Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 261020
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
320 AM PDT Wed Apr 26 2017

.SYNOPSIS...Moist unsettled flow from the Pacific into Western
Washington will maintain showery weather with some sunbreaks at
times through Thursday. A developing Puget Sound Convergence Zone
will enhance showers later today in the central Sound area. Higher
pressure is expected to build Friday for a respite from the wet
weather that may hang on into Saturday. Another weather system is
forecast to arrive by Sunday for more rain before higher pressure
rebuilds early next week for drier weather heading into May.

&&

.SHORT TERM...The old saying - April showers bring May flowers -
we certainly have had plenty of April showers with more in store the
next few days. A flat upper level ridge resides southwest of the
Pacific NW this morning with a 140 knot plus jet extending from near
the dateline into the region. There are a sequence of shortwaves
embedded in the jet that will maintain moist unsettled weather into
Thursday.

Tuesday`s weakening front moved onshore overnight with spotty rain.
The next shortwave approaching 130W appearing well on water vapor
satellite imagery at this early hour will move onshore and enhance
showers later today, particularly in southern sections of the area.
At the surface, a ridge of higher pressure off the coast will
increase onshore flow as well. This pattern will help create a Puget
Sound Convergence Zone later today that has the potential to sag
south from northern Kitsap and Snohomish Counties south well into
King County this evening thanks to the strong flow to develop in the
Strait of Juan de Fuca.

Another embedded shortwave is forecast to swing onshore Thursday to
keep the showers going, but mainly in the mountains and higher
terrain. High temperatures look to remain a bit below late April
averages.

Guidance strongly suggests that Friday may offer a break in the
precipitation with some sunshine as a ridge of higher pressure
builds off the coast. Have those sunglasses handy. Buehner

.LONG TERM...The next question is will the drier weather hang on
into Saturday. The Canadian and ECMWF concur with the ridge holding
for another dry day while the GFS is quicker in spreading
precipitation onshore during the day. At this point, have indicated
a growing threat of rain for the Olympic Peninsula and coast during
the day Saturday and increasing clouds elsewhere. Then this weather
system should provide more rain Saturday night with decreasing
showers on Sunday.

As the month of May begins, guidance again is not consistent. The
ECMWF rebuilds the upper ridge while the Canadian and GFS swing yet
another shortwave onshore for additional showers. But beyond Monday,
restless natives eager for some sunshine may get some as it appears
higher pressure aloft will build over the region. Buehner

&&

.AVIATION...Moderate westerly flow aloft becoming light
northwesterly flow aloft tonight as a trough shift eastward. Strong
onshore flow today with a moist and weakly unstable air mass air
mass over Western Washington through this evening. Showers will
decrease later tonight. The Puget Sound convergence zone could
become active this afternoon and evening in the prone corridor from
Sea-Pae. A wind shift to the north-northwest will occur over north
Puget Sound this afternoon and could cause southwest winds at
KBFI/KSEA to turn westerly or switch to north-northwest for a few
hours late in afternoon and early evening. Opposing flow w/ wind
speeds to 10 kt at KSEA/KBLI possible roughly 00z-05z early this
evening. Mostly MVFR cigs and isolated IFR cigs this morning
becoming predominately VFR this afternoon except lower with showers
or near the convergence zone.

KSEA...The air mass is saturated near the surface with periods of
IFR possible until around daybreak. Otherwise predominately MVFR
cigs this morning improving to mostly VFR this afternoon. Breezy
southwest winds 10-16 kt w /gusts 25-28 kt. The Puget Sound
convergence zone will develop during the afternoon and could reach
the terminal by around 00z late this afternoon. Winds could shift
more westerly and then north-northwesterly at times through 06z
before switching back to a southerly direction and decreasing. A
period of opposing flow at KSEA/KBFI could occur late this afternoon
or evening w/ wind speed up to 10-12 kt out of the south and lighter
from the north direction. dtm

&&

.MARINE...Onshore flow will increase today behind a front as an
upper level system crosses the area. Winds should gradually east
later tonight. Small craft winds expected most waters for post
frontal onshore west-southwesterly winds. A stronger westerly push
is anticipated in the central/east Strait, thus a gale warning is up
from midday through most of tonight. Small craft winds may continue
over other waters near the east entrance to the Strait. dtm

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small craft advisory all waters.
     Gale warning central and eastern Strait late this morning
through late tonight.

&&

$$

www.weather.gov/seattle/

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html


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