Small Craft Advisory 177, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
922 AM PDT Tue Jul 31 2018

SYNOPSIS...A major pattern change is getting underway. Onshore
flow will result in a significant cooling trend and eventually
lead to a few showers on Thursday. A fairly strong upper low
pressure trough will develop offshore this weekend and move across
the area around next Monday, bringing the possibility of a
wetting rain early next week.

SHORT TERM...
Pattern change underway as strong ridge of high pressure shifts
over the Great Basin today. As a result, onshore flow returned to
western WA and the marine layer made a push inland to east of the
I-5 corridor overnight. Skies should scatter out inland throughout
the day with another push onshore overnight into Wednesday. This
will begin the start of a noticeable cooling trend with daytime
highs in the 80s today and even cooler on Wednesday.

By Thursday, a weak upper level trough moves onshore and across
the area. This will further deepen the marine layer with daytime
highs struggling to make it into the upper 60s. Weak forcing may
be enough lift to squeeze out some drizzle or light showers across
much of the area so have continued the chance for precipitation in
the forecast. CEO

LONG TERM FROM PREVIOUS DISCUSSION...
Models have come in to very good agreement with the pattern
through early next week, favoring the development of a fairly deep
upper trough off the coast late this weekend, with a negative
height anomaly bullseye west of Vancouver Island and Northwest
Washington on Sunday and Monday. 500 mb heights will fall to 560
decameters or lower over the forecast area, which along with
cyclonic flow aloft, will fairly easily support showers in Western
Washington. Embedded shortwaves ejecting from the longwave axis
on Monday should bring recognizable rounds of showers by Monday.
Have introduced chance PoPs into the forecast, with room to boost
PoPs even more in later forecast updates. Bottom line is the
extended forecast will feature cooler than normal temperatures ---
and the distinct possibility of a wetting rain around Monday.
This would temporarily ease fire danger concerns locally for this
weekend and much of next week. Haner

&&

.AVIATION...
Wrn WA will remain btwn an upper level ridge centered
over the interior W and an upper level trof offshore today. Sly
flow aloft will continue. The low level flow will remain onshore.
Expect wdsprd IFR CIGs and areas of MVFR VSBYs to lift or
dissipate over the interior by 2000 UTC. Low clouds are expected
to persist over the coast thru this afternoon.

Meanwhile, expect the smoke layer to shift slightly ewd during the
day today as the flow aloft becomes SW.

KSEA...VFR conditions will return by 1900 UTC (noon PDT). Low
clouds will be back later tonight, likely arriving a couple of
hours earlier than last night. Winds will remain south or
southwest. There will be less wildfire smoke, if any, over the
area tomorrow morning.

&&

.MARINE...
Onshore flow of varying strength will persist the rest of this
week due to higher pressure offshore with low pressure east of the
Cascades. Gale force westerlies are possible again over parts of
the Strait of Juan de Fuca tonight.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from 6 PM PDT this evening through late tonight for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PDT this evening for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

&&

www.weather.gov/seattle