Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Seattle Washington
920 am PDT Tuesday Sep 2 2014
Corrected aviation and marine sections.
Synopsis...a frontal system will bring a little rain to western
Washington this afternoon and evening. High pressure will produce
sunshine and warmer weather for the remainder of the week.
Short term...a relatively vigorous and fast moving upper trough and
cold front will bring a little rain to the area later today. Rain
should reach the north coast late this morning and spread into the
interior during the afternoon. Showers will become mostly confined
to the Cascades and a Puget Sound sound convergence zone by early evening
as the system quickly moves inland. A few showers will linger over
the Cascades into Wednesday morning. Otherwise...Wednesday will be
dry with decreasing clouds as drier northerly flow aloft develops.
High pressure aloft will be in control Thursday and Friday with low
level offshore flow developing. This will give sunny weather with
highs warming 3-5 degrees each day. Highs in the 70s will be common
Wednesday and parts of the south sound should see highs near 80 by
Long term...previous discussion...high pressure aloft over the NE
Pacific will persist Friday and Saturday before possible changes in
the pattern. Thermally induced low pressure will expand northward along
the Washington coast during this period with dry offshore flow. Temperatures
will warm in this pattern...with low 80s from the central sound to
the SW interior.
500 mb heights fall Saturday night...then a weak trough arrives
Sunday. This should induce an onshore push bringing marine air
inland and resulting in moderating daytime temperatures. This system
will likely bring little if any precipitation so the main affect
will be the change onshore flow conditions. Highs will only reach
the 70s and possibly struggle out of the upper 60s near the coast.
Weak onshore flow but mostly dry conditions should prevail early
next week. The majority of models show a trough clipping the region
but at this time does not appear to be a rain producer. Mercer
Aviation...flow aloft today will remain westerly as an upper level
trough approaches the area from the north...then become
northwesterly late tonight as the trough moves into eastern Oregon.
The associated cold front will bring rain to the coast and north
interior this morning...then spreading showers into the interior
during the afternoon.
Ceilings over the area this morning remain generally MVFR over the
interior...and VFR for the coast. Expect conditions to improve over
the interior through midday as ceilings lift to VFR
levels...conversely increasing low and middle level clouds will
establish MVFR conditions for the coast through the same period.
Onshore flow will persist through Wednesday with increasing
southerly winds over the interior this afternoon and evening. Low
level flow from the east Strait through the north Puget Sound sound area
will become westerly to northwesterly this evening...with a possible
Puget Sound sound convergence zone forming over King County. The trailing
upper level trough will keep showers over the interior and Cascades
behind the front tonight...generally winding down after midnight.
Ksea...MVFR conditions will improve to VFR through midday. Light
southerly winds will become established by 18z and strengthen
through the afternoon to 8 to 10 knots. Expect showers in the vicinity
this afternoon and evening with a possible pscz forming over north
King County later this evening.
Marine...onshore flow will increase behind a cold front this
afternoon. Gale force winds are forecast through the central and
east Strait of Juan Delaware fuca tonight. Onshore flow will ease late
tonight. High pressure over the NE Pacific will maintain onshore flow
through Wednesday. Northerly flow will prevail Thursday and Friday as low pressure
forms along the coast. 33
Pz...Gale Warning central and east Strait of Juan Delaware fuca.
.Small Craft Advisory coastal waters...Western Strait...
northern inland waters...and Admiralty Inlet.
You can see an illustrated version of this forecast discussion at