Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 201628
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
928 AM PDT Tue Jun 20 2017

.SYNOPSIS...A cold front over the coast will be east of the
Cascades by midday today. Expect a Puget Sound Convergence Zone
to develop in the wake of the front and then dissipate this
evening. High pressure aloft will prevail Wednesday and Thursday
for typical late June weather.

&&

.SHORT TERM...
It looks like the front was on the coast at this time and should
be east of the Cascades by noon. Strong low level onshore flow
will develop in the wake of the front. Expect a Puget Sound
Convergence Zone (PSCZ) to develop about midday just north of the
Snohomish/King County line late this morning or early this
afternoon. The PSCZ will drift into King County by midafternoon,
possibly reaching KSEA by late this afternoon. Look for the PSCZ
to dissipate later this evening.

Strong northwest flow aloft will prevail over the region on
Wednesday. The low level flow will remain onshore but will be
weaker than today. There is some question as to how much low
clouds will be present tomorrow morning over the lowlands. The NAM
was cloudier while the GFS was sunnier. Currently, the forecast
leans in the direction of the NAM solution.

High pressure aloft will prevail on Thursday for typical late
June conditions.

.LONG TERM FROM THE PREV DISCUSSION...
It`s a split decision on Friday in the models with the NAM and
ECMWF bringing a dry trough down out of the north Thursday
night/early Friday inducing stronger onshore flow. The GFS and
Canadian not only don`t have this feature they both build the
upper level ridge over the area. Large variability in the
temperature guidance with the cooler models 5 to 10 degrees less
than the warmer solutions. Current forecast leans toward the
cooler solutions, which still have highs will above normal...in
the 70s to mid 80s. With the lack of consistency in the models
will stay with the current forecast. The weekend looks very warm
with the potential for the warmest days of the year Saturday and
Sunday (warmest day so far in Seattle 86 on May 28th). Thermally
induced trough making its way up the coast later Saturday into
Sunday. Model 925 and 850 mb winds are light but both have an
easterly component. 500 mb heights rising into the mid 580 dms.
Have bumped up the temperatures a couple of degrees over the
weekend with upper 70s to lower 90s forecast for both days. Low
level flow turns onshore during the day on Monday with 10 degrees
or so of cooling for the interior and more along the Coast. Felton

&&

.AVIATION...Weak upper level ridging in Western Montana combined
with a weak upper level trough over the northeast Pacific Ocean
will maintain moderate WSW flow aloft through Wednesday. A weak
front is moving through Western Washington this morning. This
front will dissipate as it moves east early this afternoon. A
Puget Sound Convergence Zone will form in the Seattle area in the
wake of the front this afternoon then will dissipate this evening.
Strong onshore flow behind the front this afternoon and tonight
will weaken on Wednesday.

IFR to low IFR conditions in low ceilings, fog, or drizzle
prevail along the coast and in central portions of the Puget Sound
lowlands this morning. Elsewhere, MVFR to VFR ceilings prevail.
Expect conditions along the coast and in the central portions if
the interior lowlands to improve to MVFR to VFR midday with strong
June insolation and increased mixing associated with increased
post-frontal onshore flow. MVFR conditions this afternoon will be
lowest with a Puget Sound Convergence Zone that will form around
KPAE midday then slide southward to KBFI and KSEA between 21Z and
00Z this afternoon. Conditions will improve to VFR most areas this
evening. MVFR ceilings from a cloud deck forming at the top of a
deepening marine layer will dominate late tonight into Wednesday
morning. Albrecht

KSEA...IFR conditions in stratus and some drizzle will improve
to higher end MVFR or VFR around 18Z as a cold front moves across
the area. Expect a developing Puget Sound Convergence Zone to the
north of the terminal to impact the terminal between 21Z and 00Z
with southwesterly winds 12G20KT shifting to northerly with the
passage with the convergence zone. Ceilings may fall to 015-025
with 3-5SM -SHRA around the convergence zone. Conditions will
improve after 02Z with northerly winds continuing. Albrecht

&&

.MARINE...A weak frontal system near the Washington coast will
move through the inland waters midday. Strong onshore flow will
develop in the wake of the front this afternoon through tonight. A
gale warning is in effect for the eastern two-thirds of the Strait
of Juan de Fuca from early this afternoon through tonight, and
small craft advisories for westerly to northwesterly winds are in
effect for the other waters (with exception to Puget Sound and
Hood Canal).

Moderate onshore flow will continue Wednesday, weakening on
Thursday then becoming northerly on Friday. A thermally induced
trough of low pressure will develop northward along the Oregon and
Washington coast on Saturday. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning from 2 PM this afternoon to 3 AM PDT Wednesday
 for the central and eastern Strait of Juan de Fuca.

 Small Craft Advisories are in effect for the remaining
 waters except the Puget Sound/Hood Canal.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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