Small Craft Advisory 221, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
937 AM PDT Thu Oct 4 2018

.SYNOPSIS...A weak upper level trough will exit the area around
mid-day, with a brief period of clearing this afternoon in its
wake. On Friday, widespread rain is expected, with heaviest
amounts favoring Southwest Washington, due to a low pressure wave
that will move onto the north Oregon coast. Drying trend on
Saturday with northerly flow aloft behind the front. Next system
moving down from the northwest reaching the area on Monday.

&&

.SHORT TERM...An upper trough axis is moving eastward across
Western Washington right now. It came in with little fanfare other
than for the increase in mid-high level clouds. With the trough's
exit today, clouds will decrease this afternoon.

On Friday and Friday evening, a widespread wetting rain is
expected over coast and over Southwest Washington, with lighter
rainfall amounts elsewhere. This is owing to a 1005 mb low center
that will pass E-SE across 130W on Friday morning, filling to 1010
mb upon reaching the North Oregon Coast on Friday evening. A
heavier swath of rain is expected in the deformation zone to the
left of the low track. This heavier swath of rain should pass
across the coast and across Southwest Washington, where about 1/3"
to 2/3" of rain is forecast. Locations north of Seattle should
get 1/4" or less.

Quite a high amplitude pattern will be in place on Saturday, with
a longwave ridge out around 140W-145W, and a longwave trough
carving out over the Great Basin and interior West. With north-
northwest flow aloft over Western Washington, a shortwave ridge on
Saturday and northerly surface flow will bring rapid clearing in
the morning, with sunny weather in the afternoon. Haner

.LONG TERM...From Previous Discussion: Extended model solutions
inconsistent once again with the GFS bringing another system down
in the northwesterly flow aloft on Sunday while the ECMWF is much
slower with the front keeping the area dry. ECMWF keeps the system
to the north Sunday night while the GFS brings another front
embedded in the northwesterly flow aloft close enough to the area
to spread rain into at least the northern half. The GFS is much
wetter and faster moving the system into the area on Monday versus
the slower and weaker ECMWF. There is some general agreement with
dry north northwesterly flow with an upper level low moving by to
the east Tuesday or Wednesday. Previous forecast more in line
with the ECMWF solution and with the lack of consistency in the
models no reason to change much in the extended forecast keeping
Sunday mostly dry, at least a chance of rain on Monday and Tuesday
with a dry Wednesday. Felton

&&

.AVIATION...A weak upper trough will shift east of the area today.
Light westerly flow aloft will become southeasterly on Friday. Air
mass stable and generally dry except somewhat moist near the surface
and periods of high clouds. Satellite imagery and observations
indicate only a few spots with isolated fog which will clear out the
next couple hours. VFR skies today with clouds increasing later
tonight and Friday as a low offshore moves southeast. The high
clouds will probably prevent anything more than local fog
development in prone valleys.

Light rain develops across the area Friday morning through the day,
but cigs should remain generally VFR.

KSEA...North to Northwest wind 6-10 kt becoming light this evening,
then light southerly later tonight. VFR conditions with mainly thin
high clouds today. Fog is not likely to affect the terminal late
tonight and Friday with higher clouds increasing. Stratus and light
rain Friday should still remain mostly VFR. dtm

&&

.MARINE...Weak northerly flow will become weakly onshore today. A
1008 mb surface low will move SE through the outer coastal waters on
Friday. Gale force winds to 35 kt are expected near the low center
over the coastal waters. Easterly gap flow winds could locally reach
close to gale in parts of the inner coastal waters, but generally
remain at 30 kt or less. The low will shift SE into Oregon Friday
night with light northerly winds through Saturday.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from Friday morning through Friday evening for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From James Island To Point Grenville 10
     To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm.

&&

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