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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 260317

Area Forecast Discussion
National Weather Service Seattle WA
817 PM PDT Fri May 25 2018

.SYNOPSIS...High pressure offshore and lower pressure inland is
giving strong onshore flow. The onshore flow will bring clouds
into the area later tonight into Saturday morning along with
areas of drizzle or light rain. Onshore flow will decrease
somewhat Sunday into Memorial Day resulting in less night and
morning clouds. An upper level trough will give an increase in
clouds, cooler temperatures, and a slight chance of showers toward
the middle to end of next week. 


.SHORT TERM...Clouds have been hanging tough over the central
portions of Puget Sound this evening and over the Cascades.
Elsewhere, clouds have cleared out today giving nice conditions.
With rather strong onshore flow this evening, expect clouds to
fill in quickly after 06Z, Short term models, particularly the
NAM12, show some very light rain or drizzle forming along the
coast and possibly in an area of convergence over the central
Puget Sound area later tonight into early Saturday as the marine
layer deepens to over 5000 ft and upward vertical motion within
the moist layer develops in response to the passage of a weak
upper level trough. 

The upper trough moving across the area tonight will move well
east of the area by Saturday afternoon resulting in rebounding 500
mb heights across Western Washington. With the marine air in
place, temperatures will rise into the 60s across the area.
However, it is expected to clear out during the afternoon hours
and onshore pressure gradients Saturday evening are expected to be
much lower than they are this evening. 

Expect fewer marine clouds Saturday night and Sunday. With
sunshine, temperatures on Sunday will rise into the mid 60s along
the coast and into the lower to mid 70s inland. The warming trend
will be short-lived however as onshore flow increases again Sunday
night and marine air moves back inland late Sunday night into
Memorial Day. 

Current forecasts are in good shape. No updates are required.

.LONG TERM...From the previous long term discussion: Tuesday 
looks similar to Monday. Something of a change in the patten 
occurs for the Wednesday through Friday period, as an upper trough
sets up over the region. Heights will be lower, and there will be
a chance of showers at times. Currently the best shot at a little
rain are Wednesday and again Thursday night. Highs will be in the
60s all three days. Burke 


.AVIATION...A weak upper level trough will move across Western
Washington this evening. This will help to maintain, or even
strengthen, onshore flow tonight and deepen the moist and stable 
marine layer. Expect clouds to fill in between 06Z and 09Z tonight
with fairly widespread coverage of CIGS at or below 020 later 
tonight into Saturday morning. Lift within the deepening marine 
layer will produce areas of drizzle or light rain along the coast 
and in the central Puget Sound lowlands, and the drizzle may cause
local IFR conditions in reduced CIGS and VISBYS. 

Moderate southwest flow aloft will become light northwesterly 
late tonight. Above the marine layer, the air mass will be dry and

Conditions will improve to VFR around 18Z Saturday with VFR
conditions continuing throughout the area through Saturday night.

KSEA...The above discussion applies. North wind this evening will
turn southerly 4-7 kt by or shortly after 06Z this evening. 
MVFR Cigs will fill in between 07Z and 09Z with cigs below 020 
expected. 3-5SM -DZ is possible 09Z-15Z. Clouds will lift and
scatter by 21Z Saturday. Albrecht


.MARINE...Moderate to strong onshore flow tonight is giving gales
to the central and eastern portions of the Strait of Juan de Fuca
and small craft advisory conditions to Admiralty Inlet and the
Northern Inland waters adjacent to the east entrance. The onshore
flow and wind will gradually relax late tonight into Saturday
morning. Sea breeze produced small craft advisory conditions over
the near-shore coastal waters near Grays Harbor will relax after 9

Onshore flow will weaken Saturday through midday Sunday as high
pressure builds into the waters. But diurnally driven sea breezes
will result in small craft advisory conditions in portions of the
Strait during the late afternoon and overnight hours on Saturday. 

Onshore pressure gradients are expected to increase once again
Sunday afternoon and evening. Gales are possible any evening from
Sunday through Wednesday. Albrecht


PZ...Small Craft Advisory until 9 PM PDT this evening for Coastal 
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 3 AM PDT Saturday for Central U.S. Waters 
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of 
     Juan De Fuca.

     Small Craft Advisory until 3 AM PDT Saturday for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands.