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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 191106

Area Forecast Discussion
National Weather Service Seattle WA
400 AM PDT Wed Sep 19 2018

.SYNOPSIS...A weak system will bring some clouds and a few
showers to the area the next couple days. Another system will more 
rain and breezy conditions Friday through the weekend. The coast, 
north interior, and mountains will see the most rainfall while the 
southern interior lowlands will get only light amounts. Warmer and 
dry offshore flow may develop early next week. 


.SHORT TERM...A weak elongated upper trough extends from the B.C. 
coastline south over the Pacific Northwest to CA/NV this morning. 
This spreading of energy across along the west coast inevitably 
starves embedded waves of moisture, lift and any real organization. 
The system over the area early this morning is no different. So far 
little to no rain has been observed except by the coastal radar 
which has picked up a narrow band of possible showers stalled well 
offshore. This activity has shown little movement most the night and 
appears to have formed close to the weak surface trough depicted 
offshore by MSAS analysis. The mid and high cloud from the 
approaching weak system already sheared off across the area but 
satellite does show some lower level moisture off the coast along 
the surface trough. Most models do bring abundant low level moisture 
in this evening through early Thursday. Weak lift is indicated and 
spotty QPF is shown by some models. Given the evidence for lower 
deeper moisture arriving and a source of weak lift, will leaves 
slight or low chance pops in the forecast, although most spots will 
stay dry the next couple days. 

Models agree that a stronger frontal system will bring more 
appreciable rain to Western Washington Friday into the weekend. 
While models show some fairly decent rain amounts in some areas like 
the Olympic Peninsula, far north interior, and Cascades north of 
Snoqualmie, the Seattle/central Puget Sound area southward looks to 
get much less. Highest amounts in favored areas mentioned could see 
well over 1 inch in a 48 hour period, while less than a quarter inch 
is expected around Seattle south due to rain shadow affects and the 
bulk of moisture heading inland well to the north. The onshore flow 
will keep temperatures a bit cool but not far off average in the mid 
to upper 60s. Moisture should prevent cold pocket locations from 
radiating too effectively, with lows mostly above freezing even 
around Olympia and Shelton. Showers decrease either late Saturday or 
Sunday, depending on the model with the ECMWF the slowest and most 
aggressive with northwest flow and showers. Will stick with chance 
pops and keep QPF amounts light in this pattern. 

.LONG TERM...The ECMWF is a day later than the GFS in clearing out 
moisture and showers. Most models agree that dry offshore flow will 
develop by Monday as high pressure amplifies off the Pacific 
Northwest coast in response to a larger scale broad trough over the 
central U.S. through most of next week. This is typical for 
September to get cold shots dive into the Rockies or Plains and keep 
dry offshore flow west of the Cascades. Heights peak above 5900M by 
Thursday, so it could definitely reach the 80s by the middle of next 
week. Mercer


.AVIATION...Weak upper level trough moving into Western 
Washington early this morning with light southwest flow aloft. The 
flow aloft will shift to moderate northwesterly behind the trough 
axis later today. The air mass is stable and generally dry except 
for some high level moisture. Low level flow is weak, but will turn 
increasingly onshore this afternoon and evening as surface high 
pressure builds into the coastal waters. Presence of high clouds 
over the region overnight may limit stratus formation over the 
interior of Western Washington this morning, but still expecting 
some patchy morning fog over the SW Interior and possibly the South 
Sound. Stronger onshore flow tonight should bring low clouds with 
low MVFR/IFR ceilings to the coastal areas by this evening and into 
the interior of Western Washington Thursday morning. 

KSEA...Sticking with VFR forecast today. Low MVFR ceilings could 
return tonight after 06Z as stronger onshore flow pulls some low 
level moisture inland. Surface winds light and variable early this 
morning becoming SW 5 to 10 knots after 18z.   27


.MARINE...Onshore flow will increase on Wednesday as a weak system
moves through. Small craft advisory winds are likely in the
central and eastern portions of the Strait of Juan de Fuca and
possibly the northern inland waters later this evening - tonight.
A stronger system will arrive Thursday night into Friday with 
small craft advisory winds over the coastal waters and Strait. 
Onshore flow will continue Friday night and Saturday. CEO