Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 292229
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 PM PDT Sat Apr 29 2017

.SYNOPSIS...Rain will spread inland this evening. Showers linger
Sunday in the Cascades and in a Puget Sound convergence zone. A weak
weather system will brush the south part of the region on Monday. An
upper ridge will likely keep Tuesday through Thursday dry. A weak
front may arrive Friday, followed by an upper trough Saturday.

&&

.SHORT TERM...Rain is falling on the coast but most inland sites
have recorded only a trace or less. Radar trends indicate rain will
develop 00Z-03Z this evening over inland areas. Westerly flow aloft
may keep the Seattle area in a rain shadow, so while there are
categorical pops most places, Seattle has likely pops. A Puget Sound
convergence zone will develop behind the front late tonight; models
keep it going all day Sunday and into Sunday evening even as most
other places dry out.

Showers will end by Monday morning as heights rise a bit. Models
have a blob of rain moving into Oregon and clipping the southern
part of the CWA Monday afternoon. Have adjusted the forecasts to
include chance pops in the mountains, on the coast, and over the
south to account for this feature. Seattle north will be dry. Highs
Sunday and Monday will be mostly in the mid to upper 50s.

Heights rise into the 570s on Tuesday. Even as this strong ridge is
building, models show a weak weather system reaching the coast in
the afternoon and even raining a bit over some inland areas. This
seems overdone, so have limited pops to the coast on Tuesday
afternoon and have slight chance pops over western sections Tuesday
night. Even with some cloudiness, soaring heights on Tuesday should
mean highs reach into the 60s. Burke

.LONG TERM...An upper ridge will probably keep Wednesday and
Thursday dry. However the models are not quite dry so have kept
slight chance pops in some areas. On Thursday night and Friday an
upper trough, seen in both the GFS and Euro, will lower heights and
bring moisture. There may be some drying on Saturday.

The good news is the upper ridge will probably bring some highs in
the 70s on Wednesday and Thursday, possibly including the first 70
degree day this year at KSEA. The upper trough will lower temps to
around 60 Friday and Saturday. Burke

&&

.AVIATION...The tail end of an upper trough and a cold front will
push eastward across Western Washington tonight. Strong westerly
flow aloft tonight will turn to moderate northwest flow aloft on
Sunday. The air mass will be stable and increasingly moist
through this evening. On Sunday, the air mass will be unstable and
somewhat moist below 10,000 feet msl but dry and stable above.
Cigs will come down quickly later this evening with the arrival of
low-level onshore winds behind the cold front. Expect a Puget
Sound Convergence Zone (PSCZ) from late this evening through most
of Sunday in its usual spot between Seattle and Everett.

KSEA...Cigs will lower steadily this evening. Cigs of 025-035 and
light rain are the most likely conditions during the busier 03z-
05z time frame. The larger area of rain will exit late this
evening, but will then be watching PSCZ showers to the north of
the terminal from late tonight through most of Sunday. Cigs will
lift with daytime heating on Sunday but will never really scatter
out on Sunday. S-SW wind through Sunday morning. Haner

&&

.MARINE...A cold front will pass eastward across the waters this
evening. It is being preceded by increasing southerly winds. It
will be followed late this evening by moderate to strong onshore
flow. The current forecast has 20-30 knot westerlies through the
Strait late tonight and Sunday, but a gale of 34 knots is a
reasonable possibility. Moderate to strong westerly flow will
continue through Sunday, then ease on Sunday night. Light wind
(under 20 knots) will prevail over all the waters from Monday
through Wednesday. A return of moderate to strong onshore flow is
expected on Thursday.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 9 AM PDT Sunday 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 until 3 AM PDT Monday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 9 PM PDT Sunday for Northern Inland
     Waters Including The San Juan Islands.

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

     Small Craft Advisory from 9 PM this evening to 3 AM PDT Monday
     for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until midnight PDT Sunday night for
     Admiralty Inlet.

     Small Craft Advisory until midnight PDT tonight for Puget Sound
     and Hood Canal.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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