Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 am PDT Friday Mar 27 2015
Synopsis...a front will bring rain to the coast late this afternoon
and to the interior by this evening. A weak upper ridge will steer
weather systems mostly into British Columbia over the weekend. A
cold front will reach western Washington late Monday. A cool showery
upper trough will be over the area Tuesday and Wednesday.
Short term...visible satellite imagery and observation show some fog around
The Lowlands with mainly light flow at the surface. Gradients are
turning positive ahead of a front offshore so fog will mix out but
some middle/high clouds will be around. High temperatures today should
top out a few degrees cooler than yesterday...mainly middle 60s.
Current infrared satellite shows cooling cloud tops near a developing
surface low feature tracking toward southern b.C. This morning. This is
where the rain will be focused today ahead of the main front.
Models are in good agreement that rain associated with the cold
front will reach the Washington coast by late this afternoon....then spread
inland during the evening. The upper low is expected to close off
and dig southeast across western Washington tonight ushering in cooler air aloft. Snow
levels around 7000 feet ahead of the front should fall to 4500 to
5000 feet behind the front after midnight. Higher passes like
Stevens could get an inch or two and possibly 2-5 inches at spots
like Mount Baker above pass level through Saturday morning. Will
make some minor adjustments to the mountain zones for
It will become breezy behind the front later tonight and Saturday
morning. Surface ridging along the coast will become conducive for a
Puget Sound sound convergence zone to form...mainly north of Seattle. There
will be some showers elsewhere with rain or snow at times in the
mountains. It will be cooler with highs in the upper 50s.
The next system will arrive Saturday night and Sunday. Models show
the bulk of moisture hanging up over southern b.C. And extreme northwest
Washington...but there is a chance that some light rain could extend further
S at times. Mercer
Long term...from previous discussion...a front will reach the area
late Monday or Monday night for a period of rain. Cooler showery
weather will prevail Tuesday and Wednesday with a broad upper trough
over the Pacific northwest. Models suggest drier weather is possible
Thursday as an upper ridge moves over the region. Schneider
Hydrology...flooding is unlikely the next 7 days.
Aviation...patches of fog should lift and mix away this morning as
the low level flow turns southerly and picks up ahead of a front
just offshore. The front will move onto the coast during the
afternoon and inland this evening. Rain will spread inland with the
front and turn to showers late tonight.
Ksea...fog made it into sea tac but should lift out by 18z a southerly
flow starts to increase and mix the air mass. Clouds will increase
pretty rapidly this afternoon with rain by evening and perhaps a
fairly breezy SW wind tonight with the front.
Marine...a cold front will move into the coastal waters this
afternoon and to the inland waters this evening. Small craft
advisories are up for the winds expected with this front. High
pressure will build over the region on Saturday with a break in the
winds. An approaching frontal boundary will bring increasing
southerlies to the coastal waters Saturday night and Sunday.
Pz...Small Craft Advisory central and northern coastal
waters...central Strait...east Strait...Puget Sound sound/Hood
Canal...and northern inland waters.
You can see an illustrated version of this forecast discussion at