Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 290352
Area Forecast Discussion
National Weather Service Seattle WA
852 PM PDT TUE JUN 28 2016
.SYNOPSIS...High pressure aloft will weaken on Thursday. Dry weather
will continue the next couple days with weak onshore flow resulting
in areas of late night and morning low clouds. A series of upper
level troughs will brush the Pacific Northwest Friday through
Independence Day. These systems will bring increased clouds, a
chance of a few showers, and cooler daytime temperatures. Drier
weather will develop Monday night and Tuesday.
.SHORT TERM...Low level onshore flow will prevail tonight with
marine stratus clouds pushing inland. The lowlands should be
cloudy by mid morning but the stratus will burn back to the coast
during the afternoon. A weak upper level trough will trigger
isolated showers and thunderstorms near and east of the crest in
the North Cascades Tuesday afternoon. Otherwise, the rest of the
region will remain dry. Temperatures will be close to normal. The
500 mb height field is nearly zonal as we move into Thursday but
heights remain high and in the 580s. There will be morning low
clouds again then scattering in the afternoon. The air mass is a
little drier in the mid levels for dry weather across the board.
Friday will be cloudier and slightly cooler as an upper level
trough moves in. There will be a few showers too, mainly on the
coast and in the Cascades. 33
.LONG TERM...Previous discussion...Most long range models are in
agreement in showing a series of upper level systems crossing
mainly through B.C. Saturday through Independence Day. None of the
models show a well defined front but rather moist onshore flow
with weak short waves inducing just enough lift at times for
spotty light showers. The convergence zone could be briefly active
at times and orographic lift will enhance shower activity along
the Cascades/foothills and Olympics. Precipitation amounts will be
quite light and generally spotty each day.
Models indicate an eastward shift in the trough pattern by
Independence day. Timing is highly uncertain, but if models are
correct, the chance of showers will decrease or end during the
afternoon. Low clouds will plague the lowlands but would likely
break up by evening in time for firework viewing. Highs will be in
the low to mid 70s. Tuesday looks to be dry with mostly clear skies
and slightly warmer temperatures as a ridge builds offshore. Mercer
.AVIATION...An upper level ridge will remain centered over Idaho
with southwest flow aloft over Washington tonight and Wednesday.
At the surface, onshore flow will continue. The air mass is dry
and stable except for marine stratus over the coast. Stratus will
move inland tonight bringing IFR or low end MVFR ceilings to most
terminals. The ceilings will lift late Wednesday morning and the
stratus will evaporate back to the by afternoon.
KSEA...Southwesterly wind 4-8 kt will become variable by 06Z.
Stratus with ceilings around 1k ft is expected late tonight. It will
burn off Wednesday morning similar to this morning. CHB.
.MARINE...Onshore flow will prevail through Friday with high
pressure offshore and lower pressure inland. Gale force westerlies
are occurring tonight in the strait with small craft advisory
strength winds in adjacent waters.
At least small craft advisory strength west winds will occur each
day in the central and eastern Strait with gale force westerlies
also possible. Gales are likely Wednesday night. CHB
PZ...Small Craft Advisory until 5 AM PDT Wednesday for Admiralty
Inlet-Coastal Waters From Point Grenville To Cape
Shoalwater Out 10 Nm-Northern Inland Waters Including The
San Juan Islands.
Small Craft Advisory until 5 PM PDT Wednesday 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 Wednesday afternoon through late Wednesday night
for Central U.S. Waters Strait Of Juan De Fuca-East
Entrance U.S. Waters Strait Of Juan De Fuca.
Gale Warning until 5 AM PDT Wednesday 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