Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 201105
Area Forecast Discussion
National Weather Service Seattle WA
405 AM PDT MON JUN 20 2016
.SYNOPSIS...A weak upper level low just offshore will bring a chance
of showers back into the area by tonight. Weak high pressure will
build over the region Tuesday. Another upper level low will move
through the area Thursday and Thursday night. A weak upper level
ridge will build over Western Washington for the weekend.
.SHORT TERM...Satellite imagery shows upper level low still
spinning around off the Washington coast early this morning. High
level clouds spread over the area Sunday evening and remain over
the area early this morning. The cloud cover has kept temperatures
in the 50s at 10z/3am.
Not much change in the pattern today with the low remaining
offshore. Air mass becoming unstable in the afternoon especially
over the mountains with lifted indexes as low as -3 and convective
temperatures in the lower 60s. With the mid level moisture present
will keep the idea of a chance of thunderstorms in the mountains
later this afternoon in the forecast. With the mid and high level
moisture over the area most of the day have trimmed another couple
of degrees off the high temperatures with mid 60s to mid 70s
Upper level low finally moves inland tonight for a chance of
showers over the entire area mainly in the evening hours. The low
will move into the Cascades late tonight. At the surface, low
level onshore flow pattern with the best chances for showers after
midnight in a convergence zone over Snohomish and possibly Skagit
county. Lows will most be in the lower to mid 50s.
Low well east of the area Tuesday morning with a weak upper level
ridge building into the area. Low level onshore flow continuing
with 500 mb heights increasing to the mid 570 dms by 00z
Wednesday. The low level onshore flow will keep high temperatures
near normal Tuesday...in the mid 60s to mid 70s.
Upper level ridge does not last long over the area. Upper level
low starts to dig south off the British Columbia coast. Weak
front associated with the low still offshore by 12z Wednesday.
Lows will again be in the mid 40s to mid 50s.
What is left of the front moves onto the coast Wednesday afternoon
for a chance of rain. For the interior just increasing clouds
during the day. Highs will once again be in the mid 60s to mid
.LONG TERM...Extended models in good agreement with the upper
level low approaching the coast Thursday afternoon. Low level
onshore flow and a destabilizing air mass will result in
increasing shower activity in the afternoon. With the cooling
aloft associated with the low and the afternoon precipitation
highs on Thursday will be cool for early summer...only in the 60s.
Upper level low moving through the area Thursday night into Friday
morning keeping showers in the forecast. Upper level ridge
building offshore Friday with drier northwesterly flow aloft
eventually taking over. Friday will be another cool day by first
week of summer standards with highs in the 60s and lower 70s.
General agreement in the models over the weekend with a ridge
building into Western Washington. The gfs and Canadian are more
aggressive with the ridge building with 500 mb heights rising into
the lower 580 dms. The ecmwf is a little weaker with the ridge
with 500 mb heights only in the mid 570 dms. All of the models are
dry. Will stay with current cool trend and go for highs closer
to the ecmwf guidance...70s for the most part...for Saturday and
.AVIATION...Southerly flow aloft over Western Washington early
this morning will continue through this evening, as an upper low
remains just offshore. The low will move inland tonight, and the
flow aloft will veer to westerly. The air mass will be fairly
moist above 10,000 ft today, and become unstable this afternoon
and evening especially over the mountains. The air mass will
stabilize later tonight, and the lower levels will become moist
as onshore flow develops.
Aside from the possibility of patchy stratus and fog along the
coast this morning, there should be no significant cloud cover
below 12000 ft. Increasing mid and high moisture and instability
should lead to cloud cover beginning late this morning or early
afternoon, something like SCT035 BKN090. Isolated thunderstorms
could develop over the mountains. Low level moisture should
increase tonight in onshore flow, with conditions deteriorating to
MVFR over most of the area by late tonight.
KSEA...North wind 6-12 kt, becoming northwest 5-10 kt around
midday, then backing to southwest 6-12 kt tonight. Isolated
thunderstorms could develop over the Olympics, Cascades, and south
interior of Western Washington late this afternoon and evening,
but they are not expected in the vicinity of the terminal. Marine
stratus should push inland to KSEA tonight, giving BKN-OVC012
beginning around 11Z. McDonnal
.MARINE...A thermal trough over the Western Oregon interior will
shift east of the Cascades this afternoon. As a result, the
northerly flow over Western Washington and its waters will give
way to onshore flow late today and tonight. Small craft advisory
winds are likely over the outer coastal waters, the central and
eastern Strait of Juan de Fuca, and the adjacent parts of
Admiralty Inter and the northern inland waters.
The gale watch that was in effect for the central and eastern
Strait has been changed to a small craft advisory with a forecast
of 20 to 30 kt, because the model trend has been for a slightly
weaker push and the Western Washington interior will not be
especially hot today. This doesn`t absolutely rule out the
possibility of marginal gales in the Strait, but it looks
Onshore flow will continue Tuesday and Tuesday night, and then
somewhat weaker onshore flow will prevail Wednesday through
PZ...Small Craft Advisory from noon today to 3 AM PDT Tuesday 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.
Small Craft Advisory from 4 PM this afternoon to 9 AM PDT
Tuesday 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 3 AM PDT Tuesday
for Northern Inland Waters Including The San Juan Islands.
Small Craft Advisory from 6 PM this evening to midnight PDT
tonight for Admiralty Inlet.
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