Small Craft Advisory 187, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
230 AM PDT FRI AUG 5 2016

.SYNOPSIS...An upper level low is centered on the British Columbia
coast near northern Vancouver Island early this morning. The low
will move south and showers will increase as the low center arrives
over Western Washington on Sunday. The low center will shift east of
the Cascades on Monday and the low will linger over Eastern
Washington on Tuesday.

&&

.SHORT TERM...There is not much moisture in the air mass yet but
as the low center slowly drops down over the area the chance for
showers will increase over the weekend. Sunday through Monday
morning looks like the coolest weather with the best chance of
showers and there is a chance of thunderstorms in the forecast for
Sunday afternoon and evening.

.LONG TERM...Showers could linger through Monday and there could be
wrap around moisture feeding around the low through Monday. I think
by Tuesday the threat of showers should probably be out of the
forecast as the low departs, but for now the forecast is unchanged.
Sunnier weather with warming temps will kick in around midweek as
heights recover.

&&

.AVIATION...SW flow aloft over W WA will continue into Saturday as
the fairly deep upper level low currently over the central B.C.
coast moves slowly S over southern Vancouver Island Saturday
afternoon. The air mass over W WA is dry and cloud free this
morning, except for some patchy stratus along the coast and in the
Strait of Juan de Fuca. The stratus along the coast has reached the
KHQM terminal with CIGS OVC008, but should burn off around midday.
Stratus in the Strait of Juan de Fuca could reach KCLM for some
brief IFR CIGS. Otherwise good VFR conditions are expected today
through this evening.

Strong onshore flow developing this evening will result in a marine
push which will bring low level marine moisture across the W WA
interior tonight and Saturday morning. This should produce areas of
stratus with IFR/MVFR cigs. Skies should clear Saturday afternoon.

KSEA...Clear skies today through this evening. MVFR/IFR stratus will
reach the terminal late tonight. Surface winds will be northerly 4-
10 KT this morning will shift westerly after 16Z then continue
through this afternoon. Kam

&&

.MARINE...Onshore flow will continue through tonight, between a
large surface high over the NE Pacific and lower pressure over E WA.
Onshore flow is expected to be stronger this afternoon and evening
and borderline gales are expected in the central and east strait of
Juan de Fuca. The gale watch has been upgraded to a gale warning.

NW flow out of the strait of Georgia early this morning has risen to
24 KT at Sandheads, just NW of Point Roberts, which should affect
the area north of the San Juan islands.  Mesoscale models did pretty
well with this event although they were too fast to develop the
winds. A SCA has been issued through 15Z/8 AM.

SCA winds are still blowing over the outer coastal waters and should
continue through this afternoon. Only the near shore waters between
Point Grenville and Cape Shoalwater are expected to have winds
occasionally reaching SCA levels through this afternoon.

The main forecast problem is the increase in onshore flow this
afternoon and evening ahead of the upper level low moving slowly
down the central B.C. coast. UWWRF and Canadian HRDPS models both
indicate borderline gale winds in the central and east strait late
this afternoon and this evening. Confidence in a gale is only so-so
but still tilted enough toward the higher winds so that a gale
warning has been issued. The adjacent waters over the southern San
Juans and northern Admiralty inlet should have companion SCA winds.

The above mentioned fairly deep upper level low will affect the area
this weekend. The weak surface trough reflection of the upper level
low will move over the WA coastal waters Saturday morning. This will
be enough to weaken or even disrupt the onshore flow Saturday
through Sunday morning. Models take the upper low slowly E on
Sunday, which will allow weak onshore flow to redevelop Sunday
evening. Kam

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Gale warning central and east strait this afternoon and tonight.
     Small Craft Advisory north inland waters and Admiralty Inlet
  tonight.
     Small Craft Advisory outer coastal waters from Cape Flattery
  to Cape Shoalwater through this afternoon.
     Small Craft Advisory near shore waters from Point Grenville to
    Cape Shoalwater through this afternoon.
     Small Craft Advisory northern inland waters north of the San
  Juan Islands until 8 AM this morning.

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