Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
905 am PST Tuesday Dec 1 2015
Synopsis...an active weather pattern will persist through the
weekend. This will begin with a frontal system that will bring
precipitation and gusty winds to the area this afternoon and
tonight. A second frontal system will follow Wednesday afternoon and
evening with a cold front on Thursday. A third...potentially wet
system...is expected to affect the area over the weekend.
Short term...showers continue to linger over the coast and
Olympics this morning ahead of an occluded front that is approaching
the area from the offshore waters. Klgx starting to pick up rain
ahead of the front...with the expectation that it will move onshore
later this morning and spread slowly inland through the afternoon
hours ahead of the front. Precipitation for this system looks
nominal...with the most significant impact being snowfall over the
Cascade mountains. Snow levels in the 2000 to 3000 foot snow range
coupled with cold easterly flow through the passes will help 6 to 11
inches of snow to fall over the Cascades from Snohomish County to
Lewis County through tonight. In addition...the lower passes...such
as Snoqualmie Pass...may see a winter mix of snow/sleet/freezing
rain early Wednesday as the snow level rises above the cold easterly
flow through the pass.
Continue to expect a brief break in the weather Wednesday morning
before stratiform precipitation pushes into the area with a warm
front in the afternoon. A trailing cold front will move onshore and
inland Thursday...bringing breezy to windy conditions as well as
locally heavy precipitation. Winds may reach advisory levels
especially over the coast and north interior. Precipitation will
transition to showers Thursday night with some potential for a Puget Sound
sound convergence zone in the Post frontal environment.
Long term...from previous discussion...a wet and active pattern
will continue over the Pacific northwest during this period due to a
parade of low pressure systems moving across the region. An
atmospheric river may set up over the area this weekend for the
potential of heavy rain on the southwest slopes of the olympic and
north Cascade ranges.
Aviation...the next in a series of frontal waves is just off the
coast and clouds will fill in...thicken up...and rain will develop
again this afternoon and evening. Southwest flow aloft. High
pressure over eastern Washington combined with lower pressure
offshore will give low level southeasterly flow.
Ksea...clouds and light rain will increase again later this
afternoon and evening as the next frontal system arrives.
Marine...the next front is just off the coast and will move through
western Washington this afternoon and evening. A lull between systems is
expected after midnight tonight until about noon on Wednesday...and
then another front will arrive. Another frontal wave on Thursday
looks like it could be the strongest. So far these first two fronts
look like they have had a tendency to split and I think they have
not been as strong as forecast while the systems Wednesday night and
Thursday look like they could be stronger than the current forecast.
The forecast will need to be updated a bit today after the uw wrfgfs
river flooding is not expected through Friday. There is a possibility
that a few rivers may flood this weekend with a wet Pacific system
that is expected to move onshore Saturday.
Washington...Winter Weather Advisory in effect from 4 PM this afternoon until
4 am Wednesday for Cascades of Snohomish County to Lewis
Pz...gale warnings are in effect until 4 am PST Wednesday for the
coastal waters...East Entrance to the Strait of Juan Delaware fuca...
and the northern inland waters.
Small Craft Advisory in effect until 4 am PST Wednesday for the
Admiralty Inlet and the central/western Strait of Juan Delaware fuca.
Small Craft Advisory in effect for rough bar until 4 am PST
Wednesday for the Grays Harbor bar.
An illustrated version of this forecast discussion can be seen at