Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
902 am PDT Sat Aug 27 2016
Synopsis...a cool upper level trough moving across British Columbia
combined with strong low level onshore flow will end the recent hot
spell, bringing temperatures back to normal ranges this weekend. A
large upper level trough forming just offshore on Sunday will
continue this stretch of seasonable weather and probably bring a few
showers to the area in the coming week.
Short term...a strong SW marine push is underway this morning,
triggered by the upper level low moving across b.C. And the
associated dry front that is currently moving southeast across West Washington.
Stratus currently pushing northward through Puget Sound has made it
as far as Everett but will probably come to a halt within the next
couple of hours, as daytime heating starts to erode the stratus. The
stratus will likely burn off back to around Shelton early this
afternoon. Even though most of the stratus will burn off for a
mostly sunny afternoon, the cooler marine air mass flooding the area
will keep high temperatures today in the mid 60s to mid 70s, which
is near normal.
A broad upper level longwave trough will remain over the region
tonight with westerly flow aloft. Onshore flow will continue
deepening the marine layer, although the flow will be weakening with
time. At the same time on Sunday the small upper level shortwave
trough currently offshore near 54n/144w will be approaching
Vancouver Island. The models show a weak frontal system south of the
trough approaching West Washington with a weak area of warm advection ahead of
the cold front. Mid level clouds associated with the warm advection
will spread over West Washington Sunday, and combined with all the moisture in
the deep marine layer will make for a mostly cloudy day. High
temperatures will be about the same as today, perhaps a degree or
two cooler. Models agree that there could be some light rain over
the coast and Olympics Sunday afternoon, caused by the warm
The cold front from the frontal system will stall just offshore
Sunday night and Monday as a large upper level longwave trough
deepens farther offshore west of the b.C. Coast. This will keep the
bulk of the front's precipitation just west of the Washington coast. There
are small differences between the models in just how far offshore
the precipitation will be, so it will probably be necessary to nudge
the pops up a little along the coast. Interior locations should have
less cloud cover and may end up having a partly to mostly sunny day,
with slightly warmer temperatures. Kam
Long term...previous discussion from the 320 am afd...the mid range
guidance is similar in maintaining the trough of low pressure aloft
off the coast Tuesday with the system swinging onshore by Wednesday.
This may be our first decent threat of light showers. After that,
the guidance is more variable. Yet the next threat of showers looks
to be on Friday with another shortwave tracking onshore. With cloud
cover and low level onshore flow at times next week, temperatures
should be near or a bit below late August and early September
Aviation...an upper trough will brush western Washington today
with more zonal flow tonight and Sunday between systems. Moderate
west-northwest flow becoming light westerly tonight and Sunday.
Stable and moist in the lower levels with strong onshore flow.
Somewhat moist in the mid and upper levels.
IFR cigs along the coast and parts of the Strait with visibility
down to 2sm during the morning. MVFR cigs over the southwest
interior and Puget Sound lowlands. Cigs lifting to near 2000 feet by
midday with the stratus deck scattering out over the interior in the
early afternoon. Clouds along the coast will only lift briefly this
afternoon then settle this back to an IFR deck. Scattered to broken
mid level cloud deck 8-10k ft associated with upper level trough.
Ksea...southwest winds 12-18 kt easing to 10-14 kt this afternoon.
Cigs near 1200 ft should begin lifting to 2000 ft by 18-19z. Clouds
are still expected to scatter by 20-21z. The MVFR cloud deck around
1500 ft is expected to develop again after midnight. Dtm
Marine...strong onshore flow will weaken later tonight and Sunday.
A Small Craft Advisory is now in effect for Puget Sound today due to
to the strong southwesterly onshore winds. A Gale Warning is in
effect for the central and eastern Strait of Juan de Fuca through
tonight for westerlies 25 to 35 kt. Small Craft Advisory
northwesterly winds are expected over the central and northern
offshore waters through this afternoon. Outflow from the Strait will
also produce small craft winds over the souther parts of the
northern inland waters and the northern part of Admiralty Inlet
later today through this evening when gradients peak. A frontal
system will stall over the offshore waters on Monday then weaken and
move inland on Tuesday. Dtm
Pz...Gale Warning to 2 am Sunday for central U.S. Waters Strait of
Juan de Fuca-East Entrance U.S. Waters Strait of Juan de Fuca.
Small Craft Advisory until 2 PM today for the northern and central
outer coastal waters.
An illustrated version of this forecast discussion can be seen at