Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 220326
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
826 PM PDT SAT MAY 21 2016

.SYNOPSIS...An upper low over the Pacific Northwest will maintain
a chance of showers for the next couple days. Weak high pressure
aloft will bring dry weather in the middle of next week. Another
upper low will drop over the area late next week for a return to
cooler and showery weather.

&&

.SHORT TERM...Showers have blossomed over Puget Sound in the last
couple hours. Models are in good agreement showing this activity
fading from the west over the next few hours. Showers should be
mostly confined to the Cascades by late evening.

A chance of showers will continue Sunday and Monday but the trend
will be for slowly decreasing chances each day. The best chances
will be over the Cascades, closest to the upper low. Weak high
pressure aloft will move over western Washington on Tuesday for a
generally dry day.

It will be generally cloudy the next few days but the trend will
be for slightly more sunshine each afternoon. Highs will be a few
degrees below normal Sunday, then warm slightly each day, back to
near normal by Tuesday. Schneider

.LONG TERM...There will probably be a period of dry weather for two
or three days centered around midweek. Night and morning marine
layer cloudiness should tend to break up for the afternoon hours.
Another upper trough will likely drop down over the Pacific
Northwest toward the end of next week for another round of showers.
The 12z gfs is possibly too dry, keeping the showers with low
pressure offshore on Friday and not kicking off any rain til
Saturday in the southeast flow aloft as the pretty deep upper level
low drops south and stalls just off Oregon through next weekend. The
ECMWF solution is different with an weaker open trough dropping
further inland and then developing into a cut off upper low over
Idaho instead of off the Oregon coast.

&&

.AVIATION...A large upper level trough will remain over the
Pacific Northwest through Sunday. The main low center over eastern
Oregon at the time of this writing will move over western MT by
Sunday morning. E flow aloft over W WA tonight will become weak
northerly on Sunday. Moderate W to SW onshore surface flow will
develop tonight which will increase the low level moisture over the
interior by Sunday morning. The air mass over W WA remains moist and
stable with cigs generally ranging from VFR to MVFR. Cigs are
expected to lower overnight to widespread MVFR conditions with some
spots dipping down into IFR by morning. Cigs should lift a little
Sunday afternoon. Kam/SMR

KSEA...Low end MVFR cigs in place now may dip down into IFR at times
during the overnight and into Sunday morning...before rebounding to
more solidly MVFR cigs by Sunday afternoon. SW onshore flow
overnight will contribute to the lowering conditions. Surface winds
will be SW 5-15 kt. Kam/SMR

&&

.MARINE...A weak surface low over Vancouver island will maintain
weak to moderate W-SW onshore flow through this weekend. The onshore
flow will rise to SCA levels in the central and east strait this
evening peaking in the 20-30 kt range then decreasing late tonight.
Current obs really only show Race Rocks getting close to
criteria...although with winds expected to pick up a bit during the
overnight...will leave current SCA in place for the strait and N
inland waters. Current obs for Puget Sound and Admiralty Inlet
however...having a hard time thinking winds will make it to criteria
there...and may ax that portion of the SCA for evening forecast
package.

The surface low over B.C. is expected to shift slightly NE early
next week, but this will not really impact the onshore flow pattern
very much. Onshore flow should continue through mid week with
varying strength, probably weaker on Monday then stronger on Tuesday
and Wednesday. SCA winds will occur mainly in the evenings. Kam/SMR

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Sunday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca-Northern Inland Waters Including The
     San Juan Islands.

     Small Craft Advisory until 6 AM PDT Sunday for Puget Sound and
     Hood Canal.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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