Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 210959
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
259 AM PDT Sat Apr 21 2018

.SYNOPSIS...A cold front will move through the Puget Sound area
into the Cascades early this morning. Low level onshore flow
behind the front with a convergence zone developing quickly this
morning. The convergence zone will last through the afternoon.
Dry upper level trough moving through Sunday morning. An upper
level ridge will build Sunday night and remain over Western
Washington most of next week with the warmest weather of the year
expected mid week.

&&

.SHORT TERM...Satellite imagery shows cold front approaching the
Puget Sound area at 09z/2 am. Narrow band of rain with the front
with Doppler radar showing post frontal showers already reaching
the Coast. Winds have picked up in the last hour over the interior
with gusts in the 25-30 mph range. Temperatures at 2 am were in
the mid 40s to lower 50s.

Front will be into the Cascades by 12z with the brief spell of
rain coming to an end for the interior. Plenty of post frontal
showers behind the front on the satellite imagery with the west
northwesterly flow aloft behind the front creating a bit of a rain
shadow over the central Puget Sound. Gradients through the Strait
of Juan De Fuca have turned positive with the winds at Race Rocks
increasing from southwest 3 knots to west 18 knots in the last
hour. Convergence zone will form this morning over Snohomish
county then drift slightly south into the afternoon hours as the
southerly gradients decrease. By mid afternoon expect the
southern portion of the convergence zone to be over northern King
county. Outside of the convergence zone, shower activity will
become pretty much confined to the mountains this afternoon.
Highs will be in the 50s.

Convergence zone dissipating this evening. Isolated shower
activity in the mountains coming to an end around sunset. Light
flow in the lower levels will make it hard to scour out the cloud
cover overnight into Sunday morning. Lows will be in the mid 30s
to mid 40s.

Dry upper level trough moving through the area Sunday morning with
northwesterly flow aloft behind the trough. This will help clear
the cloud cover out by afternoon. High temperatures will warm back
to near normal, mid 50s to lower 60s.

Upper level ridge building over western Washington Sunday night
and Monday. Low level flow turning offshore on Monday as a
thermally induced surface trough expands northward along the
Oregon coast. With the increasing offshore flow Monday afternoon
high temperatures will warm into the lower 70s in the warmer
locations with 60s for most of the remainder of the area. It will
be a chilly start to the day Monday morning with the light flow in
the lower levels as a result of the transition to offshore flow
allowing temperatures to drop into the 30s and lower 40s
overnight Sunday night.

.LONG TERM...Extended models in good agreement on the 00z run with
the warmest weather for the year so far in store for western
Washington Tuesday through Thursday. Upper level ridge remaining
over the area with the low level offshore flow that starts on
Monday lasting until Thursday morning. Model 850 mb temperatures
in the plus 8 to 10C range on Tuesday warm to the plus 14 to 16C
range by Thursday. High temperatures in the 70s will be common
Tuesday through Thursday with the warmer locations pushing 80
degrees by Thursday. The exception will be near the Strait of Juan
de Fuca where highs will be in the 60s. Low level flow turns
onshore on Friday as a shortwave spinning out of an upper level
low off the southern Oregon coast approaches the area. The
resulting marine push with the transition to onshore flow will
lower high temperatures 10-15 degrees on Friday, back down into
the upper 50s to mid 60s. Felton   RIP HarryW

&&

.AVIATION...A cold front will push through Western Washington
early this morning for rain and breezy conditions. The flow aloft
is southwesterly. The air mass is moist with MVFR conditions
possible. Post-frontal onshore flow will prevail this afternoon
and a Puget Sound Convergence Zone will likely develop in
Snohomish/King counties. Showers from the convergence zone may
linger into the evening then diminish as onshore flow weakens.
Expect drier weather on Sunday as an upper level ridge builds
inland. 33

KSEA...Showers in the region this morning with gusty S/SW wind to
20 kt. May see a break in the early afternoon, then showers from a
PSCZ forming by 21z. May see a wind flip to N/NW as the PSCZ sags
south and diminishes 00-03 tonight. 33

&&

.MARINE...Cold front currently crossing the inland waters. West
flow already developing through the Strait, with westerly gales
expected in the central/eastern Strait this morning. Small Craft
Advisories all other waters. West swell along the coast in excess
of 10 feet for a time today.

A surface high pressure ridge will nose across the waters tonight,
for quickly weakening gradients. A thermally induced low pressure
trough will develop along the Washington and Oregon coasts on
Sunday night and Monday morning, bringing east-northeast wind. It
will continue in place through Wednesday.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 6 PM PDT this evening for Admiralty
     Inlet-Coastal Waters From Cape Flattery To James Island 10
     To 60 Nm-Coastal Waters From Cape Flattery To James Island
     Out 10 Nm-Coastal Waters From James Island To Point
     Grenville 10 To 60 Nm-Coastal Waters From James Island To
     Point Grenville Out 10 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters
     From Point Grenville To Cape Shoalwater Out 10 Nm-Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory for rough bar until 6 PM PDT this evening
     for Grays Harbor Bar.

     Gale Warning until noon PDT today for Central U.S. Waters Strait
     Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De
     Fuca.

     Small Craft Advisory until 3 PM PDT this afternoon for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until noon PDT today for Puget Sound and
     Hood Canal.

&&

$$

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