Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 171556

Area Forecast Discussion
National Weather Service Seattle WA
856 AM PDT Wed May 17 2017

.SYNOPSIS...A weak ridge of high pressure over the offshore waters
will very gradually build into the Pacific Northwest and amplify
over the next several days bringing a drying trend to the area.
However, low-level onshore flow and a weak upper trough moving
through the ridge will keep clouds over the area, mainly during
the night and morning hours, through the first part of the
weekend. The upper ridge will build strongly over the area Sunday
through the first part of next week. This ridging, combined with
developing low-level offshore flow will give warm conditions,
possibly the warmest so far this year, Monday and Tuesday.


.SHORT TERM...While precipitation across the region has come to an
end this morning, visible satellite imagery shows abundant, and
rather deep looking, cloud cover across the lowlands of Western
Washington and over the coastal waters. Pressure gradients are
light this morning, but will become increasingly onshore during
the late afternoon and evening hours as the sun does its job
warming Eastern Washington. Expect clouds to develop some breaks
midday through this evening, but overall it will remain mostly
cloudy through Thursday morning with the marine layer in place and
the increase in onshore pressure gradients.

The developing ridge along about 135W with surface ridging over
the coastal and offshore waters favors abundant stratus across the
area, especially during the late night and morning hours. This is
reflected in the overcast conditions shown in the model
statistical guidance. Some areas, such as around the waters of the
eastern portions of the Strait of Juan de Fuca, the San Juans,
and around Hood Canal, will see more breaks than the coastal areas
and the western slopes of the Cascades. Breaks will be a bit more
widespread on Thursday and even more so on Friday as the ridge
builds and shifts east with time. Temperatures today will rise to
around 60, then they will fall into the 40s tonight. We will see
3-4 degrees daytime warming on Thursday and another 3-4 on Friday
as the ridge approaches the area and we see more sunshine during
the afternoons.

We made some minor grid updates for today to remove POPS from the
lowlands this morning, increase the cloud cover slightly this
afternoon, and lower afternoon highs a degree or so. The changes
were not big enough to necessitate a worded zone forecast update.
The remainder of the forecast is in good shape.  Albrecht

.LONG TERM...From the previous long term discussion: On Saturday,
the GFS and ECMWF remain consistent in driving an upper level
shortwave trough across the top of the ridge over southern B.C.,
brushing W WA in the process. This feature looks dry but some
isolated showers over the N Cascades are not out of the question.
There should be enough cooling with the trough to lower
temperatures a few degrees or at least hold them in place. The
other feature of note is an increase in onshore flow Friday
evening ahead of the trough. This could push some coastal low
stratus partway inland along with low level cooling for Saturday

The upper level ridge over the region will rebuild strongly Sunday
and Monday after the shortwave trough departs to the E. 500 MB
heights are expected to rise to around 582 DAM. Low level offshore
flow will develop, first NE on Sunday then easterly on Monday.
Highs will be in the 70s on Sunday. Monday and Tuesday should be
the warmest with most areas in the 70s and probably some spots
passing 80 for the first time this year.

As usual the warm spell doesn`t last all that long. The ridge
begins collapsing and shifting E on Tuesday. The GFS shows a
marine push developing by Wednesday. Kam


.AVIATION...Weak upper level ridging offshore will lead to mainly
dry and stable weather across Western WA today. The flow aloft is
northwesterly. Low level onshore flow will prevail with stratus
clouds over the lowlands. There is a mix of MVFR and VFR
conditions this morning with Cigs around 1500-4000 ft. Expect
mainly VFR conditions this afternoon, then ceilings lowering back
to MVFR overnight. 33

KSEA...Low clouds lifting this afternoon with VFR conditions
expected. Winds at the surface S/SW to 10 kt. 33


.MARINE...A surface ridge along the West Coast will lead to
onshore flow across Western WA today. Highest wind and waves will
be through the central/east Strait of Juan de Fuca - a Small Craft
Advisory is in effect. A weak surface trough will cross the
Coastal Waters on Thursday. Onshore flow will prevail through the
weekend with high pressure over the NE Pacific. 33


PZ...Small Craft Advisory from 5 PM this afternoon to 2 AM PDT
     Thursday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.



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