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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 241033

Area Forecast Discussion
National Weather Service Seattle WA
333 AM PDT Thu May 24 2018

.SYNOPSIS...High pressure aloft will shift east tonight. Look for
an approaching low pressure system on Friday to move across the 
area Friday night. The main impact will be a deeper marine layer 
and stronger onshore flow, leading to a few degrees of cooling on 
Saturday. The outlook for the rest of the Memorial Day weekend
calls for dry weather with slightly above normal temperatures.


There were a few showers, some briefly heavy, over the CWA at 
this time. Expect the shower threat to persist this morning before
ending this afternoon as drier air and subsidence sets in.

Meanwhile, the marine layer deepened overnight, thanks to strong 
low level onshore flow. This allowed stratus to surge inland. Much
of the lowlands will be blanketed by a solid low overcast by
sunrise. The clouds will burn off over most areas this afternoon.
Max temps across the interior lowlands will be 5-10 degrees
cooler, on average, than yesterday.

An upper level ridge will prevail over the region today before
shifting east tonight, allowing an upper level trof to approach
from the northwest. This system will move across the region 
Friday night. The main impacts from this system will be a
deepening layer and a stronger onshore pressure gradient, which
will lead to additional cooling on Saturday. There will also be a
threat of light showers over the Cascades, near the crest, Friday
afternoon and evening as the upper trof approaches. Otherwise, 
anticipate patchy light drizzle on the lowlands, especially on 
the coast and parts of the Puget Sound region, late Friday night 
through Saturday morning.

Temperatures during this period (Thursday-Saturday) will be near
or slightly above normal.

High pressure aloft or nearly zonal flow aloft will prevail over
the region Sunday and Memorial Day. This combined with weaker 
surface onshore flow will result in temps being a few degrees 
above normal.

Beyond Memorial Day, confidence in the forecast was not high due 
to timing differences in the medium range models concerning a 
strong upper level trof. This trof may impact the Pacific
Northwest about the middle of next week. Have chosen to 
compromise between the slower (GFS/Canadian) and faster (ECMWF) 
solutions. With that in mind, anticipate the upper level trof to 
approach the region on Tuesday and initiate a marine push. 
Cloudier and cooler conditions, along with a chance of showers, 
are anticipated on Wednesday. Temps on Wednesday are currently 
forecast to be below normal. The snow levels are forecast to 
lower into 5500 (west) to 7500 feet (east) range.


.AVIATION...Stratus surging inland early this morning with 
ceilings in the 1000-1500 foot range with local areas of ceilings 
below 1000 feet. Ceilings will begin to improve mid morning with the 
ceilings below 1000 feet gone by 18z. Ceilings in the 1500-2500 foot 
range with scatter out over the interior in the 19z-21z time frame. 
MVFR ceilings will continue along the coast. Ceilings in the 1000-
2000 foot range will redevelop between 08z and 12z overnight.

KSEA...Ceilings below 1000 feet early this morning lifting to 1000-
2000 feet around 16z. Ceilings scattering out early this afternoon 
with MVFR ceilings once again early Friday morning. Southwest wind 4 
to 8 knots becoming northwesterly late in the afternoon through the 
evening hours. Felton 


.MARINE...Strong onshore continues this morning with high pressure
offshore and lower pressure inland and east of the Cascades.
A slight decrease in onshore flow this afternoon and tonight as
compared to Wednesday night will likely result in winds that are 
a few knots lighter over the inland waters. However, an increased 
northwesterly pressure gradient will give small craft advisory 
conditions to the southern third of the Washington coastal waters 
late this afternoon and tonight. 

Onshore flow is expected to increase Friday afternoon and evening
as an upper level feature moves across the region. Winds are
likely to reach gale force again over the eastern two thirds of
the Strait of Juan de Fuca during the late afternoon and overnight
hours, and small craft advisory conditions are expected over most
of the remaining waters except for Puget Sound and Hood Canal.

The rather strong onshore flow pattern is expected to persist over
the weekend. Albrecht


PZ...Small Craft Advisory from 5 PM this afternoon to 6 AM PDT Friday 
     for Coastal Waters From Point Grenville To Cape Shoalwater 
     10 To 60 Nm-Coastal Waters From Point Grenville To Cape 
     Shoalwater Out 10 Nm.

     Small Craft Advisory from 4 PM this afternoon to 4 PM PDT Friday 
     for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 7 AM PDT this morning for Central U.S. Waters 
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of 
     Juan De Fuca.

     Small Craft Advisory until 7 AM PDT this morning for Northern 
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 4 PM this afternoon to 4 AM PDT Friday 
     for Admiralty Inlet-Northern Inland Waters Including The 
     San Juan Islands.