Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 271021
Area Forecast Discussion
National Weather Service Seattle WA
320 AM PDT SAT AUG 27 2016
.SYNOPSIS...A big change in the weather pattern will unfold
today. Low level onshore flow into western Washington will bring
much cooler air inland this weekend. Developing lower pressure aloft
off the coast next week may circulate some light showers onshore at
The big weather pattern change is underway. The offshore upper level
ridge has shifted west to near 160W with NW flow aloft across the
Gulf of Alaska. The first shortwave was tracking SE in the flow
through British Columbia early this morning with the following one
moving across the northern gulf.
At the surface, low level onshore flow was picking up through the
night. Marine stratus had blanketed the coast and was surging toward
the interior. Expect it spread into much of the western Washington
interior through the morning with some areas breaking out offering
some sunshine during the afternoon. The shortwave passing just to
the north will likely spread some mid level cloudiness through the
area during the day as well.
Temperatures today will be much cooler - as much as 15 to 20 degrees
cooler in the interior. Some weak ridging aloft ahead of the next
shortwave Sunday will likely ease the onshore flow resulting in a
morning cloud and some afternoon clearing regime with daytime temps
nudging up a bit from today.
The latest guidance suggests lower pressure aloft digging off the
coast Monday with a more southerly flow aloft pattern over the area.
The trend has been to keep any precipitation threat off or just near
the coast through Monday. So have cut back on the shower threat
through Monday. Buehner
The mid range guidance is similar in maintaining the trough of low
pressure aloft off the coast Tuesday with the system swinging
onshore by Wednesday. This may be our first decent threat of light
showers. After that, the guidance is more variable. Yet the next
threat of showers looks to be on Friday with another shortwave
tracking onshore. With cloud cover and low level onshore flow at
times next week, temperatures should be near or a bit below late
August and early September readings. Buehner
.AVIATION...Northwesterly flow aloft through tonight with an upper
level trough moving through British Columbia today. Low level
onshore flow continuing into Sunday.
Stratus along the coast and moving though the lower Chehalis valley
at 10z. surface gradients onshore with KSEA-KOTH near plus 6.5 mb
and KHQM-KSEA approaching plus 3 mb. With the strong onshore flow
expect the stratus to keep surging inland reaching the puget sound
area in the 14-15z time frame. Ceilings over the interior with the
onset of the stratus near 1000 feet. Ceilings lifting to near 2000
feet later this morning with the stratus deck scattering out over
the interior in the early afternoon. Mid level cloud deck associated
with upper level trough to the north moving across the area later
this morning into this afternoon.
KSEA...With the onshore gradients will stay with the idea of a late
arrival of the stratus at Sea-Tac this morning...14z-15z. Ceilings
with the stratus will be near 1000 feet initially with ceilings
improving late morning to near 2000 feet. conditions improving with
the cloud deck scattering out in the early afternoon leaving just a
mid level deck. Low level convergence tonight could produce a 3500-
4500 foot cloud deck at the terminal late in the day. Southerly
winds 8-12 kt. Felton
.MARINE...Onshore flow increasing today. A gale warning is in
effect for the central and eastern Strait of Juan De Fuca through
tonight for westerlies 25 to 35 kt. The westerlies will reach gale
force speeds later this morning. Small craft advisory northwesterly
winds are expected over the Central and Northern offshore waters.
Onshore gradients will ease late tonight into Sunday. A frontal
system will stall over the offshore waters on Monday then weaken and
move inland on Tuesday. Felton
PZ...Gale warning to 2 am Sunday for Central U.S. Waters Strait Of
Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.
Small craft advisory until 2 pm today for the Northern and Central
outer coastal waters.
An illustrated version of this forecast discussion can be seen at