Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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717
FXUS66 KSEW 292225
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 PM PDT Wed Mar 29 2017

.SYNOPSIS...Onshore flow with showers and breezy conditions will
gradually decrease tonight. An upper trough will bring a chance of
showers and a possible Puget Sound convergence zone to prone areas
tonight into Thursday. Parts of Western Washington should get at
least some sunbreaks by Thursday afternoon as the trough shifts
east. High pressure will build across the area for dry and somewhat
milder weather Friday. A weak system should bring some light rain to
the area over the weekend, with a chance of showers early next week.

&&

.SHORT TERM...Radar and obs indicate showers across much of Wrn Wa
this afternoon and also breezy to locally windy conditions. The
strongest southerly winds are currently over central/north Puget
Sound up to Admiralty inlet as expected due to lee side meso low
northeast of the Olympics. The pdx-bli gradient peaked at near +8 mb
a couple hours ago so winds have likely peaked with just some gusts
to maybe 35 mph over the next hour or two. There is a pretty good
westerly push of wind down the Strait and localized areas along the
easterly Strait and west side of Whidbey island could get a quick
burst of gusty winds to 35 or 40 mph until around sunset. There
looks to be a sw to ne oriented convergence zone band trying to
develop from the Kitsap to Skagit/Snohomish counties. Most meso
models were showing a period of convergence zone activity over
Snohomish county this evening which could drop into north King
county later tonight. The nam-12 even brings this precip band into
central/south king county before dissipating late tonight/early
Thursday morning.

The main 500 mb trough axis is now shown by models to move through
much quicker than previous solutions were showing, perhaps shifting
east of the Cascades after midnight tonight. This could shut off
residual showers quicker on Thursday as higher pressure aloft beings
to build in from the west, albeit slowly. Shower coverage was
reduced a bit and pops bumped down for most areas although a
residual convergence zone could produce lingering showers in King
county Thursday morning. Most areas will see afternoon sunbreaks or
even clearing toward the west.

High pressure still looks to dominate the Pacific Northwest Thursday
night and Friday with dry weather and milder temperatures. Forecast
highs were pushed up several degrees above a blend of MOS guidance,
mostly upper 50s across greater Puget Sound and the southwest
interior. It would not be surprising to see some low 60s around the
area if we get a full day of sun with light flow.

The gfs/ecwmf both bring a weak trough and decaying frontal band
across the area Sunday. Mostly warm advection occurs ahead of the
system but eventually some onshore flow and clouds develop in the
afternoon. This may hinder high temperatures from reaching 60, but
most MOS guidance shows Saturday being the warmer day so we could
still crack 60 in some spots. There will be a chance of a few
showers but amounts look quite light and spotty.

.LONG TERM...The gfs/ecmwf solutions are a bit closer together today
and show a fairly typical spring like pattern Sunday through the
middle of next week. A trough will brush the area Sunday with a
chance of light rain or showers. Again, this will not be a big rain
producer but should bring some cooler temperatures and clouds. There
is a good chance for a break in the rain sometime Sunday night
through Monday night but there is still enough uncertainty on timing
of any brief ridging to keep some chance pops in the forecast.
Another system arrives around Tuesday and Wednesday. Neither of
these systems look very wet or cold, just typical systems this time
of year with some clouds, light spotty rain, and near average
temperatures. Mercer

&&

.AVIATION...Southwesterly flow aloft this evening will become
northwesterly late tonight as an upper level trough passes, then
northerly on Thursday. At the surface, strong onshore flow behind a
front will ease tonight but remain moderate on Thursday. The air
mass is moist and slightly unstable with scattered showers.

Most ceilings and visibilities are MVFR this afternoon, though there
are still some IFR ceilings in showers and there are also some clear
patches. Slow improvement should continue into the evening, then
late tonight conditions will likely fall back to low end MVFR.

KSEA...Discussion above applies. Southwest wind 15g25 KT will ease
tonight. CHB

&&

.MARINE...Strong onshore flow behind a front will ease to moderate
tonight. A brief southerly gale in Puget SOund will end this
evening. Westerly gales in the strait will fall to small craft
advisory level winds by 06Z. Onshore flow will persist on Thursday,
with small craft advisory strength winds over most waters. Gradients
will weaken Thursday night, and a ridge of high pres will give
lighter winds on Friday. Another front will arrive Saturday. CHB

&&

.HYDROLOGY...The heaviest rainfall has passed with just showers or
periods of light rain in post-frontal onshore flow through tonight.
The south and west facing slopes of the Olympics received the most
rainfall, up to 5 inches, and up to 2 or 3 inches in parts of the
Cascades, mainly central portions in King/Snohomish counties. Rivers
will run high or near bankfull in some basins, mainly those running
off the Olympics. The Bogachiel appears to have crested below flood
stage and will gradually come down tonight.

The Skokomish river near Potlach surpassed flood stage last evening
and crested this morning. It should slowly fall through tonight,
then probably drop below flood stage by Thursday morning. Only minor
flooding is occurring and should remain limited. Forecasts should
continue to be monitored closely, and always refer to the current
flood statement for the most current information.

Otherwise, flooding is not expected on area rivers during the next 7
days.

Significant rainfall since Tuesday increased the threat of
landslides across Wrn wa the past 24 hours. However, additional rain
tonight will not be as heavy, generally a quarter inch or less.
While the saturated soil across Wrn Wa will maintain an elevated
threat of landslides over the next several days, the highest risk is
passing as precipitation decreases overnight. Will issue one more
special weather statement addressing the last round of rainfall and
indicate a lower but continued threat as a somewhat less active
pattern develops.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 PM PDT Thursday for Admiralty Inlet-
     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-Northern Inland
     Waters Including The San Juan Islands-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory for rough bar until 11 AM PDT Thursday for
     Grays Harbor Bar.

     Gale Warning until 11 PM PDT this evening for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory until 5 AM PDT Thursday for Puget Sound and
     Hood Canal.

     Gale Warning until 6 PM PDT this evening for Puget Sound and
     Hood Canal.



&&

$$

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