Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 302236
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 PM PDT Sun Apr 30 2017

.SYNOPSIS...A convergence zone will keep showers over the Seattle
area this evening, with scattered showers elsewhere. A weak weather
system will probably bring showers Monday morning. A short drying
trend will follow Monday night and Tuesday morning, but another weak
system will probably bring light rain Tuesday night. Wednesday
and Thursday will be mostly dry. An upper level trough will bring
a chance of showers Thursday night through Saturday.

&&

.SHORT TERM...Radar still shows a good convergence band along the
King-Snohomish county line, with scattered showers elsewhere. Have
kept likely pops this evening for the mountains and the central
Puget Sound area, and chance pops over the same areas late tonight.
Most other areas should see good drying.

Models bring a short wave into Washington Monday morning but move it
east rapidly Monday afternoon. Have included some likely pops for
Monday morning, with at least chance pops everywhere. There will be
some drying in the afternoon. Highs Monday will be in the 50s.

An upper ridge will build over the area Tuesday, with heights in the
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 have introduced likely pops everywhere, especially
Tuesday evening. The mountains, coast, and north are most likely to
get some rain. High temperatures will be in the mid 50s to mid 60s.

Wednesday does appear to be mostly dry. The Tuesday system shifts to
the north as the upper ridge strengthens. There is still a chance of
showers in the far north, but the sun will make an appearance and
highs should reach the 70s over a broad swath of the interior. If
KSEA reaches 70 degrees, it will be the first time this year. Burke

&&

.LONG TERM...Thursday will be mostly sunny with highs in the 70s.
The upper flow will turn southerly, though, and by afternoon clouds
will be streaming from the south. This is a good thunderstorm
pattern so have introduced a chance of thunderstorms to the Cascades
Thursday afternoon and evening. Showers will continue into Friday.
Heights will definitely fall, so highs on Friday will be near 60.
The showery pattern will continue Saturday as a weak upper trough
passes. Sunday looks drier. Highs both days will be in the mid 50s
to lower 60s. Burke

&&

.AVIATION...A flat, transient upper ridge axis will
pass eastward across Western Washington tonight. It will be
closely followed by a weak upper disturbance on Monday morning,
bringing deeper moisture and a few hours of rain. The air mass
through this evening 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 on Monday morning, with
lifting on Monday afternoon into the 030-045 range.

KSEA...Puget Sound Convergence Zone (PSCZ) will be the main
weather challenge for the next several hours. Confidence is
increasing that surface wind will maintain a southerly component
at the terminal, with not much worse than light and variable at
nearby BFI. Any north wind at BFI will last less than 2 hours.
PSCZ should retreat back north of downtown Seattle starting around
05z-06z. On Monday morning, a warm front will brush by, bringing
light rain and a couple hours of cigs below 020. Moist air mass
will not allow much scattering of lower clouds on Mon PM, but
cigs will lift into the 035-045 range. Haner

&&

.MARINE...Buoy obs along the coast showed a sudden increase of
swell into the 10-11 foot range, so had to hoist a Small Craft for
Hazardous Seas, including at the West Entrance. Waves are already
diminishing at Canadian buoy 206, so this bout of 10+ foot swell
will hopefully last but a few hours. Otherwise, Gale Warning
continues for westerlies in the central and eastern Strait. Trial
Island earlier had gusts to gale force. Winds should start
diminishing this evening, earlier than the normal diurnal trend.

A warm front brushing by on Monday PM will bring a modest increase
in S-SE winds over the coastal waters. Another warm front on Tue
will aim more at Vancouver Island and bring another modest
increase in SE wind. Weak gradients and winds for Wed. On Thu and
Fri, low pressure will develop east of the Cascades and instigate
strong onshore flow, with westerly gales appearing likely in the
Strait.  Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory for hazardous seas until 9 PM PDT this
     evening 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 for rough bar until 11 PM PDT this evening
     for Grays Harbor Bar.

     Small Craft Advisory from 9 PM this evening to 5 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 9 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 3 AM PDT Monday 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.

&&

$$

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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