Gale Watch 64, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
947 AM PST Sat Nov 12 2016

.SYNOPSIS...A low pressure trough is moving across western
Washington today, accompanied by windy conditions and some
showers. A break between systems is expected later tonight.
A cold front will slowly sag southward through western Washington
on Sunday, bringing more rain and locally windy conditions on
Sunday and Sunday night. A broad upper level trough will slowly
move across the region from Tuesday through Thursday, supporting
cool showery conditions.

&&

.SHORT TERM...Just before 8 am, LGX radar showed a cyclonic spin
moving east-northeast onto the coast near Kalaloch. Around the
same time, the pressure at Destruction Island bottomed out at
1004.9 mb, highlighting an apparent triple point trailing from a
1000 mb low center that has moved across the northern tip of
Vancouver Island this morning. As the overall low pressure trough
lifts northeast through the forecast area for the rest of this
morning, southwest gradients will pick up and bring windy
conditions to the Puget Sound lowlands. The 12z NAM is forecasting
southwesterly 925 mb winds over the Seattle metro area from 18z-
21z as low-level cold advection behind the surface trough kicks
in. The Olympia-Bellingham gradient was already +5.0 mb at 9 am
and trending stronger. These factors support the current forecast
of peak gusts near 45 mph around mid-day over the Puget Sound
region. If Sea-Tac even gusts to 40 mph today, it would be the
strongest gust since Oct 14. So there is a Wind Advisory up for
parts of the forecast area. Along the coast and over the North
Interior, the advisory is geared more toward ongoing southeast
winds. For the Puget Sound region, the advisory is more geared to
the southwest winds that are currently in the process of coming
up.

A break between weather systems is expected later tonight, for a
mostly dry and benign overnight period. On Sunday PM, an open
cold front will slowly sag south-southeast through the forecast
area. Preceding the front, warm advection lift and southwest 850
mb winds near 50 kt will support a fair amount of rain. The slow
movement of the front will contribute further to enhancing
rainfall amounts. 2-3 inches of rain appear likely over the
Olympic Mtns, with the interior lowlands expecting 0.5-1.25
inches.

By Monday morning, the front will have pushed to our south over
northwest Oregon. But then it appears that a frontal wave will
ride up the front, with rain shifting back northward later Monday
into Monday night in the more strongly southerly flow aloft in
advance of the frontal wave.		Haner

.LONG TERM...From Previous Discussion: The axis to the
aforementioned trough will pass through Western Washington
Wednesday with showers lingering into Thursday morning. Models
hint at an upper level ridge forming soon afterward...but the
ridge axis shifts quickly east...allowing for a very brief pause
before another Pacific system brings rain back into the forecast
for the finale of next week...as it looks to be another wet
weekend. SMR

&&

.AVIATION...Flow aloft will be strong westerly today, easing to
moderate tonight. At the surface gusty southwest wind today will
ease tonight. The air mass is moist and stable with light rain
changing to showers by 20Z. There will be periods of MVFR ceilings
with rain this morning. Conditions will trend toward VFR this
afternoon.

KSEA...Rain at times this morning becoming showers by midday. Mostly
VFR stratus with periods of brief MVFR ceilings this morning.
Southwest wind 15 to 25 kt with gusts to 35 kt will ease late in
the afternoon. CHB

&&

.MARINE...Marginal southerly gales will continue this morning over
the coast and over northern inland waters, with small craft advisory
strength wind elsewhere. Westerly gales will occur in the strait
late this morning through the afternoon. Winds will ease this
evening over all areas.

A front will probably bring gales to the coast Sunday; a gale watch
is in effect.  Another low pressure center is forecast to move
northeast onto Vancouver Island Monday night. This could bring gales
to the coast and some inland waters. CHB

&&

.HYDROLOGY...About 1.5 to 2 inches of rain has fallen in the
Skokomish Basin since late yesterday. This is supporting ongoing
rises, but the Skokomish should not reach flood stage today. A
frontal system Sunday afternoon and Sunday night will produce an
additional 2 to 3.5 inches of rain in the Olympic Mtns over a 24
hour period. By late Sunday night or Monday, this amount of rain
could push the flood-prone Skokomish River to its flood stage of
17.0 feet. Otherwise, Sunday night's rain will be heavy enough to
cause rises on most area rivers, but it is not expected to cause
river flooding elsewhere.

USGS landslide indices are currently just below thresholds for
the occurrence of shallow landslides, based on rainfall data at
Puget Sound area airport stations. However, locations further
north toward Bellingham and along the coast have received
considerably more rain in the past week and likely still carry a
landslide threat in favored locations.

On Sunday PM and Monday AM, the interior lowlands are expected to
receive 0.50 to 1.5 inches of rain, which should be enough to once
again exceed thresholds for triggering shallow landslides. Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory until 3 PM PST this afternoon for Bellevue and
     Vicinity-Bremerton and Vicinity-Everett and Vicinity-Hood
     Canal Area-Seattle and Vicinity-Southwest Interior-Tacoma
     Area.

     Wind Advisory until noon PST today for Central Coast-North Coast-
     San Juan County-Western Whatcom County.

PZ...Gale warning coast, central and east entrance to the Strait, and
     northern inland waters.
     Small craft advisory west entrance to the Strait, Admiralty Inlet,
     and Puget Sound and Hood Canal.
     Small craft advisory for rough Grays Harbor bar.

&&

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