Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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689
FXUS66 KSEW 181018
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
318 AM PDT MON JUL 18 2016

.SYNOPSIS...An upper level low will bring another round of showers
and thunderstorms today, especially in the Cascades. The low will
weaken as it moves overhead Tuesday and Wednesday. A weak trough
will pass through the area on Thursday. Expect warmer and drier
weather this weekend as a ridge builds over the Pacific
Northwest.

&&

.SHORT TERM...An upper level low offshore will maintain a moist
and unstable air mass across Western Washington today, due to diffluent
southerly flow aloft. The highest instability is over the Cascades
where showers and thunderstorms will fire up once again. It looks
like it will be a little more active compared to yesterday with
stronger winds aloft. There have been a few strikes early this
morning down in Oregon. This wave will reach Western WA this
afternoon in prime time for convection. There is a slight chance
of showers and thunderstorms in the interior but it`s likely that
most of the activity will remain in the Cascades. The Olympics may
also see building cumulus cells this afternoon which may also
drift north into the Strait of Juan de Fuca for a chance of
thunderstorms too. Showers and T-storms will diminish this evening
with a few showers lingering in the North Cascades past midnight.

The upper level low will move inland and weaken as we move into
Tuesday, then weaken further on Wednesday. Expect just isolated
showers across the area. The showers may be limited to just the
Olympics and North Cascades on Wednesday.

A second, weak trough will push through the area on Thursday but
it still looks mainly dry across Western WA. A stronger onshore
push will bring marine clouds farther inland Thursday night. 33

.LONG TERM...A broad upper level ridge will build over the West
late this week into the weekend for warmer and drier weather
across Western WA. Temperatures will trend up a few degrees with
highs back into the upper 70s and maybe low 80s. The low level
flow will remain onshore for areas of late night and morning
clouds...with sunbreaks in the afternoon. A trough may break the
ridge down early next week. 33

&&

.AVIATION...A broad upper low will remain over the offshore waters
through Tuesday with weak southerly flow aloft today becoming
southwesterly on Tuesday. Low level onshore flow will prevail with
high pressure offshore and lower pressure inland. An upper level
disturbance rotating around the upper level trough will work to
destabilize the atmosphere later today for the potential of a few
thunderstorms across the interior...especially over the mountains.

VFR conditions encompass much of Western Washington early this
morning with areas of ceilings mostly in the 5000 to 6000 foot
range except for a few pockets near 3000 feet along the coast.
Areas of MVFR ceilings are expected to develop toward daybreak but
should dissipate by late morning. An upper level disturbance
pivoting around the upper level trough centered offshore will bring
a chance of thunderstorms to the region this afternoon into this
evening. The best potential will be over the Cascades and
Olympics.

KSEA...A scattered to broken stratocumulus deck near 5000 feet will
linger through the morning. MVFR low clouds may develop in the
vicinity of the terminal this morning near sunrise...dissipating by
late morning. The above mentioned upper level disturbance will
spread additional mid and high level moisture into the area by late
morning. Numerical guidance is not in good agreement on thunderstorm
potential. For the time being, the threat appears too low to mention
in morning forecast. Surface winds south to southwest 5 to 10 knots.
27

&&

.MARINE...Onshore flow will prevail for the next several days
with high pressure offshore and lower pressure inland. Small craft
advisory west winds are forecast again this evening in the Central
and Eastern Strait of Juan De Fuca. Onshore gradients will be
marginally stronger this afternoon and evening than yesterday, but
have kept central and east strait winds in the 20 to 30 knot range
for late today.

Weaker onshore flow is expected Tuesday and Wednesday with
no headlines expected, then onshore flow will increase again
Thursday and Friday for more small craft advisory westerlies late
in the day in the Central and Eastern Strait. 27

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT early this morning for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 2 PM this afternoon to 5 AM PDT
     Tuesday for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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