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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 241011
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
311 AM PDT Sat Aug 24 2019

.SYNOPSIS...A weak upper level trough will move by to the north
today and tonight. An upper level ridge will begin building
offshore on Sunday. The ridge will continue to build and shift
over Western Washington Tuesday and Wednesday with the low level
flow going light offshore. The ridge will weaken Thursday and
Friday with low level onshore flow returning to the area.

&&

.SHORT TERM /TODAY THROUGH MONDAY/...Satellite imagery shows
cloudy skies over Western Washington at 3 am/10z. Most of the
cloud cover is of the mid and high variety. Temperatures were in
the 50s to lower 60s.

Lots of cloud cover today into tonight as a weak trough moves by
to the north. Could be enough lift with the trough to produce a 
few showers from about a Bellingham to Astoria line westward today
and mainly over the Cascades this evening. Any precipitation that
does develop will be light. With the cloud cover high temperatures
will continue to be a little below normal, in the mid 60s to mid 
70s. Lows tonight mostly in the 50s. 

Increasing low level onshore flow later tonight into Sunday with 
the trough east of the area. Low clouds developing along the coast
spreading inland Sunday morning. Marine layer not deep enough to
get the stratus all the way to the Cascade crest but deep enough
to keep skies mostly cloudy through the morning hours. Upper level
ridge beginning to build offshore with 500 mb heights rising over
Western Washington but too late to warm high temperatures Sunday
with readings once again in the mid 60s to mid 70s.

Upper level ridge continuing to build offshore Sunday night into
Monday with the surface gradients going northwesterly. This will 
limit the cloud cover in the morning on Monday. Monday morning
the coolest morning of the month with lows in the mid 40s to mid 
50s. Highs on Monday warming a little, into the 70s.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Upper level ridge moving
over Western Washington Tuesday and Wednesday with 500 mb heights
in the lower to mid 580 dms. Low level flow turning offshore but
00z runs are weaker with this development versus previous runs.
Models still have 850 mb temperatures warming into the plus 16 to
plus 20C range but with the weaker offshore flow scenario will 
have the warmest locations near 90 with the remainder of the area
in the mid 70s to upper 80s. The weaker offshore flow scenario
leads to a quicker return to low level onshore flow on Thursday 
as the upper level ridge begins to weaken. Upper level ridge 
continues to weaken on Friday with low level onshore flow. The 
models have a weak surface low over the Oregon and Washington 
coastal waters limiting the strength of the onshore flow. For now 
will go with a 2 day weak push scenario with around 5 degrees of 
cooling both Thursday and Friday. Felton

&&

.AVIATION...Westerly flow aloft will become more northwesterly today 
as an upper level trough departs. Onshore flow will continue at the 
surface. VFR conditions across the area currently with mid to high 
level clouds. Low level stratus may develop along the coast and push 
slightly inland this morning, with perhaps a brief reduction in 
ceilings at a few sites such as HQM and OLM. Elsewhere expect mid 
level clouds and a continuation of VFR conditions. 

KSEA...VFR conditions through the period. A few MVFR stratus may 
move into the vicinity of the airfield by daybreak Saturday. Light 
N/NE winds veering S/SW by this afternoon before shifting back to 
the N/NE under 10 kts this evening. 

CEO

&&

.MARINE...Onshore flow continues this morning. Winds in the Strait 
just barely touched small craft advisory overnight as gradients were 
less than model forecasts indicated. Winds will ease this morning. A 
weak front will move across the area today with another westerly 
push down the Strait this evening into tonight. Gradients look 
stronger with this push but still under gale criteria so have issued 
a small craft advisory for the central and eastern Strait. Onshore 
flow continues through Monday before shifting to offshore for much 
of the week. 

CEO

&&

.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 4 PM this afternoon to 5 AM PDT Sunday 
     for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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