...Satellite imagery shows mostly cloudy skies over
much of western Washington but there is a surprisingly large
hole in the clouds centered over south Puget Sound.  The radar
shows scattered showers in the cloudy areas. A convergence zone
has faded but models indicate it could form again at times through
tonight.

An upper level trough that is mainly over southern British
Columbia will affect western Washington weather through about
Sunday. Today it is likely to re-energize the convergence zone and
probably also fill in the currently clear areas with a cumulus
field. High temperatures today will be around 60.

A weak front will reach the coast Saturday morning and spread rain
inland during the day. The NAM is dry over Puget Sound but has rain
in the Cascades Saturday afternoon. Highs will continue to be on the
cool side...mid 50s to lower 60s.

The last in the series of shortwaves moves into western Washington
on Sunday, keeping a chance of showers in the forecast. Surface
pattern is once again favorable for a convergence zone over Snohomish
and northern King county in the afternoon. Highs will remain below
normal...mid 50s to mid 60s. Burke/Felton

.LONG TERM...Previous discussion...Some changes to the forecast
beginning Sunday night. Model trends continuing with the idea of
faster upper level ridge development offshore with the upper level
trough moving east faster as well. Have dried out the forecast
Sunday evening west of Puget Sound and taken the pops out of the
forecast overnight Sunday night/Monday morning. Flow at the lower
levels light Monday morning and with plenty of low level moisture
still around will be hard to get rid of the cloud cover. By
afternoon with the ridge building in and the low level flow going
northwesterly expect the sun to break out. With more sunshine in the
forecast have raised the high temperatures a few degrees with the
warmer locations getting into the lower 70s. Rapid warm up still on
tap for Tuesday as the upper level ridge moves overhead and the low
level flow goes offshore as a thermally induced trough builds along
the coast. Warmer locations on Tuesday should reach the lower 80s.
Another warm day on Wednesday with the thermally induced trough
moving inland. Combination of the upper level ridge shifting east
with plenty of high level moisture moving over the area in southerly
flow aloft and a transition to light onshore flow at the surface
will result in some cooling on Thursday. Felton

&&

.AVIATION...
An upper level low over the southeastern Alaska panhandle with a
secondary trough over north-central Washington will give moist and
stable northwest flow to the area through this afternoon. The
flow aloft will become more westerly tonight and Saturday as the
upper low over southeast Alaska drops into southwestern British
Columbia.

Showers and cigs around BKN020 are associated with a convergence
zone from SeaTac airport into northern King County. While ceilings
will improve to BKN035 midday, the convergence zone will linger
through about 06Z this evening. The convergence zone will shift
north and dissipate later tonight as weak low pressure and an
associated frontal zone moves into the coastal waters. The frontal
system offshore will then shift eastward onto the coast late
Saturday morning then inland Saturday afternoon. MVFR with local
IFR conditions in light rain and fog can be expected ahead of the
front, especially along the coast, beginning 10Z tonight then
continuing until the front passes midday Saturday. Albrecht

KSEA...SCT-BKN015-020 with another layer around BKN070 will
improve to BKN035 with showers in the vicinity about 18Z. Ceilings
will fill back in to BKN-OVC025-030 about 10Z tonight. Wind
21007-12KT through 06Z backing to 18007-12KT 06Z-18Z.  Albrecht

&&

.MARINE...A trough of low pressure aloft is inducing onshore flow
this morning with a convergence zone over central and northern
portions of Puget Sound. Forecast pressure gradients and mesoscale
models show inflow through the Strait of Juan De Fuca increasing
by mid afternoon and continuing until about midnight tonight...so
a Small Craft Advisory remains in effect for the central and
eastern portions of the strait through about 3 AM early Saturday
morning.

A 1017 mb low well offshore will deepen to about 1013 mb as it
approaches central Vancouver Island 18Z Saturday. A frontal system
to the east of the low will move through the coastal waters during
the morning hours on Saturday then will push eastward through the
inland waters Saturday afternoon. Small Craft Advisory conditions
are expected to develop over the coastal waters by 12Z Saturday
and continue until the front passes. At this time it appears that
winds will increase to about 20 kt from northern Admiralty Inlet
into the Northern Inland Waters during the morning hours ahead of
the front, and there is about a 50 percent chance that winds could
reach 25 kt there. The incoming GFS20 shows a mesoscale low
forming around Hood Canal during the afternoon hours on Saturday
behind the front, so there is a 50 percent chance that southerly
winds could reach Small Craft Advisory criteria in the central
Puget Sound from Alki Point to West Point Saturday afternoon and
early evening. Onshore flow and convergence zone activity will
likely resume Saturday evening through Sunday afternoon.

High pressure will increase over the offshore waters on Memorial
Day. This will give the potential for 20-25 kt nwly winds over the
coastal zones Monday afternoon and evening. Moderate to strong
inflow is also possible Monday evening through the strait.

The offshore ridge will likely shift east into the coastal and
inland waters mid week resulting in winds becoming more north to
northeast by Wednesday. Albrecht

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory from 5 AM to 3 PM PDT Saturday for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     Out 10 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory from noon today to 3 AM PDT Saturday 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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