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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 250302
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
802 PM PDT Thu May 24 2018

.SYNOPSIS...High pressure offshore and lower pressure inland will
give varying degrees of onshore flow for the next several days.
Expect areas of late night and morning clouds and partly sunny
afternoons on most days. An upper trough could bring some showers
around the middle of next week.

&&

.SHORT TERM...An upper level ridge sits from southeastern British
Columbia into Utah while a weak upper trough sits along 130W. At
the surface, a strong ridge of high pressure extends from the 
Queen Charlotte Sound southward to 40N 138W while a trough of low 
pressure sits over Eastern Washington. Onshore flow is moderate to
strong this evening and will strengthen Friday afternoon and 
Friday night as the weak upper trough offshore lifts ENE across 
the area. 

Satellite imagery this evening shows a few clouds over the central
Washington Cascades and the Olympics and broken clouds over the
coastal waters. Cloud cover appears more moth-eaten than this 
time yesterday and onshore flow isn't as strong, so expect 
stratus coverage to be less expansive and to burn off a bit
earlier on Friday than today. A quick sky coverage update was
issued this evening to accommodate these minor changes. With the 
onshore flow and marine air in place, high temperatures will be 
near normal for the latter part of May. 

The stronger onshore flow late Friday will bring a deeper marine
layer inland Friday night into Saturday and may result in clouds
persisting later into the afternoon on Saturday. But upper level
ridging will build into the area late in the day and the end
result will likely be very little change in temperature with late
day highs. Higher heights and continued, but weaker, onshore flow
on Sunday will likely bring less night and morning clouds and a 
few degree increase in daytime highs. Albrecht 

.LONG TERM...From the previous long term discussion: An upper 
trough will gradually settle over the area for the first half of 
next week with low level onshore flow continuing. There are no 
fronts or organized weather systems expected but there will be 
slight chance of showers Tuesday and beyond. By Thursday the 
models become quite inconsistent adding to the uncertainty and 
justifying low chance PoPs for now. Schneider

&&

.AVIATION...An upper level ridge to the east of the area combined
with troughing offshore will give southwest flow aloft through
Friday. The offshore trough will move east across the area Friday
evening. At the surface, high pressure offshore combined with
lower pressure east of the Cascades will give low level onshore
flow through Friday. The onshore flow will increase Friday evening
as the upper trough moves across the area.

Satellite imagery shows broken stratus over the coastal waters
this evening with VFR conditions across the interior. Expect the
clouds to develop at the top of the marine inversion late tonight
into Friday morning resulting in widespread MVFR conditions across
the area from about 12Z-19Z. Expect scattered cumulus to give VFR
conditions Friday afternoon. Albrecht

KSEA...The above discussion applies. Expect marine stratus with
bases 025-030 around the terminal from about 12Z-19Z on Friday. 
NW wind 4-6 kt will turn north this evening then will back to NW 
8-10 kt Friday afternoon. Albrecht

&&

.MARINE...Moderate to strong onshore flow is giving near gale to
gale conditions to portions of the eastern two-thirds of the
Strait of Juan de Fuca this evening, so the small craft advisory
there was upgraded to a gale warning for the overnight hours.
Marginal small craft advisory conditions remain likely over the
portions of Admiralty Inlet and the Northern Inland waters that
border the eastern entrance to the Strait. An evening sea breeze
is also bringing marginal small craft advisory conditions to the
nearshore portion of the southern third of the coastal waters this
evening. These diurnally driven winds will continue this evening
then slowly diminish during the early morning hours.

Onshore flow is expected to strengthen Friday afternoon and Friday
night as an upper level trough along about 135W swings east across
the waters. A gale watch is in effect for Friday night across the
eastern two thirds of the Strait and will likely be upgraded to a
warning later tonight. 

Varying degrees of onshore flow will continue through the 
Memorial Day weekend and into early next week. At this time the
forecast is for high-end small craft advisory strength westerlies
through the Strait during the late afternoon and overnight hours
each day through the Memorial Day weekend. In typical late spring
fashion, gales are possible on just about any evening in the
Strait. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from Friday afternoon through late Friday night 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 AM 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 4 AM 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 until 4 AM PDT Friday for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 6 AM PDT Friday for Coastal Waters 
     From Point Grenville To Cape Shoalwater Out 10 Nm.

&&

$$

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