Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 211018
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
320 AM PDT SUN AUG 21 2016

.SYNOPSIS...Increasing low level onshore flow will spread a cooler
marine air mass across the Western Washington, bringing temperatures
back to normal ranges. An upper level trough will bring a little
more cooling this afternoon and tonight as it moves across the area.
Another upper level ridge will gradually build over the Pacific
Northwest this week with onshore flow turning more offshore by
around Wednesday. This may allow for a return to hot temperatures
by the weekend.

&&

.SHORT TERM...Cooler marine air continues to push inland this early
morning with most locations seeing temperatures in the 50s. Some
isolated locations...mostly in the Seattle Metro Area and the I5
corridor still clinging to temperatures in the lower to mid 60s but
fully expect current push to erode those temps down by sunrise.

The combination of this cooler marine air and low level cloudiness
pushing inland as well will get the usual daytime heating off to a
late start and will result in temperatures much cooler than those of
the past couple of days. Helping things out even further is the
advance of an upper level trough through southern portions of B.C.
This will maintain the cooling trend into Monday as well as pose a
limited risk for showers tonight and Monday...mainly confined to the
North Cascades. Unfortunately...the trough does not linger as model
progs show it moving east of the area by Monday night.

This will give way to yet another building ridge that will initiate
yet another warming trend. The first indication of this will be the
nearly 5 degree jump in high temperatures area-wide Tuesday. SMR


.LONG TERM.....That will be pretty much the going rate for the
following two days as well...Wednesday shaping up to be on average 5
degrees warmer than Tuesday and Thursday being 5 degrees warmer than
Wednesday. As one would expect with climbing temperatures...not
expecting much in the way of cloud cover for the middle of the week.
Also contributing to the rising temperatures...will start to see
surface flow shifting offshore Wednesday as another thermal pressure
trough looks to develop over the area and remain in place before
moving out of the area late Thursday.

Long range models start to become a bit divergent as the end of the
week approaches. The GFS is starting to indicate...much like the
current pattern...that the upper level ridge will be pushed out of
the area by a trough with a very similar track...thus resulting in
similar cooling and perhaps even better chances for precipitation
for next weekend. The ECMWF however really locks that ridge into
place. While it also has a similar system...the ridge keeps that
system more to the north and thus minimizing its impact...if
any...on the area. The Canadian...while echoing the ECMWF for the
most part...does allow the trough to dip into Western Washington
next Sunday. Opted to split the difference between the disparate
model solutions...going with cooler temperatures but not really
biting on to the prospect of any kind of precip just yet. Will be
interesting to see next few model runs to see if a consensus arises.
SMR

&&

.AVIATION...Westerly flow aloft over Western Washington early this
morning will increase through this evening, an upper trough moves
southeast into British Columbia. The flow will veer to northwest
later tonight as the trough moves into the Canadian Rockies. At
the surface, onshore flow will strengthen through this evening, with
an increasingly deep layer of marine moisture moving into the
Western Washington interior.

Marine stratus covers the Washington coast, and it had spread
through the Chehalis Gap to the extreme south end of Puget Sound
and the western part of the Strait of Juan de Fuca as of 2 am.
Conditions are IFR or low IFR where there is stratus. It will
continue to push inland during the next several hours, probably
covering most of the lowlands of the Puget Sound region and south
interior for several hours. Stratus should burn off to the coast
in the afternoon. Stratus will return tonight as the marine layer
deepens.

KSEA...Southerly wind 3-8 kt, rising to 8-14 kt midday and
probably shifting to northwest for a while this evening. Winds
should become southerly 6-12 kt again later tonight. Opposing
winds at KBFI and KSEA are possible at times late this afternoon
and evening. Marine stratus will probably reach KSEA early this morning,
with several hours of IFR or low-end MVFR conditions roughly 13Z-
18Z. McDonnal

&&

.MARINE...Onshore flow early this morning will become strong
this afternoon and tonight. Confidence is high that westerly
gales will occur in the central and eastern Strait of Juan de Fuca
this afternoon and night with 30 to 40 kt winds. A gale warning
remains in effect. Small craft advisory winds are likely tonight
over the waters of Admiralty Inlet and the northern inland waters
adjacent to the Strait, and small craft advisory northwest winds
over much of the coastal waters.

Onshore flow will ease Monday and Tuesday, and northerly flow will
prevail Wednesday and Thursday. McDonnal

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory until 3 AM PDT Monday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 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.

     Small Craft Advisory until noon PDT today for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Gale Warning from noon today to 3 AM PDT Monday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory from 2 PM this afternoon to 3 AM PDT Monday
     for Admiralty Inlet-Northern Inland Waters Including The
     San Juan Islands.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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