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Forecast Discussion for Seattle/Tacoma, WA

000
FXUS66 KSEW 291629
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
930 AM PDT Sat Apr 29 2017

.SYNOPSIS...Rain will develop on the coast this afternoon and spread
inland this evening. Showers will 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.

&&

.SHORT TERM...High clouds are increasing this morning as a front
moves toward the coast. Radar indicates light rain may be falling on
the north coast already but surface observations have not recorded
any precipitation yet. Rain will develop on the coast by noon or so.
Models generally bring light rain inland 00Z-03Z this evening,
though westerly flow aloft may keep the Seattle area in a rain
shadow. A Puget Sound convergence zone will develop behind the front
late tonight; models keep it going all day Sunday even as most other
places become dry. Highs today and Sunday will be in the 50s.

Showers will end by Monday morning as heights rise a bit. Models now
feature a blob of rain moving into Oregon and clipping the southern
part of the CWA Monday afternoon. Will adjust the forecasts with the
afternoon package. Highs Monday will be mostly in the mid to upper
50s. Burke

.LONG TERM...Previous discussion...It has been a rare thing this
spring to be able to chain together more than one dry day, but the
ridge for late Monday through early Thursday morning just might be a
chance for doing so. Before completely grasping onto that optimistic
solution, however, it must be pointed out that a system does pass
just to the north of the CWA in this time span on Wed morning. The
GFS has it just nicking the very northernmost portions of the CWA
while the ECMWF remains true to form and offers a wetter solution.
While it may end up being a gambler`s fallacy, currently siding with
GFS solution for no better argument than W WA`s luck has to change
sometime. Plus, cannot turn away from the prospect of afternoon high
temperatures getting close to 70 on Wed. Late Thursday morning shows
an upper level ridge passing through B.C. which may allow for precip
for the northern half of the CWA into Thursday evening before
ridging over the Pacific allows for conditions over W WA to dry once
more for Friday. The ECMWF, however, shows a fairly deep trough over
the western US at this time, so, as has been the case for at least
the past couple of weeks, long term models remain generally
uncertain and as such lowering any degree of confidence in the
extended forecast.  SMR

&&

.AVIATION...Upper ridging will flatten this afternoon over the
Pacific Northwest, allowing the tail end of an upper trough and a
cold front to push across Western Washington tonight. Moderate
northwest flow aloft today will become strong westerly this
evening, then moderate northwest flow aloft again 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 onshore
post-frontal winds. Expect a Puget Sound Convergence Zone from
late this evening through most of Sunday.

KSEA...Thickening clouds at or above 100 until 00z (5 pm), then
cigs will lower more quickly this evening. Cigs of 025-035 and
light rain are the most likely conditions during the busier 03z-
05z time frame. S-SW wind for the next 30 hours. Haner

&&

.MARINE...A cold front will pass east across the waters this
evening. The front will be preceded by increasing southerly winds
this afternoon, with speeds of up to 20 to 30 knots forecast over
the coastal waters. It will then followed by westerly flow later
this evening, strongest in the central and eastern Strait.
Westerly flow will continue through Sunday.

Onshore flow will weaken later Sunday evening with a lull in wind
across the area starting Monday. The next front may reach the
waters by Thursday but timing is highly uncertain. Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 3 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 from 1 PM this afternoon to 9 AM PDT Sunday
     for East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 PM this afternoon to midnight PDT
     tonight for Northern Inland Waters Including The San Juan
     Islands.

     Small Craft Advisory from 1 PM this afternoon to 3 AM PDT Sunday
     for West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 7 PM this evening to 9 AM PDT Sunday
     for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 1 PM this afternoon to midnight PDT
     tonight for Admiralty Inlet-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

NWS Seattle/Tacoma, WA (SEW) Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather