Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
944 am PST sun Nov 29 2015
Synopsis...a strong upper high will move east across the Canadian
rockies and slowly start to loosen its grip on the weather in
western Washington. In the meantime...a stagnant air mass will
remain in place today...with areas of stubborn fog lingering over
The Lowlands. A weak cold front will reach the coast with some
light rain on Monday. Active and sometimes stormy southwest flow
aloft will become established starting Tuesday...continuing
through the rest of the week ahead.
Short term...a strong upper high over southeast b.C. This morning
continues to dominate our local weather. Warm air aloft is
responsible for strong low-level temperature inversions...trapping
the air beneath with a stagnant low-level air mass. Fog coverage
has been expanding each of the last 4 mornings...and this morning
is no exception. Dense fog is occurring in most places from
Olympia to Everett...including the Kitsap peninsula and Hood Canal
area. With so little mixing and anemic solar insolation...the fog
today will be particularly stubborn. Over the central Puget Sound sound
region...the 15z hrrr does not show visibility improving above 1
mile until about 22z...or 2 PM.
Fog will re-develop this evening...with development taking place
in similar locations to what is seen this morning. However...the
fog should be quite a bit quicker to dissipate on Monday...given
increasing southerly flow across the top of the boundary layer and
the dissipation of the days-old low-level temperature inversion.
With the exit of the upper ridge and a change to S/SW flow aloft
on Monday...an upper level impulse will finally be able to brush
western Washington on Monday. This feature will be weak...with only a few meager
hundredths of an inch of rain expected as a dissipating front
moves inland. Looks like the first drops of rain should reach the
coast around mid-day. If any rain reaches the interior before
dissipation...it would be in the 4-8 PM time frame.
Though tomorrow's weather will lack panache...the front will pave
the way for more active SW flow aloft to start aiming toward the
Pacific northwest with stronger fronts to come. Looking like a respectable
warm front will spread across western Washington on Tuesday PM...accompanied by
heavier rain and wind. As with many warm fronts...it will be
windiest in southeast-wind prone areas...such as the coast and
north interior. Could be strong enough to eventually necessitate a
Wind Advisory. Haner
Long term...from previous discussion...no major changes to the
previous thinking. A series of Pacific storms will keep this
period quite active. It looks like there will be windy conditions
on Wednesday...with speeds possibly reaching advisory criterion over the
coast and northwest interior.
Aviation...the strong upper level ridge over the Pacific northwest
will drift slowly east through tonight. A weak surface ridge over
southern British Columbia and eastern Washington will maintain weak
dry low level offshore flow today. A very strong inversion is in
place over western Washington making the air mass very stable.
A large area of fog and stratus is producing mainly LIFR conditions
over the greater Puget Sound sound region from around Olympia up through S
Whidbey Island. There is patchy stratus along the Strait of Juan Delaware
fuca. Due to the strong inversion the Puget Sound sound region fog will be
persistent and probably not dissipate until 21z-22z...and even then
there will be areas in the central sound that will not clear today.
Ksea...sea-tac is on the periphery of the big area of fog and stratus
centered over Puget Sound sound this morning. That increases the odds that
it will clear out...but that will not likely happen until 21z or so.
It could take a little later than that. Visibility will likely
remain 1/4sm or less until after 18z. Surface winds will be light.
Marine...a weak cold front will approach the region today and shift
winds to southerly. Small Craft Advisory strength winds ahead of the
front are likely over the coastal waters today. The front will move
inland on Monday and dissipate with borderline advisory level winds
possible over the north interior waters.
A stronger front will reach the area on Tuesday...with very Little
Break in the winds between the Monday front and tuesdays system.
Gales are likely on the coast and over the north interior waters. Gales
could start on the coast as early as Monday morning...and over the
interior waters as early as Monday evening.
The GFS European model (ecmwf) and Canadian models agree that the rest of the week
will be an active weather pattern with gales possible at times.
However confidence in the details like timing and strength of
each system is still low. Kam
Hydrology...there is a possibility of minor flooding on the
Skokomish River on Thursday. Rainfall amounts of 3 to 5 inches are
forecast for the skokomish basin during the 36-hour period ending
4 PM PST Thursday. Elsewhere...flooding is unlikely for the next seven
Washington...air stagnation advisory until noon PST Monday for Admiralty
Inlet area-Bellevue and vicinity-Bremerton and vicinity-
east Puget Sound sound lowlands-eastern Strait of Juan Delaware fuca-
Everett and vicinity-Hood Canal area-lower Chehalis valley
area-San Juan County-Seattle and vicinity-southwest
interior-Tacoma area-western Skagit County-western Strait
of Juan Delaware fuca-western Whatcom County.
Dense fog advisory until 10 am PST this morning for Bellevue and
vicinity-Bremerton and vicinity-Hood Canal area-Seattle and
vicinity-southwest interior-Tacoma area.
Pz...Small Craft Advisory coast and West Entrance.
Small Craft Advisory for rough Grays Harbor bar
gale watch coast...East Entrance...north inland waters and
You can see an illustrated version of this discussion at