Small Craft Advisory 4, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
846 PM PST Fri Jan 4 2019

.UPDATE...
Evening water vapor imagery indicates that yesterday's system has
dampened into a weak trough across southern BC, with a very
picturesque area of low pressure deepening offshore of the CA
coast around 130 W. A shortwave ridge of high pressure is briefly
building atop the PAcific Northwest as this trough deepens. Can
see a sfc reflection of this low in close proximity with a warm
front extending to the E/NE and a cold front draped slightly ahead
of the low center.

For the remainder of tonight: scattered showers leftover from
yesterday's system will continue to diminish in coverage
overnight. However the low pressure system west of California will
begin to lift NE, allowing the warm front to push northward in
the vicinity of the western Washington coastline. This will push a
band of showers northward which can already be seen in MRMS radar
imagery and is picked up well by most hi-res model solutions.
This will not amount to much significance overnight and should
mainly remain just offshore but a few showers on its eastern
periphery could graze the peninsula. Will see a period of dry
weather as this band of rain lifts north and we see the best
effect of the mid level ridge Saturday morning. As the low
continues to pull north it will drag deep layer moisture but also
weaken into more of an open wave trough with several notable areas
of stronger vorticity. Also evident as this happens are two sfc
waves, one that pulls north offshore the WA coast and one that
moves essentially right up Puget Sound. Although the Puget Sound
low is the weaker of the two, guidance shows a noticeable increase
in winds from the sfc-850mb. There could be a brief period
Saturday night for some breezy winds so will need to monitor for
the potential for wind headlines. Deep moisture will move across
the area with rain likely from west to east Saturday afternoon
into Sunday. Snow levels are in the 3500-5000ft range thru
Saturday but drop dramatically into Sunday to around 2000ft with
some showers lingering in lowlands and snow in the mountains-will
need to watch for winter weather headlines for mountains.

Active pattern with periodic breaks between systems still looks
likely next week with little change in the longwave pattern across
the Pacific and CONUS.

Kovacik

&&

.PREV DISCUSSION... /issued 300 PM PST Fri Jan 4 2019/

.SYNOPSIS...Showers will diminish this afternoon and tonight with
most locations getting a dry start to Saturday. Next system
moving up from the south arriving Saturday night into Sunday.
Another system moving into the area from the south arriving
Tuesday or Wednesday. Unsettled weather will continue through the
end of next week.

&&

.SHORT TERM /FRIDAY THROUGH MONDAY/...Some scattered showers
still showing up on radar...but not nearly the coverage when
compared to this morning. This drying trend will continue through
the evening with most locations in W WA looking to start the day
Saturday dry. As is always the case though...this break will be
short lived as the leading edge of the next system looks to reach
the coast by early Saturday afternoon...spreading inland Saturday
evening. This system looked rather non-spectacular but 12Z data
introduced a new solution where pressure gradients tighten
considerably during the overnight hours Saturday and into early
Sunday morning where wind speeds may reach Wind Advisory criteria.
Given how strongly this differs from previous solutions...while
the forecast has nudged wind speeds upward from inherited
values...confidence simply is not there to go forward with any
headlines at this time. Hopefully additional runs will provide
some degree of consistency before a decision needs to be made.

Rain looks to continue through at least Sunday morning...but models
disagree on coverage beyond that...with the current GFS run trending
drier with activity tapering off while the ECMWF remains fairly wet
into early Monday morning. Given the pattern that has been observed
over the area...decided to lean toward the wetter Euro solution.
Models re-align as they agree on a minor ridge making for generally
dry conditions for the area for Monday.

Accumulations in the mountains from the above discussed system looks
to fall under or at the very low end of Winter Weather Advisory
criteria. Opting to lean more below these thresholds at this time
given the discrepancy in the models and thus will not be issuing any
additional headlines at this time.  SMR

.LONG TERM /TUESDAY THROUGH FRIDAY/...Models continue to show an
upper level low out over the Pacific making its presence known to W
WA Tuesday. The discrepancy deals mostly with the amount of precip
associated with this system...as the ECMWF is more progressive with
the heavier precip by ejecting it up into B.C. by Wednesday morning
while the GFS keeps it lingering over the Olympic Peninsula through
the day Wednesday and into Thursday morning. This discrepancy will
certainly have hydrologic impacts with the flood prone Skokomish and
additional rivers flowing out of the Olympics. Aside from amounts
though...models do agree that conditions will remain soggy
throughout much of the day Thursday before upper level ridging once
again stakes a brief claim to the area late Thursday night and into
Friday.  SMR

&&

.AVIATION...Isolated to scattered shower activity is still
evident across western Washington this evening.As yesterday's
system continues to dampen over BC and a weak ridge builds north
in its wake, should see activity continue to diminish overnight and
continue into early Saturday morning. All sites are VFR this
evening aside from HQM and OLM and it appears VFR will generally
be the rule for just about everyone thru this TAF cycle. Some fog
at OLM tonight will reduce vsbys and allow for period of very low
cigs. Winds will be on the east side through much of tonight and
Saturday. These nice break in calm weather will be short-lived as
the next system just off the CA/OR coast pulls north and brings
moisture with it. There is growing consensus on the possibility of
a quick surge of gusty southerly winds Saturday night-will have
to monitor. Wind shear does not appear to be a concern with this
system right now.

KSEA...Showers will remain in the vicinity over the next several
hours before a brief break in precip is expected. Rain will return
late in the day Saturday. VFR cigs the rule thru this period with
winds E/NE at 10kts or less but will likely increase Saturday
night. No reductions to vsbys expected.

Kovacik

&&

.MARINE...Flow will become light offshore overnight tonight as
high pressure exists in the interior of the CONUS and low pressure
deepens offshore of the CA coast. Latest observations indicate
seas subsiding under 20 ft so have let High Surf Advisory expire
at 03z Sat. Can generally expect winds to diminish after 06z
across the interior waters but given closest proximity of
deepening low near CA, the coast will remain SCA through Saturday
morning with hazardous seas for the west Strait. The tricky part
of the forecast will come as the low moves north, remaining
offshore the Washington coast. A surge of gales will be possible
across coastal waters but also noticing a decent surge of
southerly wind across the interior waters in the 06-09z timeframe
on Sunday. This may produce gales across these waters as well.
Onshore flow will continue in the wake of this system with an
active pattern continuing into next week.

Kovacik

&&

.HYDROLOGY...No changes made with this update...The Skokomish
River at Potlatch remains just above moderate flood stage early
this afternoon. The river has crested and will continue to recede,
falling below flood stage later this weekend/early next week. The
Bogachiel River near La Push fell back below flood stage this
morning. River flooding is not expected on any other rivers
through the weekend.

A system arriving on Tuesday has the potential to cause yet another
round of minor flooding problems for the rivers flowing off the
Olympics but details remain uncertain at this time.

CEO/Kovacik

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 4 PM PST Saturday for 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.

     Small Craft Advisory for rough bar until 10 AM PST Saturday for
     Grays Harbor Bar.

     Small Craft Advisory until 10 PM PST this evening for Admiralty
     Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands-Puget
     Sound and Hood Canal.

     Small Craft Advisory for hazardous seas until noon PST Saturday
     for West Entrance U.S. Waters Strait Of Juan De Fuca.

&&

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