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Area forecast discussion 
National Weather Service Seattle Washington
325 PM PDT Monday may 25 2015

Synopsis...a surface ridge offshore will maintain moist low level
onshore flow across western Washington this week. Moderate onshore
flow will keep skies mostly cloudy through Tuesday...but weaker
onshore flow from middle week Onward along with higher pressure
building over the region will allow more afternoon sunshine and a
warming trend. The air mass over the Cascades will continue to
become slightly unstable late in the day for a chance of showers
near the crest.


Short term...satellite imagery shows the upper level low is still
hanging out just across the central Washington border over S central b.C.
Moisture continues to rotate around the low...but aside from the
area of rain that crossed the north interior this morning...nothing has
reached West Washington. All models except for the arw keep the north interior and
the rest of the West Washington lowlands dry tonight. A chance of showers will
continue for the Cascades tonight along with a slight chance of
evening thunderstorms over the far NE corner. However confidence is
only moderate that no showers will creep S over the north interior
lowlands overnight.

Weak onshore flow continues this afternoon despite surface pressure
gradients being slack early this afternoon. Onshore pressure
gradients will increase this evening bringing in a little more low
level moisture. GFS/NAM time-height sections show some thinning of
the marine layer Tuesday afternoon...but mean relative humidity values in the
marine layer do not lower enough to expect more than some afternoon
sunbreaks. Models show a little low level warming...enough to nudge
maximum temperatures up into the middle 60s.

The upper level low will move southeast over southeast Washington Tuesday afternoon then
cease to be a factor Wednesday as it continues moving east. An upper
level ridge offshore will build slowly inland behind the departing
low. The air mass over the Cascades will continue become unstable
each afternoon for a chance of showers. I will leave thunderstorms
out of the forecast for now since the chance is very low.

Onshore flow should be weak enough and westerly enough to allow
partial clearing Wednesday afternoon...and probably hopefully mostly
sunny skies Thursday afternoon. Continuing weak onshore flow will
still produce areas of morning low clouds. Warming from the upper
level ridge combined with more sunshine should allow maximum temperatures to
reach into the lower 70s on Thursday. 500 mb heights will approach
5760 meters over Puget Sound sound with 850 mb temperatures close to 14c. Kam

Long term...GFS/ECMWF/Canadian all bring another upper level low
down the b.C. Coast Wednesday through Friday morning...when per the
European model (ecmwf)/Canadian it will be centered over the central b.C. Coast. The
GFS does not bring the low as far S and is quicker in sending it
east over the weekend. None of the models show any precipitation reaching West
Washington...but it will probably prevent the upper level ridge over the
Pacific northwest from building any higher than 5760 meters. Onshore flow
should remain fairly weak for partly to mostly cloudy mornings and
afternoon sunshine. Maximum temperatures are expected to top out in the 70s
each day this weekend.

All three models agree on progressing the ridge quickly east Sunday
night...allowing an upper level trough to approach the region from
the SW on Monday. Kam


Aviation...a weak...closed upper low will move S-southeast through eastern Washington
from tonight through Tuesday...reaching NE Oregon late Tuesday night. This will
bring light north-northwest flow aloft over western Washington through Tuesday evening. Moderate
onshore flow at the surface will keep the air mass moist in the lower
levels...and somewhat moist aloft. Weakly stable air mass...except
unstable over the Cascades during the afternoon and early evening hours.

MVFR and VFR ceilings prevailing this evening...then lowering to mainly IFR
ceilings overnight and Tuesday morning. The trend of lifting ceilings on Tuesday
afternoon will follow closely to trends that are being observed today.
Significant scattering of clouds is not likely for the rest of
today...and it looks unlikely again on Tuesday. Patchy drizzle is
possible on Tuesday morning over The Lowlands but especially along the

Ksea...the deep marine layer will remain intact for the next 30
hours along with broken-overcast cloud coverage. VFR ceilings near 040 are
expected this evening...falling to below 020 tonight and Tuesday morning. Ceilings
lifting again on Tuesday to near 040. SW wind 6-10 knots expected to
persist near the terminal...with the Puget Sound sound convergence zone
getting about as far south as Elliott Bay between 03z and 06z this
evening. Haner


Marine...high pressure will remain offshore with lower pressure inland all
week long. Small Craft Advisory strength winds of 15 to 25 knots will
occur this evening and most evenings this week...simply as a
function of diurnal strengthening and weakening of onshore pressure
gradients. The lowest chance of small craft westerlies will be on
Thursday...when onshore flow should be weakest. Haner


Sew watches/warnings/advisories...
Pz...Small Craft Advisory central and east Strait.




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