Heat Advisory 2, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
926 AM PDT WED AUG 17 2016

.SYNOPSIS...High pressure aloft and low level offshore flow will
bring and extended period of very warm weather later this week. An
upper trough and onshore will cool things off early next week.

&&

.SHORT TERM...An upper ridge over the Northeast Pacific will give
dry weather to Western Washington through Friday. For today, low
clouds and fog over the southwest interior and down the Strait of
Juan De Fuca will burn back to the coast by afternoon. Otherwise,
it will be sunny. Weak low level onshore flow will continue with
highs a few degrees above normal.

Low level flow will switch to offshore tonight and Thursday as a
thermal trough develops along the coast. This will cause high
temperatures to rise several degrees Thursday with mid 80s to
lower 90s common, even at the coast. The same general pattern will
continue on Friday - the thermal trough will remain near the
coast resulting in another hot day for all areas.

An excessive heat watch is currently in effect for the coast and
interior from Everett south Thursday afternoon through Saturday.
This will likely be converted to either a heat advisory or
excessive heat warning with the afternoon forecast package
depending on how warm the latest guidance looks. Schneider

.LONG TERM...Previous discussion...A weakening upper level ridge
will remain over the region on Saturday for another warm day
across the interior. Weak low level onshore flow will return
Saturday afternoon, providing the coast with relief from the heat.
Temperatures on the coast will likely peak early in the afternoon
before falling.

Beyond Saturday, the operational medium range solutions were quite
a bit different from the earlier runs. The models now indicated a
trough over the Pacific Northwest on Sunday. This would result in
temperatures being cooler than forecast. In other words, the
models were faster with breaking down the ridge. For now,
decided to hold off on making wholesale changes to the forecast.
Would rather wait to see if this idea persists.

It looks like western WA will lie between an upper level ridge
centered offshore and a mean upper level trough over the Rockies
Monday and Tuesday. Heights/thicknesses will still be rather high
but onshore flow should keep temps close to climatology.

&&

.AVIATION...A high pressure ridge will strengthen over Washington
the next couple days. Light northerly flow aloft becoming
northeasterly thursday. Dry and stable conditions will remain in
place over Western Washington...except residual near surface marine
moisture this morning coast...strait...and the sw interior/s Puget
Sound. This low stratus pattern is not expected to develop again
Thursday morning as the low level flow turns more northerly or
offshore keeping any coastal stratus at bay.

KSEA...Few-Sct clouds near 500 ft through 10 am pdt. Otherwise VFR
conditions remain in place with west-northwest wind 5-9
kt...becoming north 10-13 kt after 20z. Stronger surface north winds
are expected by Thursday afternoon. dtm

&&

.MARINE...High pressure over the NE Pacific will maintain
light onshore flow across Western WA. Diurnal northwesterly
afternoon winds continue again today, strongest over the outer part
of the coastal waters where a small craft advisory remains in
effect through at least tonight.

The flow will turn northerly on Thursday then offshore Friday and
Saturday as low pressure forms along the coast. dtm

&&

.FIRE WEATHER...Red flag conditions are likely over the interior
lowlands and the west slopes of the northern Cascades on Thursday.
Red flag conditions will likely spread into the west slopes of the
central Cascades, the Olympics, and the coastal lowlands late
Thursday night into Friday. Fire weather watches have been issued
for most of Western Washington to cover this event.

Light to moderate onshore flow will continue today, so a
continuation seasonably warm weather that has been occurring for
the past several days.

A rather major change in the pattern will begin early Thursday
morning as surface high pressure develops over the interior of
central British Columbia. Forecast models continue to show warm
and dry continental air from the British Columbia interior moving
southwest into the Bellingham area in a Fraser outflow pattern as
Bellingham-Williams Lake pressure gradients fall to around -8 mb.
The air will compress and warm as it moves into the area
resulting in dewpoint temperatures falling into the 30s and 40s on
Thursday. Temperatures just away from the Sound will rise to the
80s to mid 90s giving relative humidity values 10-20 percent
during the afternoon and early evening hours. 20 foot winds will
rise to north 10-20 mph in several locations in the interior and
in the northern Cascades.

Later Thursday night into Friday winds will veer to easterly
between 2000 and 5000 feet and will increase. This will result in
Red Flag conditions developing Friday morning in the Olympics,
portions of the coastal zones, and in the central portions of the
Cascades. It will remain rather windy in the lowlands with
north to northeast flow and relative humidity values will once
again fall to 10-20 percent.

Fire weather watches are now in effect for most western Washington
zones Thursday through Friday night.

Thermal low pressure will remain over the area on Saturday and
temperatures will remain warm. Temperatures aloft will cool some
which will increase instability. Red flag conditions for
instability and dry air aloft may occur in the mountains for the
first part of the weekend.

Conditions will begin to moderate Saturday night and Sunday as
temperatures gradually cool and relative humidity increases.
Albrecht/Bower

&&

.SEW Watches/Warnings/Advisories...
WA...Fire Weather Watch from Thursday morning through Friday evening
     for Black Hills and Southwest Interior Lowlands-Central and
     South Puget Sound Lowlands-Northeast Puget Sound Lowlands
     Generally Below 1500 Feet-Southeast Puget Sound Lowlands
     Generally Below 1500 Feet-Strait of Juan De Fuca and
     Northwest Interior Lowlands-West Slopes of the North
     Cascades Generally above 1500 Feet.

     Excessive Heat Watch from Thursday afternoon through Saturday
     evening for Bellevue and Vicinity-Bremerton and Vicinity-
     East Puget Sound Lowlands-Everett and Vicinity-Hood Canal
     Area-Lower Chehalis Valley Area-Seattle and Vicinity-
     Southwest Interior-Tacoma Area.

     Fire Weather Watch from late Thursday night through Friday
     evening for Central Coastal Lowlands-East Portion of the
     Olympic Mountains-West Portion of the Olympic Mountains-
     West Slopes of the Central Cascades Generally above 1500
     Feet.

     Fire Weather Watch from Thursday morning through Friday evening
     for North Coastal Lowlands.

     Excessive Heat Watch from Thursday afternoon through Friday
     evening for Central Coast-North Coast.

PZ...Small Craft Advisory until 5 AM PDT Thursday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm.

&&

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