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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 250342

Area Forecast Discussion
National Weather Service Seattle WA
842 PM PDT Mon Jun 24 2019

.SYNOPSIS...Light low level onshore flow through tonight. An upper
level trough will approach the area Tuesday and be over the
Pacific Northwest through Friday. The trough will move east of the
area on Saturday.


.UPDATE...Quiet evening across the region. Minor changes were made
to the sky and wind forecast based on the latest trends and short
range guidance. Otherwise the forecast is in good shape and no 
further updates are needed at this time. -Wolcott-


.SHORT TERM /TONIGHT THROUGH THURSDAY/...Satellite imagery shows
partly clear skies with cumulus clouds this afternoon as a 
result of daytime heating. Temperatures are currently in the low 
to mid 60s across the area. Light low level flow will continue 
through tonight. Light flow will limit the expanse of the stratus 
deck late tonight into Tuesday morning with low clouds generally 
along the coast and into the Southwest Interior. 

An upper level low will slide south along the British Columbia
coast tonight into Tuesday morning. Given the location of the
upper low well offshore, dry conditions will continue through 
early Tuesday afternoon. Increasing clouds along with a few 
showers may develop later Tuesday afternoon ahead of the system. 
Temperatures will be a few degrees warmer on Tuesday with a mix of
clouds and sun. Highs will be in the upper 60s to low 70s.

The upper low will begin to slide east over W WA late Tuesday
night into Wednesday. Showers will likely become more widespread
during the day on Wednesday with the chance of heaviest rain in
places late Wednesday through Thursday as the upper low slowly 
moves northeast over the area. Temperatures on Wednesday and 
Thursday will be in the 60s across the area. JD

.LONG TERM /FRIDAY THROUGH MONDAY/...Guidance is in decent
agreement that the upper low will slowly push northeast on Friday
with continued showers likely across W WA as it does so. Model
differences exist Saturday through Monday as the ECMWF suggests a
weak upper level trough over the area while the GFS has a weak 
ridge build into the area. At least a chance of POPs kept through
Saturday for this reason. Temperatures will likely warm through
the weekend with highs in the mid 70s by Sunday and Monday across
the area. JD


.AVIATION...Westerly flow aloft over Western Washington will become 
southwesterly tonight as another upper trough digs southward over the 
offshore waters. The air mass continues to gradually dry out this 
afternoon as low level flow turns weak northerly late this afternoon. 
VFR expected the area through late tonight. Some stratus is expected to 
reform near the coast tonight and may push locally inland toward 
daybreak. Mid and high level moisture will be increasing across the 
area on Tuesday ahead of the next system.

KSEA...VFR through tonight. Cumulus should dissipate after the sun 
sets. Not expecting stratus for Tuesday AM at this time. Mid and 
high clouds will increase through the day Tuesday. Surface winds 
light northwesterly this afternoon shifting to northerly less 
than 10 knots this evening. Light and variable wind on Tuesday. 27


.MARINE...As a broad trough of low pressure develops over the 
offshore waters and remains in place through much of the week, 
the strength of onshore flow will be generally held in check over 
the coming days. Onshore flow will be strong enough to produce small 
craft advisory westerlies in the central and eastern Strait of Juan 
de Fuca most afternoons and evenings, but winds look otherwise 
sub-advisory level across area waters.   27


.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.


PZ...Small Craft Advisory until 3 AM PDT Tuesday for Central U.S. 
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters 
     Strait Of Juan De Fuca.