367 
FXUS66 KSEW 100316
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
815 PM PDT Tue Oct 9 2018

.SYNOPSIS...A ridge of high pressure building into British Columbia 
Wednesday will expand over Washington Thursday, then will shift west 
just offshore on Friday as a trough drives south into the interior 
Pacific Northwest. This high pressure will strengthen over the 
entire region this weekend into early next week. Light offshore or 
northerly flow will maintain dry weather with enough moisture at the 
surface for areas of fog during the night and morning hours. High 
temperatures will be near to slightly below average.

&&

.SHORT TERM...Current radar and obs are finally clear of precip and 
even some breaks in the clouds were visible prior to 
sunset...allowing for some basking in the sunlight before the autumn 
gloom settles into the area. Current IR satellite shows some high 
clouds moving over the area and with plenty of moisture near the 
surface from the previous rains...not difficult to imagine some low 
level clouds or isolated fog in the more prone areas developing 
overnight.

Looking at the models...they appear to be drying out the lower 
levels a little too quickly as satellite trends seem to imply the 
aforementioned stratus/fog threat. Expecting these low clouds and 
fog to be fairly widespread by morning only to clear out around noon 
or a little after which should allow highs to reach the low 60s over 
the interior. 

The ridge shifts more directly over the region Thursday with 
residual moisture remaining trapped near the surface. The resulting 
subsidence will transition conditions toward less stratus and more 
fog due to radiational cooling Wednesday night and Thursday morning. 
Most exposed areas should break out of the fog by afternoon but 
sheltered valleys including the southwest interior could hold on to 
fog longer into the afternoon hours. This may cause some of the 
valleys to stay noticeably cooler during the afternoon while areas 
with clearing reach the mid 60s.

The ridge will amplify and shift westward just off the British 
Columbia and WA/Oregon coast Friday as a trough drives southward 
east of the Cascades. Conditions are expected to remain dry as the 
path of the trough keeps W WA out of any of the associated moisture. 
Weak onshore flow may bring a slight influx of moisture thus 
continuing the pattern of morning low clouds and fog clearing to 
reveal partial afternoon sunshine. Highs should be near normal in 
the low to mid 60s.  SMR/Mercer

.LONG TERM...From Previous Discussion...Global models are in good 
agreement that the ridge of high pressure will continue to dominate 
the pattern through the weekend into the early to middle part of 
next week. Offshore flow may strengthen and dry out the low level 
enough to inhibit widespread fog. Sheltered prone valleys will still 
get fog, but more of the area should see sunshine with better 
mixing. MOS shows only slightly warmer temperatures, with mid or 
even upper 60s possible Monday and Tuesday of next week. Models do 
not show any significant fronts by this time next week. Expect a 
prolonged period of dry weather and seasonable temperatures. Mercer

&&

.AVIATION...Moderate northwest flow continues into Wednesday as
high pressure builds offshore. VFR conditions currently in place for 
most W WA terminals except for HQM reporting MVFR. With plenty of 
surface level moisture due to previous rains...does not take a lot 
of imagination to expect stratus development during the overnight 
hours tonight. Models are currently a bit too optimistic regarding 
cigs while inherited forecast may be a touch too pessimistic. 
Certainly expecting MVFR conditions to become more widespread 
overnight with the more cloud/fog prone areas dropping into IFR and 
persisting into Wednesday morning with improvement expected as the 
day progresses into afternoon. Lighter winds through the period. 

KSEA...Above discussion applies. VFR cigs this evening will 
gradually erode as the night progresses with MVFR to IFR conditions 
expected during the overnight and into early Wednesday morning. 
Improvement to VFR expected after 18z late Wed morning. Light and 
variable winds overnight into Wednesday morning becoming northerly 
in the afternoon with speeds 4-7 knots. SMR/CEO

&&

.MARINE...Winds have eased over the coastal waters as high pressure 
builds into the waters and remains in place through Thursday. This 
will result in lighter winds and areas of fog. As such...will cancel 
the inherited small craft advisory with the evening forecast 
package. This high pressure remains in control through the end of 
the week and into the weekend with northerly low level offshore 
flow keeping winds fairly benign. SMR/CEO

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.

&&

$$

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