Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 190333

Area Forecast Discussion
National Weather Service Seattle WA
830 PM PDT MON JUL 18 2016

.SYNOPSIS...An upper low will produce scattered showers through
Tuesday night, with isolated thunderstorms over the Cascades in
the afternoon and evening hours. The low will weaken as it moves
slowly inland Wednesday, and the threat of showers will decrease
as a weaker trough sits well offshore. A weak upper ridge should
bring dry and warmer weather this weekend.


.SHORT TERM...Weakly diffluent southerly flow aloft continues and
the air mass is unstable and moist below 10000 ft. A few showers
continue to move through the area but most of been firing off in
the warmth of the day and that will probably be true again Tue.
The upper low will move slowly northeast Tuesday, reaching the
Washington coastal waters late Tuesday night. It will keep a
chance of showers across the forecast area, and we have decided to
add a slight chance of thunderstorms to the Cascades for the
afternoon and evening hours. On the other hand, the combination of
weaker onshore flow and the instability should bring partly sunny
skies to much of the interior lowlands in the afternoon.

The remains of the low will weaken and move inland Wednesday,
leaving a broader upper trough offshore out near 130W. It will
maintain somewhat unstable southwest flow aloft over the forecast
area, although the air mass should be less moist during the
afternoon. The threat of showers on Wednesday should be limited to
the coast, northern lowlands, and mountains. And again, the
morning clouds should give way to partly sunny skies over parts of
the interior lowlands during the afternoon. Highs will again range
from the mid 60s at the coast to the mid 70s inland.

The offshore upper trough will progress slowly eastward Wednesday
night and Thursday, probably moving into Western Washington and
Oregon Thursday evening. The chance of showers looks limited to
parts of the Olympics and Cascades, with partly to mostly sunny
skies elsewhere.

.LONG TERM...A rather flat upper ridge will begin building
offshore on Friday. It looks like a final weak shortwave will move
through Western Washington, weakening though as 500 mb heights
rise to around 5820. It should be a dry day, with morning marine
stratus mainly at the coast. Upper heights will rise over the
area on Saturday, then fall a bit Sunday as an upper trough digs
into British Columbia and Monday as it moves into the Canadian and
Northern Rocky Mountains. The weather is likely to remain dry as
the trough moves by. For now we`re forecasting minimal effects
associated with it, but it could trigger a marine push Sunday
night or Monday night. McDonnal


.AVIATION...A broad upper low over the coastal waters will remain
stationary through Tuesday giving light southerly flow aloft. A
ridge of high pressure extending northward from the Oregon coastal
waters along the Washington coastline will give low level south to
southwest flow. The air mass will be moist through Tuesday. The
lower levels will be mainly stable with a marine layer while
conditions in the mid and upper levels will be mostly unstable
through Tuesday.

Showers are decreasing this evening, although there are still some
over the Cascades. Stratus will likely develop in the marine layer
overnight with ceilings most locations falling to around OVC020.
Lower stratus will likely scatter out around 20Z Tuesday.

KSEA...Cigs around 050 this evening have recently scattered out.
They may reform and fall to OVC020 about 11Z tonight. It appears
that most shower activity will over the Cascades late this evening.
Winds will be Southerly 5-9 KT. Albrecht/13


.MARINE...A weak ridge of high pressure extends from the Oregon
coastal waters northward into the Washington coastal waters near
the immediate coastline. This will result in southerly flow over
the coastal waters and south to southwest flow inland. Westerly
inflow through the Strait of Juan de Fuca will remain near or below
small craft advisory criteria through this evening. Onshore flow
will weaken Tuesday and Wednesday.

The surface ridge will move into the offshore waters later Wednesday
through Friday resulting in a more west to northwest flow over the
coastal waters. Expect daily inflow through the Strait of Juan de
Fuca resulting in typical late afternoon and overnight small craft
advisory conditions Thursday through Saturday. Albrecht/13


.SEW Watches/Warnings/Advisories...



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