Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 270258

Area Forecast Discussion
National Weather Service Seattle WA
800 PM PDT Thu Apr 26 2018

.SYNOPSIS...Onshore will bring stratus clouds inland tonight. An
upper level low will approach from the southwest on Friday with
showers spreading over the area, mainly in the evening. The upper
level low will maintain a cool and showery air mass through this
weekend. A ridge will bring drier weather by the middle of next week.


.SHORT TERM...Surface flow has already turned onshore, with westerly
wind at KSHN and down the strait. Satellite imagery shows marine
stratus moving well onshore at 03Z. The cooler marine air will reach
inland areas tonight. Low level stratus clouds will reach the south
interior by morning. Moisture wrapping around an upper level low
will spread north into the area Friday afternoon and evening for
a chance of showers. Temperatures will be closer to normal with
highs in the lower 60s.

The upper level low will slowly drift inland this weekend for a
continued chance of showers and cooler conditions. Highs on Saturday
and Sunday will be a few degrees below normal with temperatures
mainly in the mid 50s. Snow levels will lower to around 4500 feet in
the mountains. Burke

.LONG TERM...Previous discussion...As the main upper low exits on
Monday, a second weaker system will clip Western WA in the NW flow.
This means another round of showers in the forecast along with
temperatures continuing to trend below normal. An upper level ridge
will build offshore on Tuesday then shift inland Wednesday and
Thursday for warmer and drier conditions. The ECMWF keeps the ridge
over the Pac NW into the weekend but the GFS kicks the axis toward
the Rockies. Given the uncertainty, little changes were made from
the previous forecast. 33


.AVIATION...An upper low off the coast of northern California will
move slowly east/northeast tonight and Friday. Southerly flow aloft
tonight will become more east to southeast on Friday. At the
surface, a thermally induced trough of low pressure will shift
inland tonight; onshore flow has already developed. Onshore flow
will increase on Friday. The air mass is stable.

Stratus has filled in the coastal counties and is threatening
to reach KSHN by 05Z or so. Stratus around 1k ft is forecast to
reach KSEA at 14Z and KPAE by 16Z. Low level moisture will deepen
across the area on Friday. Mid and upper level moisture will move up
from the south later in the day as the upper low nears. Schneider/CHB

KSEA...Clear tonight. MVFR low clouds should reach the terminal a
little after sunrise Friday morning. It looks like low clouds will
hang around most of the day Friday as moisture deepens across the
region. Northerly wind 5-10 knots will ease tonight and become
southerly Friday morning. Schneider


.MARINE...Onshore flow has developed this evening, with 10-20 kt
westerlies in the strait and 10-20 kt southwest winds in the
southern Hood Canal area. There is a chance either or both of these
could tip upward into small craft advisory, so will watch the obs.
For now have updated the forecast for 10-20 kt.

Onshore flow will increase during the day Friday, until gale force
westerlies in the strait and small craft advisory southwest winds
elsewhere are a real possibility. A gale watch remains in effect for
the strait.

Onshore flow will continue, with diurnal variations, each day
through Tuesday. Small craft advisories are likely each day in the
strait, with gale force westerlies possible at times. CHB


PZ...Gale Watch from Friday afternoon through late Friday night for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.


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