Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
911 am PST Thursday Nov 26 2015
Synopsis...a strong upper ridge will bring dry weather to western
Washington through at least Monday...with cool sunny days and cold
clear nights. A weak frontal system may bring rain to the area
again on Tuesday or Wednesday.
Short term...a strong blocking upper high is currently centered
near the coast of b.C.. it will take all weekend for the high to
drift across southern b... the air mass over western Washington dry
and very stable all the way through the end of Thanksgiving
weekend. Hardly a cloud is expected...with just some patchy
freezing fog each morning in favored locations such as the SW
interior and the Snoqualmie valley.
The only concern through Sunday will be with stagnant conditions.
Very warm air aloft due to the strong upper ridge is leading to
strong low-level inversions...and they will persist through the
weekend. For today...there is still some air movement due to
moderate NE pressure gradients. However...gradients are on a
weakening trend...so air movement and smoke dispersion will be
much more limited from tonight through at least Sunday. The
poorest dispersion days will be Saturday and Sunday. An air
stagnation advisory GOES into effect this evening. Some of our
local clean air partners have already declared burn bans. Please
do your part to keep the air in your neighborhood clean. Haner
Long term...the trend of both the 12z GFS and 00z European model (ecmwf) is to
keep dry weather going for a bit longer. The 00z European model (ecmwf) now holds
the next chance of precipitation off until next Tuesday...and the 12z GFS now
holds off precipitation until next Tuesday night. Both models are pointing
toward a splittier pattern...with the next weak trough aiming
further to our south. Unless the European model (ecmwf) later this morning throws
US a surprise...the forecast update this afternoon will likely push off
the next chance of precipitation. Haner
Aviation...a strong upper level ridge centered offshore will keep
cool dry northeast flow aloft over western Washington through this
weekend. A surface ridge over southern British Columbia will
maintain dry north to northeast low level flow through Friday and
weak flow the rest of this weekend.
The air mass will remain dry and stable through the weekend with
minimal fog or stratus in the morning. Morning air temperatures will
be near or below freezing so isolated freezing fog is possible.
Ksea...clear skies. Northerly wind 4-7 knots through 02z then light
Marine...a strong surface ridge over southern British Columbia will
maintain moderate northeast flow today. The 06z Canadian lam model
indicates east winds will reach Small Craft Advisory strength 15-25 knots
at the West Entrance and adjacent coastal waters at times through
tonight...and possibly on Friday as well. A Small Craft Advisory is
in effect. Elsewhere wind should be less than 20 knots through the
weekend...although the lam indicates brief periods over the north
interior and central Strait where the east-NE winds could approach
advisory levels through tonight.
Around Monday a front will arrive and turn the winds southerly. Some
Small Craft Advisory strength winds are possible around that time.
The current period of King tides will have the highest tides this
year. Fortunately strong high pressure over the area and relatively
light winds appear likely to keep the tidal anomalies minimal. No
coastal flooding is forecast during the high tides. Kam
Hydrology...flooding is not expected for the next seven days.
Washington...air stagnation advisory from 9 PM this evening to 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
Pz...Small Craft Advisory until 4 PM PST Friday for coastal waters
from Cape Flattery to James Island out 10 nm-West Entrance
U.S. Waters Strait of Juan Delaware fuca.
You can see an illustrated version of this discussion at