Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 131022
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
322 AM PDT Sun Aug 13 2017

.SYNOPSIS...A cold front moving across the area early this morning
will weaken as it moves east of the Cascades. Post frontal onshore
flow will give a convergence zone and chance of showers to the
west slopes of the Cascades and to mainly the Central Puget Sound
lowlands through early this evening. An upper level ridge will
gradually move into the area toward the latter half of the
upcoming week then will move east of the area next Friday or
Saturday. The ridging will give a gradual warming trend and dry
weather Monday through Thursday or Friday.

&&

.SHORT TERM...Changes to the forecast early this morning are
mainly seen in the extended period from about Wednesday through
Friday where temperatures are expected to be a few degrees warmer
than previously expected. Otherwise, forecasts are consistent with
the previous package with only minor changes.

The frontal system moving through Western Washington early this
morning is shearing apart with upper level clouds and moisture
racing eastward across the region and the low level front lagging
somewhat, sitting from about Central Vancouver Island southward
through the outer portions of the coastal waters. The ripped apart
front is resulting in very light rainfall amounts across the area
with interior zones only seeing 0.05 inches or less. However, the
record-long dry streak at Seatac Airport, (56 days with no rain
which beat the old record of 51 days set in 1951) came to an end
before midnight PST when they report 0.02 inches. Light rain will
continue at times during the pre dawn hours as the lower level
portion of the front drags eastward across the area.

From about mid morning on, westerly onshore flow will increase
behind the surface front and flow aloft will increasingly interact
with the terrain to form a Puget Sound convergence zone in the
area from the Kitsap Peninsula eastward to the Seattle/Everett
area and into the Cascades. Precipitation along the coast, in the
Chehalis Gap area, and from the Strait of Juan de Fuca to the San
Juans and coastal portions of the north interior will see skies
rapidly become partly sunny and showers become rather isolated
during the morning hours. The convergence zone shower activity in
the Seattle/Everett area appears that it will likely linger into
the afternoon and possibly into the early evening hours per the
various forecast models. High temperatures will be tricky as
upper 60s to mid 70s can be expected with any sunshine. The
coastal zones, areas near the water in the north interior, and
portions of the central Puget Sound area from downtown Seattle to
Everett may fail to hit 70 degrees today.

The air mass will gradually dry this evening as onshore flow
decreases and the upper trough behind the incoming front pushes
off to the east.

Monday and Tuesday will see areas of marine clouds during the
night and morning hours, partly to mostly sunny afternoons, and
high temperatures 1-3 degrees below normal for this time of year.
Nighttime lows will generally fall into the 50s. Albrecht

.LONG TERM...Upper ridging forming over the eastern North Pacific
Tuesday will shift eastward across Western Washington Thursday or
Friday, then will push eastward into western Montana next weekend.
The GFS solution is more robust with the warming and ridging than
the ECMWF, but even the ECMWF has decent 500-mb height rises by
about Thursday with heights over 580 DAM. Temperatures were warmed
to 4-6 degrees above normal by the latter portion of the upcoming
week and morning low clouds were cut back mainly out of the
interior lowlands by Thursday and Friday.

Troughing will approach the area next weekend bringing cooler
temperatures and an increase in marine moisture. Albrecht

&&

.AVIATION...A front moving onshore will push through Puget Sound
early this morning. The upper trough traversing the region today
will bring light southwest flow aloft becoming northwesterly later
today and tonight. Air mass stable and moist with some gradual
drying in the mid and upper levels. Low level moisture will linger
through tonight. Areas of light rain should end this morning, except
where any convergence zone develops in the favored KPAE-KSEA/KBFI
corridor today.

Cigs will be highly variable this morning as the front passes.
Linger Widespread MVFR cigs with some local IFR will improve to
predominately VFR stratus by later this morning. Clouds may scatter
in some areas late in the day. Less extensive local MVFR stratus
will be possible later tonight/early Monday morning.

A convergence zone is expected to form behind the front over north
Puget Sound around KPAE during the morning, then drift south
reaching the KBFI midday and KSEA during the afternoon. Expect
showers and a wind shift from southwesterly to northerly at these
terminals. A couple hours of opposing flow at KBFI/KSEA is possible
between 19-23z.

KSEA...Spotty light rain ending by mid morning. Mostly MVFR cigs
becoming VFR by late morning. A convergence zone is expected to
drift south through the KSEA terminal this afternoon with southwest
winds 8-12 kt shifting to the north 5-8 kt. Winds shifting more
northeasterly early this evening. dtm

&&

.MARINE...A front moving inland will dissipate as it pushes east of
the inland waters early this morning. Westerlies will gradually ramp
up in the Strait with Gales likely in the central/east portion from
late morning through this evening. Winds could reach gale force a
little earlier than expected, possibly as early as 8 am but more
likely toward midday. Small craft westerlies will reach the waters
adjacent to the eastern Strait later this morning. Winds will ease
later this evening as high pressure builds inland over the waters.
Varying degrees of onshore flow will prevail Monday through Thursday
as surface high pressure persists. dtm

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning until 11 PM PDT this evening for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory from 9 AM this morning to 11 PM PDT this
     evening for Admiralty Inlet-Northern Inland Waters
     Including The San Juan Islands.

&&

$$

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



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