Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
920 am PST Monday Nov 30 2015
Synopsis...the weak low pressure system that will brush the area
this afternoon and evening will help scour out the low levels and
bring a threat of light precipitation to the area this afternoon and
tonight. Active weather will return to the region as a series of
Pacific storms will impact the Pacific northwest beginning Tuesday
evening through the weekend.
Short term...the weak low pressure system lifting northeast over
the offshore waters has been helping lift fog this morning...and
will provide enough mixing today to help improve air quality today
such that the air stagnation advisory will be allowed to expire at
noon PST. In addition to mixing...clouds will increase today as the
system approaches from the southwest. Light precipitation will begin
to work onto the central Washington coast and into the southwest
interior this afternoon. Best bet for precipitation will be the
coast and the south interior...however cannot rule out spotty light
rain over the north interior late tonight. The threat of light
freezing rain this afternoon as the precipitation works inland looks
very minimal as mixing out ahead of the system should warm the low
levels enough that precipitation will fall as rain. Weak upper level
ridging will build over the area late tonight ahead of the next
system expected to begin to move into the area later Tuesday.
Precipitation with this system will begin to work onto the coast
Tuesday afternoon and through the interior Tuesday evening. This
system continues to look pretty benign...with the heaviest
precipitation over the mountains. Cold easterly flow through the
passes will continue through Thursday...helping maintain the threat
of snow through much of this period.
Long term...from previous discussion...the weather pattern will
remain active during this period. An atmospheric river may set up
over the area this weekend for the potential for flooding on a few
rivers. It also appeared that it will become windy...with advisory
criterion speeds possible over the coast and northwest interior on
Aviation...an approaching front will give southerly flow aloft
today followed by westerly flow aloft tonight. Until the front
arrives late Monday the dry stable air mass will continue along with
the strong surface based inversion. At 9am there were areas of fog
and low clouds over a good part of the inland marine areas and
Strait of Juan Delaware fuca and the satellite image shows eastern Washington full
of stratus and fog...and there is an area of middle level clouds with
the first front in a series arriving as well. The areas that
Do have low clouds or fog should see it lift and disperse in the
afternoon as the weather system moves into the area and southerly
Ksea...at 830am the Airport was surrounded by fog but the hill was
in the clear...with increasing flow today as the first front moves
into the area if any fog gets up into the Airport it should only
last for a short time this morning.
Marine...a windy week begins today as a front approaches the coast.
Southerly gales will commence today on the coast and continue at
times through Tuesday night. Gales will spread to the East Entrance
of the Strait and the northern inland waters by Tuesday when a
somewhat stronger front arrives. Winds will seldom fall below Small
Craft Advisory strength. Models agree that the rest of the week will
have an active weather pattern with gales possible at times. Details
and precise timing will vary among the model runs.
Hydrology...river flooding is not expected through Friday. Medium
range guidance has been hinting at some flood potential this
weekend...however confidence remains low with respect to timing and
impacts due to model differences. At this time the Skokomish River
remains the most likely to experience flooding...but cannot
completely rule out the threat on other rivers this early.
Washington...dense fog advisory remains in effect until 10 am this
morning for parts of the Puget Sound sound region.
Air stagnation advisory remains in effect until noon today for
the interior lowlands.
Pz...Gale Warning in effect for the coastal waters...East Entrance
to the Strait of Juan Delaware fuca...and northern inland waters.
Small craft advisories are in effect for the western/central
Strait of Juan Delaware fuca and the Admiralty Inlet.
Small Craft Advisory for rough bar until 4 am Tuesday for the
Grays Harbor bar.
An illustrated version of this forecast discussion can be seen at