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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 182152

Area Forecast Discussion
National Weather Service Seattle WA
252 PM PDT Tue Sep 18 2018

.SYNOPSIS...A zonal flow in the mid levels will give way to a
series of troughs by late this week and weekend. The weather will
remain dry through most of Wednesday, then become cooler and
showery by late week. Rain chances will increase over the weekend.
A drying and warming trend by next week with a mid level ridge of
high pressure building in across the Eastern Pacific. 


A pleasant day is on tap across Western Washington, if
temperatures in the mid 60s with plenty of sunshine are your
thing? For now the cold and damp weather is confined to areas far
from here, like Iceland. While here we have just a zonal flow
making for some quiet late summer days. Current visible satellite
picture shows increasing mid and high level clouds from the west,
and all of this morning's fog and stratus has dissipated. The 
clouds to the west are increasing in advance of a mid level trough
of low pressure swinging southeast into portions of the BC coast 
through Wednesday. While it will bring an increase in cloudiness, 
the overall moisture is not impressive. So the rainfall with the 
system should be confined primarily to showers Wednesday and 
Wednesday night and significant rainfall is not expected. 
Temperatures will continue close to seasonal normals, from the mid
to upper 60s in most spots.

Key Messages in the short term:
No significant impacts in the short term 
Dry and Cool Today 
Becoming more unsettled Wednesday through the weekend


A more substantial system will dive into the southwest BC coast 
by Friday and into the weekend. This system will have more 
moisture and be stronger so there will be a greater chance of rain
by the weekend. Temperatures will trend a few degrees below normal
this weekend as well, owing to the increased cloud cover, rain,
and onshore flow. Troughing will ease Sunday and then quickly
transition by Monday. A strong mid level ridge of high pressure
will build in across the eastern Pacific. 500mb heights will 
climb quickly early next week and the pattern will feature a 
drying and warming trend. Heights will exceed 580dm by late Monday
and into Tuesday, which would mean highs well into the 70s and
possibly even some low 80s south if the current forecast of
ridging and rising heights holds true!

Key Messages in the long term:
No significant impacts in the long term.
Warming and drying trend next week some 70s/80s?



.AVIATION...The air mass across Western WA is dry and stable with
onshore flow. The flow aloft is westerly. Expect VFR conditions
this evening. Low level clouds will likely reform in the interior
early Wednesday morning for a return of MVFR conditions. A weak
cold front will bring scattered showers to the Olympic Peninsula
Wednesday afternoon. There is a chance of showers in the interior
Wednesday night as the cold front continues to push through
Western WA. 33

KSEA...VFR conditions this evening. Low clouds possibly forming
12-15z for MVFR cigs. N/NE winds to 10 kt becoming S/SW by 12z. 33


.MARINE...Light W/NW flow will prevail across the waters tonight.
Onshore flow will increase on Wednesday behind a cold front - 
Small Craft Advisory winds are likely through the Strait of Juan 
de Fuca. 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. 33