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Area forecast discussion 
National Weather Service Seattle Washington
915 am PDT Thursday Sep 3 2015

Synopsis...an upper level low over southwest British Columbia will
move slowly south over western Washington today and tonight.
Scattered showers will continue in the cool unstable air mass along
with isolated mainly afternoon and evening thunderstorms. The upper
low will move south over Oregon on Friday allowing the air mass to
gradually dry and warm. A weak frontal system will bring a chance of
showers on Sunday. Additional weak systems will maintain an
unsettled weather pattern early next week.

&&

Short term...the cold upper level low has moved S over SW b.C. This
morning and is expected to continue moving S across West Washington...reaching
northwest Oregon Friday morning. The air mass with the low remains moist
and unstable. Water vapor imagery shows a small weak upper level
shortwave trough rotating around the southern quadrant of the
low...which has pushed an area of showers and isolated thunderstorms
over the coast at 8 am this morning. Models bring the shortwave
trough east across the West Washington interior late this morning. Scattered
showers and isolated thunderstorms will continue this
afternoon...although the NAM bufr sounding for ksea shows a little
weaker instability than yesterday.

Scattered showers will continue over West Washington this evening with the low
overhead...then gradually diminish over The Lowlands late tonight as
the low continues S. Drier north-NE flow aloft will develop over the
area Friday which should result in gradual clearing Friday
afternoon. Maximum temperatures will be about 10 degrees cooler than normal
with highs in the lower to middle 60s today and in the 60s on Friday.
Lows tonight and especially Friday night will be quite cool...40s to
lower 50s. With enough clearing Friday night it is possible that
some of the very coldest spots could reach the upper 30s.

The upper level low will be over east Oregon and headed east on Saturday
while an offshore upper level shortwave ridge moves southeast over West Washington
Saturday afternoon. Sunny weather is still expected Saturday with
maximum temperatures warming into the upper 60s and lower 70s. Kam

Long term...moderate northwest flow aloft will develop over the region
after Saturday. The GFS and European model (ecmwf) have been consistent the past
several runs in bringing a weak upper level shortwave trough and the
associated weakening cold front across West Washington on Sunday. This system
does not look that potent so probability of precipitation remain in the chance range for
now. Both models have a weak looking upper level shortwave ridge
moving over the area on Labor Day...however the European model (ecmwf) has a little
weaker ridge and tries to spread a little precipitation over the
area in the afternoon. Confidence is not particularly high for
Monday since the models over the past few days have been
inconsistent for early next week. Kam

&&

Aviation...the upper level low over the Pacific northwest will
continue to dig south along the Washington and Oregon coast. There
remains enough instability to support showers and
thunderstorms...though the it looks a little more limited than
yesterday. Abundant low level moisture keeping ceilings down around
MVFR levels through much of the interior while a line of showers and
thunderstorms moving onto the Washington coast has ceilings there solidly
IFR. Expect MVFR ceilings...with possible IFR conditions associated
with showers this morning. Conditions will generally improve through
the late morning to VFR...though showers...thunderstorm...and lower
ceilings and visibilities remain possible the afternoon. Light
westerly flow aloft will become northerly as the upper level low
works south. The air mass will remain moist and somewhat unstable
today.

Ksea...VFR ceilings this morning will gradually become MVFR. A band
of showers and thunderstorms along the coast at 15z will move into
the metropolitan area by 16/17z bringing showers...possibly lightning and
lower ceilings. Light southerly winds will strengthen somewhat this
afternoon to 8 to 10 knots before becoming northerly late in the
afternoon. Gusty winds are possible with thunderstorms today.

&&

Marine...very weak low level onshore flow today will become
offshore Friday morning as a surface low moves south through the
offshore and coastal waters. An upper level trough over the Pacific northwest
will continue to support scattered thunderstorms and showers over
area waters today...with gusty variable winds possible. Onshore
gradients and flow will strengthen Friday afternoon and
evening...with northerly winds over the coastal waters. High
pressure will build over the waters Friday into the weekend.

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...none.

&&

$$

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