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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 191540

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

.SYNOPSIS...A weak upper level trough will give a few showers to
the Olympic Peninsula today and to most of the area tonight
through Thursday. A cold front associated with a stronger low
pressure system will give rain and breezy conditions to the area
Friday into Saturday. Showers will decrease on Sunday, then drier
offshore flow will develop early next week. 


.SHORT TERM...Radars show a line of showers extending from the
central coast of Vancouver Island southward through the coastal
waters. These showers will spread onto the Olympic Peninsula later
this morning, and a few showers may develop over the Cascades this
afternoon. Shower chances will increase somewhat over the rest of
the interior tonight. 

Model guidance shows a developing warm front spreading rain onto
the coast on Thursday then lifting northeast into the northern
interior Thursday night. Most of the interior from about Everett
southward will see little in the way of precipitation with this
feature as westerly flow aloft provides some downslope off the
Olympics and the main energy with the front remains north. 

We will be closely watching the next feature upstream, a
developing low with its associated cold front, as it moves into
the area later Friday into Friday night. Model solutions continue
to show quite a spread on how much this feature develops and on 
the eventual track of the low. The rather strong looking 
deterministic run of the GFS takes the low into northern Vancouver
Island while the ECMWF is weaker and farther east with its track.
Over the past few days, some model solutions have taken a 
moderate low into Western Washington. Consistency and continuity 
of model solutions is not high on the track/strength of the low, 
but all indicate that later Friday will be wet and potentially 

A first period update was issued to lower temperatures for this
morning and increase patchy fog coverage, and to reduce POPS for
the interior today. Otherwise, forecasts look good for now. 

.LONG TERM...From the previous long term discussion: 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. 


.AVIATION...The morning visible imagery shows patchy fog and low
clouds over the Snohomish river valley, the SW interior and the
central coast. Low clouds are shallow and will burn off late this
morning. A weak cold front will move inland this afternoon and
evening for increasing clouds and isolated to scattered showers 
across Western WA. Onshore flow will increase behind this system 
tonight with MVFR conditions likely. 33

KSEA...VFR conditions today. S winds at the surface to 10 kt.
Light showers in the vicinity after 00z. MVFR ceilings by 12z with
onshore flow. 33


.MARINE...Onshore flow will increase today behind a cold front - Small
Craft Advisory winds are likely through the Strait of Juan de 
Fuca and Northern Inland Waters. A stronger Pacific frontal system
will arrive Thursday night and Friday for another round of Small 
Craft Advisory winds over the Coastal Waters, and possibly the 
entrances to the strait. The flow will turn onshore Friday night 
and Saturday as this system moves inland and weakens. Offshore
flow will develop early next week as a thermal trough forms along
the coast. 33


PZ...Small Craft Advisory from 6 PM this evening to 5 AM PDT Thursday 
     for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland 
     Waters Including The San Juan Islands.