Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
315 PM PDT Thursday Jul 2 2015
Synopsis...high pressure aloft will remain over the Pacific
northwest into early next week for sunny and hot weather.
Short term...the strong upper level ridge over the pacnw this
afternoon will weaken slightly Friday and Saturday as a large upper
level low moves across western Canada...brushing West Washington in the
process. The air mass over West Washington is hot with record or near record
high temperatures expected this afternoon in the central Puget Sound sound
metropolitan area...where highs will reach the lower to middle 90s.
500 mb heights will fall a little Friday and Saturday due to the
Canadian low brushing the area. This should provide a little cooling
so that maximum temperatures lower into the 80s to lower 90s Friday and then
mainly the 80s on Saturday.
With maximum temperatures still touching the lower 90s Friday...the heat
advisory for the Puget Sound sound metropolitan corridor...including Seattle
Bellevue Tacoma and the east Puget Sound sound lowlands has been extended
through 9 PM Friday evening. A Special Weather Statement will be
issued to supplement the heat advisory...discussing the heat through
the Holiday weekend.
Pressure gradients are currently flat or weakly offshore this
afternoon...but weak onshore flow is still expected to develop
tonight. At 2 PM there was a bank of stratus about 130 miles
offshore which the arw model brings to the coast sometime this
evening...then partway inland late tonight. The model is probably a
little fast...but even so...stratus is not expected to reach the
Puget Sound sound region. Low temperatures overnight in the advisory area
are still expected to only cool to the upper 50s and 60s. Outside
the advisory area there will be better cooling and more relief from
the heat with lows mainly in the 50s.
As the Canadian low departs to the east over central Canada on
Saturday...a surface ridge will move S over southern b.C. Saturday
afternoon and evening in the wake of the low. This surface ridge
will increase the northerly flow over West Washington Saturday evening...then
turn the low level flow to east-NE offshore Saturday night. The
offshore flow will push sundays maximum temperatures into the 80s to middle 90s
over much of West Washington including the coast...with potential record
temperatures for the date in several spots. The offshore flow will
weaken Sunday night. Kam
Long term...a somewhat broad weak upper level trough with northwest flow
aloft will be over the pacnw Monday through Thursday. Mostly sunny
weather will continue but the air mass will be a little cooler with
500 mb heights down to 5820 to 5840 meters. Onshore flow will remain
generally westerly and weak for limited stratus. Maximum temperatures on Monday
will be in the 80s to lower 90s...a step down from sundays hot
readings. Slightly more air mass cooling should drop the warmest
spots into the 80s Tuesday through Thursday. The GFS has some precipitation
brushing the far north Cascades Tuesday and Wednesday caused by weak
shortwave troughs moving across southern b.C. European model (ecmwf) shows about the
same on Tuesday...but there is still enough uncertainty to hold off
on showers for a little longer. Kam
Aviation...high pressure aloft through Friday. Westerly flow aloft
will remain light with light onshore flow at low levels. The air
mass will be stable and remain dry except for shallow surface-based
marine moisture near the coast.
Skies will be generally clear through tonight...except for areas of
late night and morning stratus along the coast.
Ksea...north wind 6-10 knots...becoming northwest midday today...then north-NE again
after sunset. Dtm
Marine...northwesterly gradients will remain strong over the outer
coastal waters through Saturday. The Small Craft Advisory remains in
effect. The flow over the inland waters will remain light or light
northerly. Westerly flow will develop in the central and eastern
Strait of Juan Delaware fuca the next couple of evenings and could reach
small craft strength the next. The flow will become more strongly
offshore by Saturday night and Sunday. Onshore flow resumes early
Washington...heat advisory for Seattle...Bellevue...Tacoma...and east Puget Sound
sound lowlands zones.
Pz...Small Craft Advisory outer coastal waters...central and
east Strait of Juan Delaware fuca.