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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 191012

Area Forecast Discussion
National Weather Service Seattle WA
312 AM PDT Mon Aug 19 2019

.SYNOPSIS...An upper level ridge will build into the area today 
with warmer temperatures and more sun into Tuesday. A front will 
move through Western Washington on Wednesday with some rain and 
cooler temperatures. Drier conditions and seasonal temperatures
will return later Thursday into the end of the week.


.SHORT TERM /TODAY THROUGH WEDNESDAY/...Upper level ridging will
build over the region today with the low level onshore flow
weakening through the day. Subsidence with the ridge will help
clear out the deck of stratocumulus covering much of Western
Washington by early this afternoon. High temperatures will be
right around climatological normals for mid August.

Upper ridging will begin to gradually weaken on Tuesday as a
fairly potent late summer trough lifts northward through the
offshore waters. High and mid level clouds should begin to
spreading over the region by later in the day, but not before
interior areas see what will probably be the warmest day of the
week with some spots from Seattle southward getting into the lower

Rain will spread to the coast after midnight Tuesday night then
into the interior by later Wednesday morning as a weakening front
moves across the area. Models continue to generate some decent QPF
with this system for late August with parts of the Olympic
Peninsula and north Cascades getting up to 1.5 inches of rainfall.
Much of Puget Sound could see a third to half inch of rainfall
over the course of a few hours during the day Wednesday. As
expected, temperatures will be cooler with 60s to around 70 for

.LONG TERM /THURSDAY THROUGH SUNDAY/...The front will shift east
of the Cascades by Wednesday night with a few showers lingering 
across the area in post-frontal onshore flow. Drier conditions 
will return Thursday as some low amplitude ridging builds back 
into the area. High temperatures on Thursday will be near normal. 
Upper ridging over the region quickly flattens later Friday as 
another system passes by mainly to our north. Apart from an 
increase in onshore flow and perhaps some drizzle near the coast, 
it shouldn't have much of an impact for most of Western 
Washington. Temperatures look seasonable for the weekend with the
likely scenario being morning clouds and afternoon sun for most.


.AVIATION...Light low level flow into Tuesday morning. Westerly
flow aloft becoming southwesterly tonight and Tuesday.

Stratus filling back in over the interior early this morning with
stratus already along the coast and Strait of Juan de Fuca.
Ceilings mostly near 3000 feet except along the coast with local
ceilings 1000-1500 feet and visibility 3-5sm in light drizzle.
Drizzle coming to and end later this morning. Ceilings lowering
another 500-1000 feet between 10z and 15z. Marine layer getting
thinner each day so expect the interior to clear out by 21z-22z 
and the coast a couple of hours later. With the light low level 
flow stratus having a hard time reforming over the interior later 
tonight into Tuesday morning. 

KSEA...Ceilings lowering to around 2000 feet by 15z. Clouds
scattering out quickly 19z-20z with clear skies expected 
overnight. Southwest winds around 5 knots becoming northerly 22z-
00z. Felton


.MARINE...Light low level flow today and tonight becoming 
southerly and increasing Tuesday as a front approaches the 
waters. The well organized front will move through Wednesday with
small craft advisory southerlies ahead of the front over the 
coastal waters, entrances to the Strait of Juan de Fuca and 
Northern Inland waters and post frontal small craft westerlies in 
the Strait. High pressure will build over the coastal waters 
behind the front Thursday. A weaker front will move by to the 
north later Friday. Felton


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