Weather Service Report

FXUS66 KSEW 201109

Area Forecast Discussion
National Weather Service Seattle WA
409 AM PDT Sat Oct 20 2018

.SYNOPSIS...Upper level ridge will remain over the area through
Monday but weaken with time. Low level flow will remain light
through Monday as well. Dissipating system moving through Western
Washington Tuesday night with a stronger system arriving 
Wednesday night and Thursday. 


.SHORT TERM...Satellite imagery shows fog has already formed over
much of the coast, lower Chehalis Valley, Southwest Interior, 
Hood Canal and San Juan Islands early this morning with areas of 
visibilities down to a half mile or less. Clear skies for now over
most of the rest of the area. Temperatures at 3 am/10z were in 
the mid 30s to mid 40s.

No change in the conditions this morning with an upper level ridge
over the area, light surface gradients and light low level flow 
( Sandpoint profiler has winds less than 10 knots up to 2500
feet ). Plenty of low level moisture around combining with light 
low level flow to produce widespread fog over Western Washington 
this morning. Visibilities of a half mile or less will be common 
through the morning hours. Sandpoint profiler also shows low level
inversion slightly weaker this morning versus Friday. The layer 
is shallow with tops 1000 feet or less so expect another 
midday/early afternoon breakout across the area. High temperatures
will be in the 50s and lower 60s. 

Upper level ridge remaining over the area tonight into Sunday. No
changes in the lower levels with light flow and light surface
gradients. Low level inversion redeveloping overnight with fog
forming once again in the late night hours. Lows will be in the
mid 30s to mid 40s. 

Upper level ridge still over the area Sunday morning but 
beginning to show signs of weakening. Conditions in the lower 
levels unchanged with light surface gradients and light low level 
flow. Expect another round of morning fog with afternoon sunshine.
Visibilities below a half mile will be common again Sunday 
morning. Highs in the 50s and lower 60s. 

Upper level ridge continuing to weaken Sunday night with the ridge
axis moving east of the area. Still no changes in the lower levels
resulting in another round of late night fog formation. Lows will
be in the mid 30s to mid 40s.

Western Washington on the backside of the upper level ridge on
Monday with southwesterly flow aloft. Low level flow still light
and with the approaching front well offshore surface gradients are
light as well. End result, you guessed it, another morning with
fog in the area and reduced visibilities followed by sunshine
in the afternoon. Highs for the third day in a row in the 50s and
lower 60s.

.LONG TERM...00z extended model runs continuing the trend of a
slower and weaker front with what is left of the system moving
through Western Washington Tuesday night. With the trend of slower
eastward movement of the front have dry weather in the forecast
Tuesday from about the Seattle area eastward and just chance pops
over the area for Tuesday night. Some differences in the solutions
start showing up on Wednesday with the GFS a few hours faster 
with the onset of precipitation ahead of the next approaching 
front. The GFS has precipitation into Western Washington by 
Wednesday afternoon while the ECMWF gives the area a little 
longer break with the precipitation not starting until Wednesday
night. The flow aloft is consolidated so can't rule out the GFS
solution at this point. Wednesday forecast has a chance of rain.
Stronger and wetter system arriving by Thursday with rain across
the entire area. Still the possibility of some windy conditions
along the coast Wednesday night into Thursday. Another system
trying to get organized quickly behind the front for Friday. Main
message of the extended is we are going back to our regular fall 
pattern next week. Felton 


.AVIATION...Strong upper level high pressure centered over eastern
Oregon and central Idaho early this morning, with moderate W-SW
flow aloft over Western Washington. Upper ridging over the Pac NW
will flatten and start to weaken through Sunday morning, but not 
enough to break the low-level temperature inversions. Therefore, 
the air mass will remain very stable, with dry air above a moist, 
shallow boundary layer. This setup will be conducive for 
development of late night and morning dense fog both this morning 
and Sunday morning. On both days, weakness of low-level flow
combined with low-level temperature inversions, and somewhat 
anemic late October insolation will amount to late burnoff of fog,
in some cases waiting until after noon.

KSEA...Dense fog (<=1/4SM) appears likely for a couple of hours
this morning and again on Sunday morning, especially in the hours
immediately following sunrise. Slow burnoff expected in the 18z-
20z time frame. North wind will be light, especially during the
more critical morning hours (5 knots or less). Haner


.MARINE...Light to moderate onshore flow will continue today. 
Westerly winds in the strait have subsided allowing for the 
inherited SCA to expire. Winds are expected to turn northerly and 
offshore Sunday into Monday.

An approaching front on Monday will turn the low level flow 
southeasterly. Advisory level winds are expected over the coastal 
waters for a time Tuesday, but the system is forecast to dissipate 
rapidly. A stronger system will arrive around Wednesday, with 
advisory level southerly wind expected to focus once again over the 
coastal waters.  SMR




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