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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 181016

Area Forecast Discussion
National Weather Service Seattle WA
316 AM PDT Sun Aug 18 2019

.SYNOPSIS...Onshore flow will persist into Monday as a weak 
weather system passes to the north of Western Washington. An upper
level ridge will strengthen early in the week for warmer 
temperatures and more sun. A front is expected around the middle 
of the week with a chance for showers and cooler temperatures.


.SHORT TERM /TODAY THROUGH TUESDAY/...Low level onshore flow is
marginally weaker this morning than the same time yesterday. There
are more breaks in the cloud cover over the interior, but clouds
are gradually filling in the remaining gaps. Conditions will be
much like yesterday as another weather system passes mainly to the
north of the region. The prospects for some afternoon sun across
the interior are a bit better than yesterday as the marine layer
is a little less organized and the weather system passing to our
north will act to mix things up a bit. 

Upper ridging will gain some amplitude over the area by later
Monday into Tuesday as a rather strong trough (for late summer) 
lifts northeastward well offshore. Low level flow will become
light...or even weakly offshore at times by Tuesday. Combined with
higher heights, this will help boost temperatures back to near or
a little above normal. 

associated with the upper trough offshore will move onshore on
Wednesday. The models haven't been very consistent with its
strength, but it does seem like the coast and Olympic Peninsula
will get some decent QPF out of it. How much is left by the time
the weakening front reaches the interior is still in question.
Plenty of cloud cover and cooler temperatures are a good bet for

The front looks to dissipate in the Cascades Wednesday night with
zonal flow and drier conditions expected for Thursday. The mean
trough position remains in place to our northwest as we approach
the weekend with another system possibly clipping the northwest
corner of the state Friday night or Saturday.    27


.AVIATION...Low level onshore flow will becoming light on Monday.
Westerly flow aloft. 

Stratus along the coast and to the north down the Strait of Juan 
de Fuca at 10z/3 am. To the south stratus already over Southwest 
Interior. Stratus along the Cascade foothills in the evening hours
has spread back over the central Puget Sound area at 10z. 
Ceilings long the coast in the 1500-2500 foot range. Ceilings in 
the interior mostly in the 3000-4000 foot range. Ceilings will 
lower another 500 feet or so from the current levels for the 
morning hours. Ceilings lifting back up to 4500 to 6000 feet this 
afternoon with the VFR ceilings continuing into the evening 
hours. Stratus spreading over the area again during the early 
morning hours on Monday with ceilings lowering back down to 
2000-3000 feet. Felton

KSEA...Ceilings near 3500 feet lowering to around 2500 feet this 
morning. Ceilings lifting back up to the 4500-6000 foot level this
afternoon into early Monday morning. Winds south southwest 4 to 8


.MARINE...Low level onshore flow through tonight will
becoming light Monday and Tuesday. Small craft advisory westerlies
expected in the central and eastern Strait of Juan de Fuca
tonight. A front will move through the waters on Wednesday with
small craft advisory southerlies over the coastal waters. High 
pressure will build over the coastal waters behind the front 


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


.CLIMATE...The chances for a 90 degree day in Seattle
significantly decrease in the last two weeks of August. Since
records started at Seattle-Tacoma airport in 1945 there have been
232 days with a high of 90 degrees or more. Only 26 of them ( 11
percent ) have occurred from August 18th to September 22nd ( the 
date of the latest 90 plus day on record at Sea-Tac ). On the flip
side, of the 26 occurrences after August 18th 10 of them ( 38 
percent ) have occurred in the last 7 years. Out of the last 7 
years Seattle has had a 90 degree plus day in the last two weeks 
of August or in September in 6 of them with the only year without
at least 1 90 degree plus day after August 17th being 2015. 

The 90 degree plus day total for Seattle so far in 2019 is 2 days.
There were 11 days in 2018, 8 days in 2017, 7 days in 2016, 12 
days in 2015 and 5 days in 2014. Felton


PZ...Small Craft Advisory from 6 PM this evening to 2 AM PDT Monday 
     for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca.