Special Weather Statement, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
341 AM PDT Fri Oct 14 2016

.SYNOPSIS...Wet windy weather will continue today as a strong low
pressure system tracks northeast across Vancouver Island. The
remains of a Pacific typhoon will arrive late Saturday and move
through Saturday night with the potential for more widespread
damaging winds. It will also bring significant precipitation with
a threat of flooding. Showery weather will continue next week.

&&

.SHORT TERM...A vigorous Pacific frontal system is moving through
Washington early this morning. The main frontal band was moving
into the Cascades at 3 am, and precipitation has decreased over
most of Western Washington in its wake. Southerly pressure
gradients peaked around midnight, and strongest wind gusts
overnight were 45 to 55 mph at the coast and in the north
interior and 35 to 45 mph in the Puget Sound region. Rainfall
amounts for the past 24 hours have been impressive across Western
Washington with 2 to 3 inches at the coast, 3 to 5+ inches in the
Olympics, generally 1 to 3 inches in the interior lowlands and 2
to 5+ inches in the Cascades.

The low pressure center of this storm is vertically stacked with
the surface and upper low near 48N/130W at 3 am, or about 250
miles west of the north Washington coast. This part of the storm
will bring more wind and rain today as it moves into Western
Washington. Its surface low will fill from around 970 mb to 980 mb
as it tracks east-northeast, moving across central Vancouver
Island this afternoon. This should produce another round of low-
end high wind for the coast and north interior, with wind advisory
southerlies for the Puget Sound region and southwest interior.
Winds should peak mid to late afternoon and ease tonight as the
low tracks northeast across British Columbia.

There will be more rain today and tonight, but less than the last
24 hours. We can expect another 0.5 to 1.5 inches in the lowlands
and 1 to 3 inches in the mountains. The snow level will be 5500 to
6000 ft. There could also be some thunderstorms at the coast.

As we have been advertising for several days, the remains of
Pacific Typhoon Songda will approach Western Washington from the
southwest on Saturday, with its surface low possibly bringing a
major windstorm to Western Washington late Saturday and Saturday
night. The low center -- probably around 970 mb -- will travel
northeast and likely pass right over the northwest corner of the
Olympic Peninsula Saturday evening and continue into British
Columbia Saturday night. The 00Z FRI GFS solution has the low just
to the east of that, tracking across the Olympic Peninsula and
then Vancouver BC. If that GFS solution is about right, then
Western Washington -- including the greater Seattle metropolitan
area -- would likely have its most significant windstorm since the
Hanukkah Eve storm of December 2006, which left many people
without power for several days. This system will bring another
period of significant precipitation, similar to or slightly less
than the last 24 hours, with the snow level around 6000 ft.

A large upper trough will move toward the region from the west
Sunday and Sunday night, for a relatively quiet period of merely
showery weather. McDonnal

.LONG TERM...More typical October weather will prevail during the
extended period. An upper trough should keep the area fairly wet
Monday afternoon through Tuesday. Wednesday should be the driest
day, as a dirty upper ridge moves through. The south end of a
system moving into British Columbia could bring more rain on
Thursday. McDonnal

&&

.HYDROLOGY..A flood warning is in effect for the Skokomish River in
Mason county, and it reached flood stage just after midnight.
Some of the more flood-prone rivers running off the west slopes
of the Cascades are also at risk, especially the Nooksack River in
Whatcom County and the Stillaguamish River in Snohomish County. A
flood watch is in effect for those two counties.

Rainfall today and tonight will be less, but the system for
Saturday and Saturday night could bring rainfall similar to what
occurred during the past 24 hours. At this point, the Northwest
River Forecast Center guidance keeps all rivers other than the
Skokomish below flood stage through the weekend, but that can
change quickly. Small landslides could also become a concern over
the weekend. With dry antecedent conditions, we are still below
the threshold for concern, but heavy rain in the lowlands Saturday
and Saturday night could push some areas above. McDonnal

&&

.AVIATION...Moist Pacific frontal systems will bring rain to W WA
through Saturday and likely into Sunday as well. The rain may seem
nearly continuous at times but there will be waves moving across W
WA from this complex system, which will produce lulls where the rain
may briefly taper off or end. Current radar imagery shows a good
example of this...with the front generally east of most terminals in
W WA. However...showers do persist west of the front...though not
nearly as organized. Moisture associated with low pressure over
Vancouver Island will continue to push into the area through the
day...allowing for the threat of rain to persist today.

Surface and low level winds have either peaked or are peaking at the
time of this writing...depending on location. However...current
forecast fully expects winds to remain somewhat increased during
this relative lull as the main low crosses Vancouver island. Ceiling
and visibility conditions will be predominantly IFR and MVFR the
next few days.

KSEA...Southerly winds will slowly diminish after 12Z this
morning...although gusts look to remain. Speeds expected to pick up
again to 20-25 KT by late this morning and persist through the
afternoon before diminishing tonight. SMR

&&

.MARINE...Impacts from initial low pressure system will linger over
the coastal waters for much of today. Storm Warning looks good to
expire and will be replaced with a Gale Warning as winds diminish
somewhat. Winds over remaining area waters are also expected to
follow this trend...but remaining Gales will only drop down into
Small Craft winds...so still plenty of headlines to go around for
much of the day today.

Next big storm coming late Saturday or Saturday night in the form of
a low that will peak in strength as a very deep low off the coast.
Models look like they are finally starting to gel on a
solution...with the low passing NE over the Olympic Peninsula. This
storm continues to look dangerous with another round of storm-force
winds possible over the coast and plenty of gales or small craft
winds for inland waters. SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...High wind warning for the coast and north interior zones until
      3 pm today.
     Wind advisory for the Puget Sound region and southwest interior
      zones until 6 pm today.
     High wind watch for all zones Saturday afternoon through
      Saturday night.

PZ...Small Craft Advisory for rough bar until 5 PM PDT for Grays
      Harbor Bar.
     Gale Warning until 2 PM PDT for Coastal Waters.
     Gale Warning until 3 PM PDT for Admiralty Inlet...East Strait
      and Northern Inland Waters.
     Gale Warning until 11 AM PDT for West Strait.
     Small Craft Advisory until 5 PM PDT for Central Strait.
     Small Craft Advisory from 11 AM PDT through this evening for
      Western Strait.
     Small Craft Advisory from 3 PM PDT through tonight for Admiralty
      Inlet...Puget Sound and Hood Canal and Northern Inland Waters.
     Small Craft Advisory from 2 PM PDT through tonight for Coastal
      Waters.

&&

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html

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