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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 241631

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

.SYNOPSIS...An onshore flow pattern will prevail for the next
several days for night and morning clouds and partial afternoon
sunshine most days. An upper trough could bring some showers
around the middle of next week.


.SHORT TERM...A marine push has brought low clouds and some 
drizzle this morning. The drizzle threat should end late this 
morning and low clouds will lift and break up a bit in the 
afternoon with daytime heating. Highs will be cooler today.

An upper trough will move over the area Friday but it looks dry. 
Low level onshore flow will continue for more night and morning 
clouds and partial afternoon sunshine. Highs will be near normal. 

More of the same is expected for Saturday with low level onshore 
flow continuing. A weak upper short wave could bring some drizzle
Saturday morning. Schneider

.LONG TERM...Previous discussion...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...An upper ridge over Eastern Washington will continue
to move east today, and the tail end of a weak upper trough will
brush by Western Washington on Friday. Moderate southwest flow
aloft will continue. Low-level onshore flow will continue to
advect marine clouds onshore, though they will burn off over the
interior this afternoon and again around mid-day on Friday. The
low-level air mass will be moist and stable, while the air mass
above the marine layer will by dry and stable.

KSEA...Current cloud depth based on pilot reports suggests
scattering of marine clouds around 21z (1 pm). A shift to north
surface wind is expected around 01z (6 pm). Should be a gradual
shift. Haner


.MARINE...Gales ended at Race Rocks around 7 am, and the UIL-BLI
gradient was down to +0.9 mb at 8 am. West wind through the Strait
will undergo a typical diurnal rise late this afternoon and this
evening. Currently forecasting high-end small craft advisory 
strength wind in the Central and Eastern Strait this evening, but 
a gale is certainly within the margin of error. Clouds and less 
heating over the interior were one reason for hedging low.

Onshore flow should be a little stronger on Friday PM as the tail
end of a progressive upper trough brushes by. Gales in the
forecast for the Strait on Friday evening. 

Moderate to sometimes strong onshore flow will continue through
the Memorial Day weekend. Haner


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 until 4 PM PDT Friday for Central U.S. 
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters 
     Strait Of Juan De Fuca.

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