Flood Watch 9, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
302 AM PDT Thu Apr 6 2017

.SYNOPSIS...A wave of low pressure will move by to the west of the
area early this morning. Showers will diminish this evening as
flow briefly becomes offshore ahead of the next low that will
approach the area from the southwest. A deep low will move by to
the west of the area on late Thursday night into Friday evening
giving windy and wet weather to the region. Unsettled weather will
continue on Saturday behind the low. After a possible brief break
in the weather on Sunday, more frontal systems will give unsettled
weather to the region early next week.

&&

.SHORT TERM...It looks like the next week will generally be
unsettled as a series of rather fast-moving systems affect the
region.

Early this morning, satellite imagery and scatterometer data show
a 1000 mb low over the coastal waters that is moving northward
toward Vancouver Island. The scatterometer data shows most of the
wind with this system remaining just off the coastline as it moves
by. Area radars show bands of rain wrapping around this tight
low and affecting the area -mainly from the Olympic Peninsula
westward. The precipitation will become more showery this
afternoon as the system moves well off to the north.

Showers will rapidly decrease this evening as low level flow turns
offshore ahead of the next system. By the period from midnight
tonight to 6 am Friday, offshore gradients will become strong
enough that breezy to locally windy conditions will develop in the
passes and in the East Puget Sound Lowlands. Short term models are
bearish in forecasting much in the way of wind, despite the 850 mb
easterly winds increasing to 30-35 knots and an approaching warm
front from the south producing a possible critical level between
5000 and 10000 ft. This may result in a brief development of
mountain wave activity late tonight. At this time the forecast
will be for breezy conditions in the East Puget Sound Lowlands,
but there is a low chance that winds could rise to advisory or
even warning levels during the early morning hours on Friday with
little notice.

The developing 991 mb low now near 39N 139W will curl northeast as
it deepens to 970-975 mb Friday morning over the Washington
offshore waters. The low will then slowly fill to around 990 mb as
it moves to the northern tip of Vancouver Island early Saturday
morning. Most of the wind with this system appears that it will
remain just offshore, but there is a good chance that marginal
high winds of south 25 to 40 mph with gusts 55 to 60 mph could hit
the coastal zones. The strongest winds would be with the front
associated with the passing low Friday morning, then again in the
afternoon and early evening hours as a bent-back occlusion lifts
northward through the coastal waters and immediate coastline.
Given the good potential for this scenario, a high wind watch will
be issued for the coastal zones from early Friday morning through
Friday evening. Given the path of the offshore low, south to
southeast winds with the passing low and associated front in the
interior should remain at advisory level or below -and should be
strongest from north of Everett to the Canadian boarder. There is
a slight chance of thunderstorms Friday afternoon over the
southern half of the forecast area along and behind the front.

Waves developing to the east of the passing low are forecast to
rise to 22-28 feet on Friday. Fortunately, we are in a neap tide
cycle on the coast and the path of the low is far enough offshore
that there is not much of a threat for coastal flooding. A High
Surf Advisory may be required for the coastal zones for Friday
through Friday evening if the threat of wave development
continues to increase.

Strong SSW winds aloft to the east of the passing low along with a
high moisture content of the air mass will result in heavy
precipitation amounts late Thursday night through Friday night on
the south slopes of the Olympics. With this, a flood watch has
been issued for Mason County, specifically the Skokomish River.

Showers Friday night through Saturday night can be expected with
strong post frontal onshore flow. The showers will gradually
decrease by Saturday night. Albrecht

.LONG TERM...A brief break in precipitation is possible Sunday as
a ridge of high pressure moves across the area. The break is most
pronounced in the 00z run of the ECMWF. Another front will move
through the area Sunday night and will be followed by showers and
onshore flow on Monday. Another brief lull is possible on Tuesday
before yet another front approaches the area on Wednesday.
Albrecht

&&

.AVIATION...Frontal system expected to resume its northeastward trek
this morning and slowly plodding along this afternoon. Activity has
tapered off some since last night...but still a fair number of
echoes...especially over the western half of the CWA...although very
light returns over the eastern half suggest that the prospect of
light rain or sprinkles continues there as well. Cigs generally VFR
over the area with the exception of CLM. As frontal system makes its
way across the area today...will expect some locations to drop down
into MVFR conditions as more widespread rainfall accompanies FROPA.
A very weak...minor upper level ridge will push into W WA by the
early evening...drying conditions briefly before another stronger
low pressure system pushes into the region during the late evening
and overnight hours...bringing a return to rain and bringing cigs
back down to a mix of VFR and MVFR.

KSEA...Lingering frontal activity over the area will continue the
threat for sprinkles or light showers in the early morning. The
front is expected to push steadier precip into the area of the
terminal by around 12Z this morning and this activity is expected to
continue before tapering off in the early afternoon. A brief period
of drying between 00Z and 06Z this evening before prospect for rain
returns during the overnight. Winds generally from the south at 7-12
kts this morning turning to more northerly once front passes this
afternoon...then becoming easterly overnight and into Fri AM. SMR

&&

.MARINE...Weak low pressure will ride up the frontal zone and
into Western WA this morning...allowing for swells to remain in SCA
for haz seas range.  As such...have extended advisory until 5 PM PDT
this afternoon. Winds...fortunately...should still remain well
behaved. A much stronger system will affect the region Friday
through Saturday morning as front and deep low pressure system move
through the area. This will result in some pretty high surf as
swells will exceed 20 ft. Will leave that headline for later...once
current headlines for swell expire so as to avoid confusion. SMR

&&

.HYDROLOGY...3 to 5 inches of rain in 24 hours is expected from
late Thursday night through Friday night on the south slopes of
the Olympics. This will likely result in flooding on the typically
flood-prone Skokomish River in Mason County. A Flood Watch was
issued for the period from Friday morning through Saturday
afternoon.

Other rivers will see rises Friday through Saturday, but flooding
is not expected for the next 7 days. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from Friday morning through Saturday afternoon for
     Mason County.

     High Wind Watch from Friday morning through Friday evening for
     Central Coast-North Coast.

PZ...Small Craft Advisory for hazardous seas until 5 PM PDT this
     afternoon 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-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory for rough bar until 5 PM PDT this afternoon
     for Grays Harbor Bar.

&&

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