Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 201559
Area Forecast Discussion
National Weather Service Seattle WA
900 AM PDT SAT AUG 20 2016
.SYNOPSIS...A strong upper level ridge over Western Washington will
bring one more day of hot weather before increasing onshore flow
spreads a cooler marine air mass across the area. An upper level
trough will bring a little more cooling Sunday afternoon and night
as it moves across the area. Another upper level ridge will
gradually build over the Pacific Northwest next week with weak
onshore flow turning more offshore around mid week, and possibly
paving the way for more hot temperatures late in the upcoming week.
.SHORT TERM...Models show a broad flat ridge over the Pacific
Northwest this morning. It appears that the ridge axis has moved
over E WA, so things are shaping up for a marine push this evening.
Second domino is the thermally induced surface trough, which is in
the process of moving over the W WA interior this morning. Third
domino is a shallow tongue of stratus that has moved up the S WA
coast over the Long Beach peninsula, indicating onshore flow and
cooling is starting to reach the coast. The OTH-SEA pressure
gradient has already gone onshore and AST-SEA is starting to trend
Temperatures across W WA have cooled off quite a bit this morning,
compared to temperatures at the same time yesterday, but they are
still within reach of the 80s to mid 90s highs forecast for this
The marine push for tonight is falling into place this morning.
Thus, the forecast for a shallow marine layer and marine stratus
spreading inland overnight and covering much of the lowlands Sunday
morning looks good. The 06z GFS shows a bit more stratus coverage
Sunday morning compared to earlier models and no longer burns the
stratus completely back to the coast in the afternoon. The latest
various MOS guidance, GFS, NAM, ECMWF, all show Sundays max temps 3-
5 degrees cooler than previous runs, so this change will be
implemented in the afternoon package.
An upper level low will move across southern B.C. Sunday night with
an upper level trough extending S over W WA. Guidance agrees that
the initial cloud band will be insignificant with moisture mainly
above 500 mb. The models still show some light precip over mainly
the N Cascades Sunday night. The best guess is that this is from
some lower level moisture between 850 mb and 700 mb. WNW winds aloft
across the Cascades may create some orographic lift, which could be
enough to wring out a little very light precip. Bottom line is that
it will be insignificant.
Onshore flow will be stronger Sunday night which will rapidly deepen
the marine layer over W WA. Stratus should just about cover all of W
WA Monday morning and will probably not start breaking up until the
afternoon. Even then the stratus may not clear out completely.
Temperatures on Monday will probably be about the same as Sunday. Kam
.LONG TERM...Previous discussion from the 315 AM AFD...The upper
level trough pushes east relatively quickly...although in its wake
Monday conditions look to remain cool. Heating starts to kick in
again starting on Tuesday as another ridge of high pressure builds
off the Pacific coast...resulting in a 5 degree jump in high temps
area-wide. While the warming takes to a slower trend after
that...the trend is still upward. Onshore flow will prove a
moderating force...but the development of another thermal pressure
trough will likely drive high temperatures into the 80s and near 90
by the Thursday to Friday time frame. SMR
.AVIATION...Upper level high pressure will weaken and flatten
today and tonight. Light northerly flow aloft this morning
becoming westerly this afternoon, then increasing on Sunday. The
air mass is dry and somewhat unstable today. Low-level marine
moisture and stratus will spread onto the coast this evening, then
reach the interior lowlands around sunrise on Sunday. Stratus will
burn back to the coast on Sunday afternoon.
Otherwise, monitoring wildfire smoke today. Biggest local source
is from the Godkin Fire deep in the interior of Olympic National
Park. Current visible satellite imagery shows an elevated smoke
layer heading southeast from Bremerton to Sea-Tac, so expect hazy
conditions through mid-afternoon as this smoke heads toward Mount
Rainier. Later today as upper flow becomes more westerly, the
experimental HRRR smoke forecast tool shows an elevated smoke
layer heading east from Sequim to Camano Island to near Glacier
Peak this afternoon, so expect reduced visibility aloft. Where the
plume intersects the Cascades around Darrington and Glacier Peak,
expect surface visibilities below 5SM in haze this afternoon.
KSEA...Clear skies and northerly surface winds through 08z
tonight. Elevated smoke layers from wildfires near the terminal
until early afternoon, when a turn from NW to W flow aloft will
push the smoke away. Visibility could briefly fall as low as 8SM
in haze late this morning, but not low enough to be reflected in
the TAF. On Sunday morning, marine stratus should just barely make
it to the terminal for a short time in the 14z-16z time frame
before burning off.Haner
.MARINE...Surface flow will turn onshore late this afternoon.
Strong onshore flow will develop tonight and continue through
Sunday night. The forecast for the central and eastern Strait for
this evening calls for 20 to 30 knot westerlies, but gales are
not out of the question. Gale force westerlies are likely in the
central and eastern Strait of Juan de Fuca on Sunday afternoon
and night, when the strength of onshore flow reaches its
maximum. In addition, small craft advisory northwest winds should
develop over the outer coastal waters this afternoon and continue
through Sunday. Weaker onshore flow will continue Monday, and the
flow will trend light northerly Tuesday and Wednesday. Haner
WA...Excessive Heat Warning until 9 PM PDT this evening for Bellevue
and Vicinity-Bremerton and Vicinity-East Puget Sound
Lowlands-Hood Canal Area-Lower Chehalis Valley Area-Seattle
and Vicinity-Southwest Interior-Tacoma Area.
Red Flag Warning until 10 PM PDT this evening for East Portion
of the Olympic Mountains-West Portion of the Olympic
Mountains-West Slopes of the Central Cascades Generally
above 1500 Feet-West Slopes of the North Cascades Generally
above 1500 Feet.
Heat Advisory until 9 PM PDT this evening for Everett and
Red Flag Warning from 8 AM this morning to 10 PM PDT this
evening for East Portion of North Cascades National
Park/Lake Chelan National Recreational Area.
PZ...Small Craft Advisory from noon today to midnight PDT Sunday
night for Coastal Waters From Cape Flattery To James Island
10 To 60 Nm-Coastal Waters From James Island To Point
Grenville 10 To 60 Nm-Coastal Waters From Point Grenville
To Cape Shoalwater 10 To 60 Nm.
Gale Watch from Sunday afternoon through late Sunday night for
Central U.S. Waters Strait Of Juan De Fuca-East Entrance
U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory from 6 PM this evening to noon PDT Sunday
for Central U.S. Waters Strait Of Juan De Fuca-East
Entrance U.S. Waters Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at