Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 260434
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
934 PM PDT THU AUG 25 2016

.SYNOPSIS...
A strong ridge of upper level high pressure over the north central
Pacific will dominate the weather for Friday. Another bout of
record high temperatures expected. A significant change on
Saturday as an upper level low pressure pushes south through BC
and allows a strong push of cooler marine air into Western
Washington. This will result in temperatures falling 15-20 degrees
from Friday`s highs. This will also herald the start of a pattern
change as an upper level trough of low pressure takes shape across
the region with showers returning by early next week.

&&

.SHORT TERM...
A record breaker across Western Washington, with SeaTac topping at
91 degrees and many spots ranging from the upper 80s to mid 90s.
Breezy conditions and low humidity were present in the offshore
flow pattern. This sent heat all the way to the coast again.
Temperatures this evening are slowly cooling, with the urban areas
clinging to the low 80s even at 9 PM. A warm night in these urban
areas with temperatures likely to only dip into the mid 60s around
sunrise. Temperatures in those urban areas will probably remain in
the 70s until after 3 AM or so, bringing little relief. Outlying
areas should fare better, but even there mid to upper 50s are
expected. The warm night should allow for quick temp rises Friday.
Our surface thermal trough near the coast early will slide
eastward by afternoon as marine influence cools the coast. Temps
for the inland areas will reach from the upper 80s to low 90s
again as the atmosphere will be about the same as today to support
those values away from the coast. 850mb temps will linger around
+18C.

Overnight Friday the flow will turn onshore and a sharp cooling
trend will commence as 500 mb heights tumble with an upper low
dropping south through BC. Highs on Saturday will be near to below
normal, and overall some 15-20 degrees cooler than Friday. Sunday
will be cooler still as 500mb heights drop to around 573dm. An
approaching trough off the coast will bring a chance of showers by
the close of the weekend into early Monday as highs will struggle
to lower 70s. Johnson

.LONG TERM...From Previous Discussion. Cool unsettled weather with more
clouds and a chance of showers is a good bet through next week.
The forecast is still pretty noncommittal about which days might
be a rain out and which will have more sun than clouds--Monday
could be wet on the coast and it could be wet in the interior of
Western Washington Monday night, and then Thursday looks pretty
showery in the latest GFS.

&&

.AVIATION...A strong upper level ridge will remain over the NE
Pacific tonight and Friday. The flow aloft is northerly. The air
mass over Western Washington is dry and stable. VFR conditions
expected with just FEW-SCT cirrus. 33

KSEA...N/NE wind 4-8 kt. VFR conditions. 33

&&

.MARINE...Northerly flow will prevail over the waters tonight and
Friday with a thermal trough along the coast. Onshore flow will
develop Friday afternoon and evening as low pressure shifts
inland. Small Craft Advisory winds are likely through the central
and eastern strait of Juan de Fuca Friday evening. Strong onshore
flow will continue on Saturday then ease on Sunday. A frontal
system will reach the area on Monday. 33

&&

.FIRE WEATHER... From Previous Discussion. Thermally induced low
pressure along the coast will shift over interior Western
Washington later tonight and Friday. Fire zones susceptible to
breezy north to northeast winds combined with the critically low
humidity will experience borderline red flag conditions this
evening. Sustained winds over the Puget Sound lowlands and the
northwest Olympic Peninsula will reach up to 10-13 mph before
easing after sunset. The red flag warning will remain in effect
until 10pm this evening for the four fire weather zones.

Partial overnight humidity recovery is expected overnight with
decreasing wind. Although humidity again falls to critical levels
Friday afternoon, lighter winds will not support red flag
conditions. Fuels have likely reached peak dryness so the potential
for large fire growth still remains elevated.

A strong onshore marine push develops Friday night, ushering in a
cool and moist marine air mass. Cool and moist conditions persist
next week as a trough stalls over the region. There will be a chance
of some rain at times. Mercer

&&

.SEW Watches/Warnings/Advisories...
WA...Red Flag Warning until 10 PM PDT this evening for Black Hills
     and Southwest Interior Lowlands-Central and South Puget
     Sound Lowlands-North Coastal Lowlands-Southeast Puget Sound
     Lowlands Generally Below 1500 Feet.

     Heat Advisory until 8 PM PDT Friday for Bellevue and Vicinity-
     Bremerton and Vicinity-East Puget Sound Lowlands-Hood Canal
     Area-Lower Chehalis Valley Area-Seattle and Vicinity-
     Southwest Interior-Tacoma Area.

PZ...None.
&&

$$

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www.weather.gov/seattle/gafd/latest_webafd.html


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