Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 242130

Area Forecast Discussion
National Weather Service Seattle WA
230 PM PDT Tue Apr 24 2018

.SYNOPSIS...Upper level ridging over Washington will keep
conditions dry through Thursday. Surface high pressure over
southeastern British Columbia will give low level offshore flow.
Warm conditions, with near-record to record high temperatures,
are expected through Thursday. An upper level low will approach
from the southwest late in the week bringing a return to
seasonably cool temperatures and an increasing chance of showers
Friday into the weekend.


.SHORT TERM...Upper level ridging centered over Eastern Washington
is showing signs of flattening somewhat this afternoon through
tonight as a shortwave trough now located inside of 135W moves
over the top of the ridge. The trough will do little more than
bring cirrus to our area and to decrease the easterly flow
overnight. We did manage to hit record high temperatures today.
Temperatures tonight will hold up in the 40s to around 50 across
the area.

Behind the upper level trough, surface high pressure is expected
to strengthen over north-central British Columbia Wednesday
morning then move southeast toward southeastern British Columbia
Wednesday night and Thursday. Expect north to northeast flow to
increase around the Fraser River valley in Western Whatcom county
early Wednesday, then flow to become more northerly across the
interior of Western Washington during the day. The area northeast
of Bellingham and the lowlands around the west and south slopes of
the Olympics will likely see the warmest temperatures on Wednesday
with the benefit of downslope flow. Other locations in the
interior with northerlies coming off cool bodies of water will
likely be about the same as they are today, and perhaps a degree
or so higher. Record highs on Wednesday are low enough that many
will likely fall.

Offshore easterly flow will increase Wednesday night as the
surface high shifts in to southeastern BC. Then the upper level
ridge and surface high will begin shifting eastward away from the
area Thursday. The increased easterlies Wednesday night will
increase the strength of the thermal trough along the coast. The
thermal trough will then migrate eastward into the interior
during the afternoon hours and east of the Cascades Thursday
evening. Temperatures on Thursday will be much cooler than
Wednesday along the coast and through the coastal gaps, and highs
will occur earlier in the day. The interior will see widespread
temperatures in the upper 70s to mid 80s, and more high
temperature records will likely fall.

Pressure gradients will rapidly become southwesterly onshore late
Thursday afternoon or Thursday evening, and this will usher in
cooler and moist marine air from the Pacific. Clouds will spread
onto the coast during the afternoon hours on Thursday then spread
inland Thursday evening. By Friday morning, clouds will cover much
of the interior. While the onshore flow develops, the upper low
now seen well off the central California coast will move
northeastward into the far northern California offshore waters.
Showers will eventually form in the diffluent southeasterly flow
over Oregon and will likely spread into far southwestern
Washington later Thursday night then into southern portions of the
forecast area on Friday. Temperatures will be 15 degrees cooler
on Friday than on Thursday. Albrecht

.LONG TERM...Global model solutions, including the US GFS, have
all come around to the ECMWF solution of the past few days. This
solution is generally slower in moving precipitation northward
through Western Washington than the earlier GFS solutions and has
lighter precipitation amounts Friday through the upcoming weekend.
Highs on Saturday and Sunday will struggle to hit 60 degrees and
there will be a good chance of showers under cloudy skies and a
cold-feeling southwest breeze.

Shower chances will diminish early next week as high pressure
builds into the area from the west. By the middle of next week,
temperatures are expected to return to normal or perhaps a few
degrees above, and conditions will dry out.

The extended forecast represents a blend of the various model
solutions. Albrecht


.AVIATION...The air mass will remain dry and stable. There will
be some cirrus through tonight. Low level offshore flow will
persist and the thermally induced low pressure trough is centered
along the Oregon coast.

KSEA...Some cirrus with an afternoon and evening northerly breeze.


.MARINE...The thermally induced low pressure trough is centered
along the Oregon coast and there wont be any substantial changes
til Thursday. The thermally induced low pressure will move inland on
Thursday with onshore flow developing Thursday night and persisting
into the weekend.



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