Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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132
FXUS66 KSEW 221009
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT Tue Aug 22 2017

.SYNOPSIS...High pressure will provide dry weather and warmer than
average temperatures today. An upper level system will approach the
region Wednesday bringing cooler onshore flow and a few showers to
the coast. The upper trough moves across Western Washington
Wednesday night and Thursday with spotty light showers and highs
struggling to reach the low 70s inland. Another ridge builds Friday
through Monday with more dry weather and above average temperatures.

&&

.SHORT TERM...Satellite imagery shows mostly clear skies across
Western Washington early this morning with just some patchy low
stratus near the coast. High pressure will provide dry weather with
highs reaching the low to mid 80s under building upper heights. The
ridge will shift east tonight with a return to onshore flow over the
area. Falling heights and onshore flow will knock high temperatures
down closer to average in the mid 70s around Puget Sound Wednesday.
The leading weak front will dissipate as it reaches the coast but
few light showers could measure near the coast.

The main upper trough has trended weaker but faster the past few
model cycles. Both the GFS/ECMWF show just spotty light
precipitation amounts with shower activity ending behind the trough
Thursday afternoon. There could be some sunbreaks late in the day
and MOS ranges are narrow giving highs in the low 70s around Puget
Sound Thursday afternoon.

If Sea-Tac reaches 70 or higher Thursday, then the record for
consecutive days reaching 70 degrees or above will be in jeopardy.
Currently, the streak is at 54 days with 70+ degrees in the forecast
through at least the next week, although Thursday could be a close
call.

.LONG TERM...High pressure builds Friday through early next week.
The pattern appears dry with a strengthening ridge. High temperatures
were trended warmer in the forecast with widespread 80s around
interior Western Washington over the weekend and possibly into
Monday. A few of the warmer locations in the southwest interior
could get close to 90 by Sunday or Monday but that`s a ways out
still.  Mercer

&&

.AVIATION...Strong upper level ridge over the Pacific NW interior
will continue to slowly drift inland through tonight. Light SW flow
aloft. At the surface, high pressure remains offshore with lower
pressure east of the Cascades for low level onshore flow. Air mass
dry except for a quite shallow marine layer along the coastline.

Skies remained clear across Western Washington overnight with the
exception of areas of fog/stratus along the beaches. The marine
layer is surface-based and quite shallow. Expect low IFR stratus to
drift onshore into the coastal areas with little if any further
inland surge this morning. Expect the shallow marine layer to
dissipate over land areas before noon today, pulling back to just
off the beaches.

Developing stronger low level onshore flow tonight should help
thicken the marine layer and bring it more into the interior
overnight tonight and Wednesday morning.

KSEA...Northerly wind 5-8 kt becoming less than 5 kt by sunrise then
becoming SW to W 5-10 kt midday today. Skies to remain clear through
this evening. Look for MVFR ceilings developing by 12z Wednesday
morning. Buehner

&&

.MARINE...Higher pressure off the coast with lower pressure east of
the Cascades is expected to continue into the weekend. This pattern
will maintain varying degrees of low level onshore flow across
Western Washington.

A weak upper level trough is forecast to approach British Columbia
tonight and Wednesday, and continue inland Thursday. This system
should increase onshore flow into Western Washington tonight and
again Wednesday night for gale westerlies in the Strait of Juan de
Fuca tonight and likely again Wednesday night. Small craft advisory
winds to develop this evening in the adjacent Admiralty Inlet and
Northern Inland Waters. Buehner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning through late tonight for Central U.S. Waters of
     Juan De Fuca-East Entrance U.S. Waters Strait of Juan de Fuca.

     Small Craft Advisory from this evening through late tonight
     for Admiralty Inlet and 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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