Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 242201
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PDT Sat Jun 24 2017

.SYNOPSIS...High pressure aloft and low level offshore flow will
produce record breaking high temperatures for much of Western
Washington again Sunday. Then the high pressure aloft will move
inland with low level flow turning onshore late in the day and
cooler air reaching the coast while the interior will have its
warmest day. Onshore flow will strengthen Monday with temperatures
cooling back to near normal with night and morning marine clouds.
The pattern of morning clouds and afternoon sunshine with
around seasonal temperatures should prevail next week.

&&

.SHORT TERM...Thermally induced lower surface pressure just off the
coast and low level offshore flow produced the warmest day of the
year thus far today. The upper ridge was approaching the coastline
from the west this afternoon as well.

One more hot day ahead before nature`s air conditioning kicks in.
Progs continue to retain the surface thermal trough near the coast
through Sunday morning. This means the low level offshore flow will
continue overnight and it will be a warm night. The Seattle record
daily minimum of 62 in 2006 may fall as well as a few other sites.

As the upper ridge continues shifting inland Sunday and SW flow
aloft develops, the thermal trough to shift with it...first to the
interior Sunday PM and then east of the Cascades Sunday evening.
Winds along the coast should swing around to onshore midday Sunday
offering some cooling relief while the interior should have the
warmest day in this hot spell. More daily record highs likely to
fall in the interior. The heat advisory to remain in place.

Following the hot day in the interior Sunday, rising low level
onshore flow should surge cooler air inland through the night into
Monday morning. Time to keep an eye on the North Bend (OR) to
Seattle pressure gradient that has been easing today as well as
marine stratus along or near the south Oregon coast that should
slide up the Pacific NW coast through Sunday. Anticipate marine
clouds to surge up the Wash coast Sunday night and make it into much
of the western interior Monday morning. The marine clouds should
dissipate by afternoon in the interior, but the big story will be
the much cooler daytime temperatures - down about 20 degs or so from
Sunday.

As the upper ridge axis moves further inland, the upper low in the
Gulf of Alaska is expected to open up into a trough and move into
Western Canada Monday. This change will drop heights aloft over
Western Washington and also increase the onshore gradient with
supporting cold air advection. So Tuesday should have a solid
blanket of marine clouds in the morning and some areas breaking out
in the afternoon, along with cooler near seasonable late June
temperatures. In addition, need to keep an eye on the possibility of
high level convection for the Cascades around the Mt Rainier area
Monday. Right now, moisture is quite limited for any storm action.

.LONG TERM...Progs agree on some ridging aloft with rising heights
Wednesday and Thursday. This change should translate into some
thinning of the marine layer resulting in a late night/morning
clouds, afternoon sunshine regime. For Friday and Saturday, the
progs differ on retaining the building height trend vs some weak
troughing off the coast. Bottom line here is likely maintaining low
level onshore flow and the marine layer, with afternoon sunshine
particularly in the interior and a bit warmer than average
temperatures. Buehner

&&

.AVIATION...An upper level ridge will remain over the region thru
Sunday morning. NW flow aloft will become SW on Sunday. Contd low
level offshore flow. VFR conditions will prevail.

KSEA...VFR. Winds will be northerly 6-13 knots, strongest this
afternoon through early this evening.

&&

.MARINE...
A thermally induced trough of low pres just off the coast will shift
E of the Cascades late Sunday afternoon, causing the flow to become
onshore or westerly. The combination of strengthening onshore flow
along with a 20-degree difference between temperatures over the
coast and the interior Sunday evening will result in gale force
winds over parts of the Strait of Juan de Fuca.

Onshore flow will strengthen further on Monday for solid gales over
parts of the Strait of Juan de Fuca. Expect weaker onshore flow on
Tuesday.

&&

.FIRE WEATHER...Hot and dry conditions this weekend will continue to
be somewhat favorable for grass and brush fires over the lowlands.
On both Saturday and Sunday, minimum RH values will be below 20% in
the afternoons. Some recovery will occur each night. Moderate
northeast wind at times will mean these weather parameters approach
red flag thresholds each afternoon. However, fuel moisture is
generally too high to give much of a threat of explosive fire
development. In addition, snow still covers much of the terrain
above 4000 feet in the mountains. The exception is so-called "fine
fuels," grass and brush, in the lowlands, which has dried
considerably in recent days. Favored areas for brush fires, such as
along highways, will have some risk of fire this weekend. The
weather will return to a more normal pattern Monday, with morning
clouds and afternoon sun, and highs in the 70s to low 80s, for the
rest of the week. Burke

&&

.CLIMATE...From previous editions - The last time Seattle had back
to back 90 degree plus days in June was June 28th and 29th, 2008.
Since records started at Seattle-Tacoma airport in 1945 there have
only been 2 days with highs 95 degrees or more in June, June 30th
1995 and June 9th, 1955. Both days the high was 96 degrees. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Heat Advisory until 9 PM PDT Sunday for Bellevue and Vicinity-
     Bremerton and Vicinity-East Puget Sound Lowlands-Hood Canal
     Area-Seattle and Vicinity-Southwest Interior-Tacoma Area.

PZ...Small Craft Advisories are in effect for much of the coastal
 waters and the Puget Sound.

  Gale Watch in effect from Sunday evening through late Sunday
 night for the central and eastern Strait of Juan de Fuca.

&&

$$

www.weather.gov/seattle/

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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