Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 010528
Area Forecast Discussion
National Weather Service Seattle WA
928 PM PST Wed Nov 30 2016
.SYNOPSIS...Weak high pressure aloft will prevail through Thursday.
Expect a weak disturbance to bring an increased threat of showers
to the area on Thursday, especially over the coast. A cold or
occluded front will sweep across the region on Friday. Expect
strong westerly flow Friday night and Saturday. A colder air mass
will arrive Sunday, leading to a chance of lowland snow early
There were scattered showers over the area at this time. Most of
the activity was over the mountains and the coast (extending
offshore). Expect scattered showers to persist overnight.
Meanwhile, a weak upper level ridge of high pressure will prevail
over the region through Thursday night. A weak disturbance will
cause an upswing in the showers during the day Thursday. The
gridded forecast was updated to show likely PoPs over the coast.
Expect the showers to diminish somewhat Thursday evening.
A warm front will approach the area late Thursday, with precip
overspreading the much of the CWA by daybreak Friday. The cold or
occluded front will move across the region during the late Friday
morning through early Friday afternoon time frame. Anticipate
post-frontal showers Friday night; although, the activity will
likely become mostly confined to the Cascades and within a Puget
Sound Convergence Zone.
Strong zonal flow will prevail on Saturday. Showers will increase
during the day as a cold upper level trough of low pressure and
associated cold front approaches from the northwest.
.LONG TERM from the previous discussion...
A stronger front will move through the area over the weekend with
a frontal passage around daybreak Sunday. There will be strong
cold advection Sunday with westerly gales coast and Strait, maybe
similar to today. The snow level will fall to around sea level,
probably Sunday night. 850mb temps over the interior of Western
Washington should be around -6C Sunday afternoon and night which
is borderline for snow in onshore flow, but with the 500mb temp
around -37C there could be decent convection and that tends to
drop the snow level down to the deck. The 1000-850mb thickness
drops to 1290m over Seattle by 12z Monday so that suggests a good
chance for lowland snow showers if there is still moisture. At
some point Fraser outflow will start up and the switch from
onshore flow to NE flow can be interesting if the moisture hasn`t
dried up by then. The 12z UW WRFGFS had spotty light snowfall at
Flooding on areas rivers is not expected through Day 7.
.AVIATION...An upper ridge off the coast is forecast to build
through Thursday night and approach the Washington coast. NW flow
aloft over western Washington. At the surface, higher pressure
to build over the area through Thursday with decreasing low level
onshore flow. The air mass is moist and slightly unstable with
scattered showers decreasing through Thursday.
The moist air mass and easing onshore flow should maintain
primarily MVFR ceilings into Thursday. The instability will also
promote some breaks in cloud cover at times as well. Expect more
cloud cover breaks and ceilings rising to lower end VFR Thursday
KSEA...Mainly MVFR ceilings through Thursday morning though may
break to low end VFR on occasion. Ceilings should improve Thursday
afternoon to mainly lower end VFR. South wind easing to 5-10 kt.
.MARINE...Onshore flow has been decreasing this evening. Gale
warnings on coast and in the strait/north inland waters down to
small craft advisories. Coastal swells in the mid teens to slowly
subside through Thursday.
Higher pressure to build onshore over western Washington through
Thursday for lighter winds by then.
The next Pacific frontal system is due to arrive Friday. It may
bring coast winds close to gale force. The following frontal system
due Saturday night and Sunday may also bring gale force winds to the
coastal water region. These systems will build coastal swells back
up into the 15-20 ft range. Buehner
PZ...Small Craft Advisories are in effect for all waters except
the Puget Sound/Hood Canal.
Small Craft Advisory for rough bar in effect until 7 AM
Thursday for the Grays Harbor Bar.
An illustrated version of this forecast discussion can be seen at