Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 242217
Area Forecast Discussion
National Weather Service Seattle WA
317 PM PDT SAT SEP 24 2016
.SYNOPSIS...A weak warm front will brush the northern portions of
Western Washington bringing clouds and a chance of light rain
tonight into Sunday morning. A strong ridge of high pressure
aloft will build into the area midday Sunday giving drying and
warming conditions. Monday will likely be the warmest day as the
upper ridge shifts into Eastern Washington. A weakening cold front
and onshore flow will bring clouds and a chance of light rain
Monday night and Tuesday morning. An upper level trough moving
out of the Gulf of Alaska later in the week will bring cooler
conditions and a threat of light rain or showers at times.
.SHORT TERM...A warm front extends east-southeast from a 1009 mb
low near 51N 135W this afternoon. Warm advection clouds and some
very light rain has spread into the northern half of Western
Washington and will continue through tonight. With the increase in
mid and high clouds across the northern portion of the state,
patchy morning fog on Sunday will be confined to favored
locations in the interior south of about Seattle.
Ridging is developing along the west coast of the US, so this
front and its rain chances will push north of the Canadian border
Sunday morning. With sunshine on Sunday and warming aloft, expect
temperatures to rise into the 70s across much of the area.
It still appears that Monday will be the warmest day across the
interior as the upper ridge shifts east and a frontal system
approaches the offshore waters. 850 mb temperatures peak on Monday
around 16C. High temperatures will reach well into the 70s on
Monday and the 78 record high at SeaTac airport could be
threatened. Expect some fog in favored inland valley locations
A rather strong marine push ahead of the incoming front Monday
evening will usher cooler marine air inland and provide an
increase in clouds. The front will be dying as it moves across the
area, but there will still be a slight chance of rain across much
of the area late Monday night into Tuesday morning. Drying is
expected starting midday Tuesday. Albrecht
.LONG TERM...Models generally agree that a long wave trough will
carve out over the eastern Pacific Wednesday through Thursday,
then will swing eastward across the area Friday through next
weekend. The general trend of the models has been to sharpen the
trough offshore and slow its forward progress somewhat. Hence,
Wednesday and Thursday should be mainly dry with some fog in
favored valleys during the late night and morning hours. Rain
chances will increase Thursday night through the weekend with the
incoming upper trough, and temperatures will fall to below normal
values for late September and early October. Albrecht
.AVIATION...High pres aloft will gradually strengthen over the
region through tonight. Strong NW flow aloft will weaken with
time. The surface flow will remain light.
Areas of MVFR CIGs are expected over the coast after 0600 UTC
Sunday, and possibly over the far northwest interior after 1200
UTC. Only mid and high level clouds are expected elsewhere. There
may be patchy MVFR VSBYs due to fog after 0900 utc from about KOLM
s and wwd.
KSEA...VFR. Winds will be light and vrbl.
A warm front will brush the outer WA coastal waters overnight as
it moves across the offshore waters. Meanwhile, a surface high
over western WA will keep the flow light through Sunday morning.
Light northerly flow will develop Sunday afternoon before turning
onshore or westerly on Monday.
The onshore flow will strengthen Monday night in response to the
passage of a cold front. This will lead to the possibility of
gale force westerlies over parts of the Strait of Juan de Fuca.
The onshore gradient will weaken on Tuesday.
You can see an illustrated version of this discussion at