Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 292155

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PDT Thu Sep 29 2016

.SYNOPSIS...Friday will be the last mostly dry day for awhile. An
upper level trough will bring showery and cooler weather Friday
night through the weekend. Western and southern areas are the most
likely to get some rain. Unsettled weather will likely continue
early next week in northwesterly flow aloft.


.SHORT TERM...Cirrus bands have kept the day from being totally
sunny, but high temperatures will still end up in the 65-70 range
over most of the area. An upper level low well off the coast will
move south Friday, then fill Friday night as it moves into Oregon.
Flow aloft will turn southerly over Washington, and clouds will
increase, but Friday will probably be almost totally dry. As the low
moves into Oregon Friday night, some light rain could clip the coast
and Olympics. Highs on Friday will be in the 60s. The most obvious
change Friday may be the lowering snow level, which by afternoon
will be 6000-7000 feet, down from its current level of 10000 feet.

Saturday looks like the wetter day this weekend. Models agree that
rain from a second upper low moving south off the coast will spread
over Western Washington on Saturday. The amounts appear light and
the more mesoscale models do not have rain over every part of the
interior lowlands. However the trend is clear so have increased pops
for Saturday and Saturday night, especially over the mountains and
coast. Highs Saturday will be in the lower to mid 60s.

On Sunday the second upper low will move into northern California.
By afternoon it will probably be far enough away to keep most of
Western Washington dry, and some clearing is likely. The GFS even
has offshore surface flow on Sunday. If that feature persists in
later model runs then more drying and clearing seems likely. Highs
will rebound a few degrees on Sunday. Burke

.LONG TERM...In the wake of the upper lows, moist northwesterly flow
aloft will develop. Models are in rough agreement that weather
systems will arrive around Tuesday and again around Thursday. The
Thursday system looks stronger. Have kept chance pops most of the
Monday to Thursday period, but bumped them up somewhat Tuesday and
Thursday. Temperatures will be near normal, with highs in the 60s
and lows in the 50s. Burke


.AVIATION...An upper level low will spin offshore tonight with
southwesterly flow aloft. The air mass over Western Washington is
dry with mainly cirrus clouds overhead. Patchy low clouds and fog
will likely develop overnight then burn off late Friday morning...
MVFR to IFR conditions are likely. The upper level low will shift
inland Friday night with showers developing along the coast. 33

KSEA...Mainly high clouds this evening for VFR conditions. Patchy
low clouds will form in the vicinity by 12-15z for MVFR to IFR
cigs. North winds at the surface to 10 kt...easing and becoming
south 12-15z. 33


.MARINE...Onshore flow will prevail across Western Washington
tonight with high pressure offshore and lower pressure inland.
Westerly winds will increase through the Strait of Juan de Fuca
this evening - a Small Craft Advisory is in effect. A weak frontal
system will spiral offshore on Friday then diminish as it moves
inland Friday night. Another weak surface low will track down 130W
on Saturday then shift into southern Oregon on Sunday. A stronger
Pacific frontal system will reach the area on Monday. 33


PZ...Small Craft Advisory until midnight PDT tonight for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.



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