Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 271046
Area Forecast Discussion
National Weather Service Seattle WA
346 AM PDT WED JUL 27 2016
.SYNOPSIS...Upper level ridging and weak low level onshore flow
will remain in place over Western Washington through Friday. This
pattern will produce sunny days with temperatures well above
normal. Temperatures will cool over the weekend as an upper level
trough moves into the area. The cooler weather will continue into
next week with another trough moving into the area.
.SHORT TERM...Satellite imagery shows stratus along the
Coast...down the Strait and through the lower Chehalis valley into
the southern portion of the Hood Canal at 3 am/10z. For the
remainder of the area just a few high clouds. Temperatures at 3
am were in the mid 50s to mid 60s.
Warmer day in store for Western Washington versus Tuesday with
weaker low level onshore flow and warmer temperatures in the lower
levels of the atmosphere. Surface gradients at 3 am were flat
with KUIL-KBLI and KHQM-KSEA under a millibar and KOTH-KSEA
slightly negative. Sandpoint profiler shows the winds in the
lower levels...below 3000 feet...are very light. Given these
variables the stratus will have a hard time making it east of
Puget Sound this morning. The stratus will burn back to the coast
later this morning. Warmer temps aloft and weaker onshore flow
will lead to warmer high temperatures today with mid 70s to mid
80s common over the interior and lower 70s along the coast.
Another well above normal temperature day on Thursday with the
flat upper level ridge over the area and light surface gradients.
500 mb heights and 850 mb temperatures peaking on Thursday with
500 mb heights approaching 590 dms and model 850 mb temperatures
in the plus 16-19c range. Thursday looks to be the warmest day in
the short term with highs in the interior in the mid 70s to near
90 and highs on the coast in the mid 70s.
Friday a transition day across the area with the low level flow
turning onshore during the day. By 00z Saturday models have the
KOTH-KSEA gradient as high as plus 5 mb and the KHQM-KSEA gradient
up over plus 3 mb. Temperatures aloft cooling just a touch on
Friday. The coast will see the greatest cool down Friday with the
increasing onshore flow resulting in 5 or so degrees of cooling.
For the interior model guidance indicating up to 5 degrees of
cooling but with the biggest increase in the onshore flow during
the afternoon persistence is a good forecast for the interior with
highs near Friday`s readings...in the mid 70s to near 90.
.LONG TERM...Extended models in good agreement with a marine push
Friday night and an upper level trough moving over the area on
Saturday for a cooler day with morning cloud cover for the
interior and just some afternoon clearing on the Coast. Model
solutions differ on Sunday with the ECMWF kicking the trough to
the east while the GFS keeps the trough over Western Washington.
Both the ECMWF and GFS ensemble solutions are similar to the
operational run. Will stay with the current forecast that leans
toward the GFS solution which calls for cooler tempers and a
slight chance of showers. Models are pretty consistent for Monday
and MOnday night with a weak upper level low reaching the area
keeping the forecast on the cool side for early August. The ECMWF
once again is quicker to kick this feature versus the GFS. For now
will stay near climatology for the highs on Tuesday. Felton
.AVIATION...Areas of morning low clouds will be less extensive
this morning than they were Tuesday. There is light westerly flow
aloft and the air is stable west of the Cascades. There is a weak
disturbance approaching the area that might give some scattered mid
and high clouds today, but otherwise it will be sunny. At 3am the
fog/stratus satellite imagery shows the low clouds on the coast,
in the Strait of Juan de Fuca and over parts of Whidbey island,
and over a good portion of southwest WA west of I-5.
KSEA...Morning low clouds might not make it as far as Seattle this
morning, but they were into Shelton at 2am and it is likely that TIW
and PWT will see low clouds for a few hours.
.MARINE...High pressure is over the offshore waters with low
pressure inland. The small craft advisory for the outer coastal
waters was delayed til this afternoon, ship obs out there have only
shown 10-20kt winds so far. There should be small craft advisory
west winds in the Strait of Juan de Fuca later this afternoon and
evening, gales are likely again late in the week as an upper level
trough arrives and marine air pushes in.
PZ...Small Craft Advisory from 3 PM this afternoon to 3 AM PDT
Thursday for Central U.S. Waters Strait Of Juan De Fuca-
East Entrance U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory from 3 PM this afternoon to 11 PM PDT
Thursday 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.
You can see an illustrated version of this discussion at