Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 301632

Area Forecast Discussion
National Weather Service Seattle WA
930 AM PDT Sun Apr 30 2017

.SYNOPSIS...A convergence zone will keep showers over the Seattle
area today, with scattered showers elsewhere. A weak weather system
will probably bring showers Monday morning. Upper level ridging will
bring mostly dry weather Tuesday through Thursday although Tuesday
night and Wednesday morning may see a little rain. An upper level
trough will bring rain Thursday night through the weekend.


.SHORT TERM...Radar and a glance out the window shows a nice
convergence zone along the King-Snohomish county line this morning.
There are scattered showers on the coast and dry, even sunny,
conditions elsewhere. Models show the convergence zone persisting
into this evening even as other areas dry up. Have updated the grids
and forecasts for higher pops in the Seattle area today and this
evening. Highs today will be in the 50s.

What had been a weak system heading into Oregon on Monday in the
models is now heading more directly into Washington. Light rain
seems likely Monday morning, with some drying in the afternoon. Have
also included this idea in the forecast update. Highs Monday will
also be in the 50s.

An upper ridge will build over the area Tuesday, with heights in the
low 570s. Normally this would be a dry pattern but models move a
weather system into the area, right through the upper ridge. This
seems overdone but the forecast does have chance pops over the whole
area by Tuesday evening. High temperatures will be in the mid 50s to
mid 60s. Burke.AVIATION...A flat, transient upper ridge axis will
reach 130W
this afternoon and pass eastward across Western Washington
tonight. It will be closely followed by a weak upper disturbance
on Monday morning, accompanied by deeper moisture and a few hours
of rain. Today`s air mass will be moist and unstable below 10,000
feet msl but dry and stable above. The air mass on Monday will
feature deeper moisture and a more stable air mass. With the moist
air mass, MVFR cigs of 015-030 will be common, with lifting both
this afternoon and Monday afternoon into the 030-045 range.

KSEA...Puget Sound Convergence Zone will be the main weather
challenge today. As of 915 am, it is evident as a band of showers
and lower cigs 10-20 miles north of the terminal. It will likely
drift south enough this afternoon and evening to bring showers to
the terminal. A northerly wind shift cannot be ruled out late
this afternoon, but winds are more likely to maintain a southerly
component at SEA. Nearby BFI is a bit more likely to get a light
north wind for 1-3 hours around 00z (5 pm). Haner


.MARINE...Moderate and briefly strong westerly onshore flow will
peak late this afternoon, then ease tonight. Currently forecasting
a brief round of low-end westerly gales for the eastern and
central Strait late this afternoon and early this evening.
Meanwhile, a Puget Sound Convergence Zone will hang out near
Bremerton and Seattle through this evening. Winds will ease
quickly late this evening.  Haner


PZ...Small Craft Advisory until 3 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.

     Small Craft Advisory from 8 PM this evening to 3 AM PDT Monday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 8 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 11 PM PDT this evening for Admiralty
     Inlet-Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 9 PM PDT this evening for West
     Entrance U.S. Waters Strait Of Juan De Fuca.



You can see an illustrated version of this discussion at

.LONG TERM...Previous discussion...Maintaining the aforementioned
reversal...GFS now wants to go with a dirty ridge over W WA for Weds
that would still allow for showers while the ECMWF remains dry. On
the bright side...the two converge on the solution that the ridging
will continue to intensify Thursday with dry conditions from both
models. This ridge intensification could also lead to many interior
locations seeing their first 70 degree day either Wed or Thu. As the
end of the upcoming week approaches...models agree an upper level
low over the Pacific will begin to impact the area starting Friday
and into the weekend. The ECMWF follows a pretty rational...standard
approach of an upper low that W WA has seen many...many times as of
late. The GFS however throws in a line of precip well ahead of the
leading edge of this trough with seemingly only a minor shortwave to
justify it. Cannot really see biting on to that solution at this
time...but did nudge pops up slightly in this time period. Would
hope models get their bits together for future runs because due to
current waffling nature...have a hard time putting a great deal of
confidence in any current solutions. SMR

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