Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 271007
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
310 AM PDT Thu Apr 27 2017

.SYNOPSIS...The onshore flow of moist unsettled air will maintain a
mix of showers and sun breaks again today. Then higher pressure
aloft is expected to build for a drier Friday into early Saturday.
Another weak weather system is forecast to swing onshore for a bit
more rain late Saturday turning to scattered showers Sunday.
Stronger higher pressure aloft is expected to start the month of May
with the prospect of more sunshine and above normal temperatures by
mid week.

&&

.SHORT TERM...Satellite imagery early this morning shows the flow
across the northern Pacific has more amplitude than a day ago,
thanks to the building upper ridge near 160E and the digging trough
near the dateline. The broad upper ridge near 140w is showing signs
of building. Still have a strong 160 kt plus jet rolling over the
ridge into the Pacific NW this morning.

Progs concur on one more embedded shortwave in the flow to swing
onshore today to enhance showers, particularly as daytime heating
kicks in this afternoon. Yet, not as many showers as on Wednesday
with sun breaks mixed in. Low level flow favors another Puget Sound
Convergence Zone to develop that should enhance the showers in the
central Sound into the Cascades as well.

The broad upper ridge should continue to build and provide a drying
trend tonight into Friday with more sunshine developing. High
temperatures should approach late April averages.

The next question is will the drier weather hang on into Saturday.
Again, guidance offers differing solutions in timing of the next
system. The Canadian and GFS bring some light rain to the NW
quadrant of Western Washington Saturday afternoon while the ECMWF
rushes the rain further inland. Will stick with the slower solution
given the higher amplitude upper ridge that develops Friday. Buehner

.LONG TERM...The next system swings onshore Saturday night with some
light rain turning to scattered showers for Sunday with most showers
in the mountains thanks to orographic flow. The showers should taper
off Sunday night as an upper ridge builds offshore.

Again, guidance is not consistent heading into early May. Ridging
aloft is anticipated. Yet with the warm advection, will there be any
accompanying light rain Monday into Tuesday despite the building
heights. In collaboration with NWS Portland and EC Vancouver, will
adjust the forecast to offer higher level clouds, a small threat of
light rain, and warming temperatures. By mid week, the upper ridge
should be more firmly in place for sunshine and perhaps the warmest
temperatures of the year thus far. Have those sunglasses handy.
Buehner

&&

.AVIATION...Moderate northwest flow aloft will weaken tonight and
Friday. Low level onshore flow will weaken this morning, then
increase again during the afternoon and evening. Air mass moist and
weakly unstable with scattered showers. Areas of MVFR cigs this
morning otherwise mostly VFR stratus lifting in the afternoon. The
Puget Sound convergence zone may develop again this afternoon and
evening from KPAE to just south of KSEA. A wind shift from
southwesterly to northerly is expected at the BFI/SEA terminal late
this afternoon. Opposing flow could occur for an hour or two at
those terminals.

KSEA...Southwesterly wind 8-12 kt increasing to 10-14 kt after 18z,
then switching to north-northwest beginning around 22-24z. Winds
should switch back to southwesterly by late evening. The Puget Sound
convergence zone may produce showers at the terminal later today and
this evening. MVFR cigs this morning should improve to mostly VFR
after 18z except MVFR possible during any showers.

.MARINE...Weakening onshore flow this morning should increase this
afternoon and evening. Small craft westerly winds in the Strait may
reach gale force late this afternoon and evening in the
central/eastern portion. The coast and other inland waters except
Puget Sound will have periods of small craft winds through this
evening. Westerly swell will also be near 10 feet today. Winds in
Puget Sound should stay below 20 kt but a switch from southwesterly
to northerly is expected during the afternoon and evening north of
Tacoma.

High pressure over the area will give light winds Friday into
Saturday morning. A front will bring increasing winds to the coastal
waters Saturday afternoon and to to the inland waters Saturday
night. Strong onshore flow Sunday will ease Monday.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory all waters except Puget Sound.
     Gale warning central and eastern Strait of Juan De Fuca from 2
   PM PDT through this evening.

&&

$$

www.weather.gov/seattle/

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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