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Area forecast discussion 
National Weather Service Seattle Washington
330 am PDT sun Aug 2 2015

Synopsis...a strong upper ridge will persist today...then weaken
tonight and shift eastward on Monday. Onshore flow will gradually
return Monday and Tuesday with moderating temperatures through middle
week. Weak upper troughs will bring a chance of showers to the
mountains Wednesday and also toward next weekend.

&&

Short term...a strong upper ridge providing the recent stretch of
dry and hot weather will persist today...then begin to weaken and
shift eastward Monday. Surface gradients are nearly flat this
morning with a thermally induced low pressure axis extending from
the Willamette Valley in Oregon north through the SW interior of Washington
over the Olympics. Light or weak offshore surface flow will prevail
east of the low axis today with sea-breeze affects hindering potential
highs just a bit near the water. 500 mb heights drop about 20m from
yesterday but without a decent onshore push...temperatures are
likely to top out close to yesterday's highs. Temperatures were
nudged higher by just a degree or two closer to the European model (ecmwf) MOS which
has performed better than other guidance the past few days...and is
the warmest of the bunch for today. If sea-tac manages to hit 90
degrees today it will again tie the record for 5 consecutive 90+
degree days for the second time this Summer.

Models agree that the 500 mb ridge axis will finally shift east of our
area over the northern Great Basin/rockies by Monday. While upper heights
begin to fall tonight...most models show a gradual increase in
onshore flow over interior western Washington rather than a solid strong onshore
push. This may result in low stratus making a debut in the Strait
and areas near the East Entrance...and also through the Chehalis gap.
While some marine air filters further inland...it appears low clouds
may hold off over much of Puget Sound sound until a stronger onshore push
Monday night. Highs Monday will be several degrees cooler given the
partial introduction of marine air and lowering upper heights. Highs
will stay in the 70s near the coast and upper 70s to middle 80s across
the interior.

Current satellite imagery and lighting detection shows a line of
elevated convection just off the Oregon coast this morning embedded
in southerly flow aloft. This will need to be watched with some
uncertainty as to how much of this moisture will reach western Washington. The
current flow at 700 mb is more southwesterly across western Washington with light westerly
flow at 500 flow. While models insist some of this moisture reaches
the area tonight and Monday...the flow pattern is not all that
favorable for thunderstorms west of the crest. Regardless...the
GFS/ecwmf do still show some light spotty quantitative precipitation forecast along the Cascade
crest late tonight and Wednesday so thunderstorms will remain in the
forecast for now. An increase in middle/high level debris clouds are
more likely.

The approaching upper trough reaches the area Monday night and moves
across western Washington on Tuesday with decent height falls. A strong onshore
push will develop late Monday and continue Tuesday. This may be the
coolest day of the week with solid morning clouds...mostly burning
off in the afternoon. Highs will be closer to average in the 70s.

Long term...upper level low pressure over the Pacific northwest will begin to
shift eastward Tuesday night and Wednesday. Models have trended toward
much less moisture so there is little threat of precipitation over
the area. Left the showers in over the mountains but if any rain
does fall it will be quite light. Highs will again top out mostly in
the 70s.

Model solutions begin to vary more starting Thursday...although the
trend is toward weak ridging. This may be transitory but the rising
heights and weaker onshore flow should allow highs to reach back
into the low to middle 80s. This may carry over into Friday but another
trough may approach the region and the affects could begin to be
felt Friday or over the weekend. Given the uncertainty of the timing
and strength of the trough...showers will remain in the forecast for
the mountains. Highs will probably start to moderate by Sat
urday...falling back to the 70s or low 80s...which is near or
slightly above average. However...confidence in the forecast this
next weekend is low. Mercer

&&

Aviation...a broad upper level ridge will remain over the Pacific
northwest today with light SW flow aloft. The ridge is moving east and
on Monday will be replaced by a large upper level trough that will
be moving over central b.C.

The air mass over West Washington will remain dry and very warm again
today...with the density altitude higher than normal this afternoon.
Some high clouds will drift over the area today...remnants of
convection to the south. As of 3 am stratus had not yet formed over
West Washington although there is still time for patchy IFR stratus to form
along the coast and in the Strait. Otherwise good VFR conditions
will continue today.

Ksea...mostly clear skies today for good VFR conditions. Surface
winds will be NE 3-7 knots through 18z then shift northwest 5-10 knots this
afternoon. Kam

&&

Marine...an offshore surface ridge combined with lower pressure
over East Washington will maintain moderate to strong onshore flow through the
middle of this week. The GFS indicates the uil-bli pressure gradient
tonight will be about the same as last night...and the Canadian lam
model keeps winds in the 20-30 knots range. I will stick with a high
end Small Craft Advisory tonight although the chance of a brief gale at Race Rocks
can never be ruled out.

Borderline Small Craft Advisory northwest winds over the outer coastal waters will continue
through late tonight so the Small Craft Advisory was extended. Small Craft Advisory northwest winds also
developed over the inner coastal waters from James Island southward
last evening...and it looks like that will repeat again late this
afternoon through late tonight. A Small Craft Advisory has been issued for this.

Strong onshore flow is expected late Monday afternoon and evening
just ahead of a weak cold front approaching the coast. There is a
high probability of a gale in the central and east Strait. The GFS has
a 4.4 mb uil-bli pressure gradient at 00z...the Canadian lam model
shows gale force winds...and the U.S. Mesoscale models show solid
Small Craft Advisory winds.

The GFS shows pretty good onshore flow continuing past middle week so
at the very least Small Craft Advisory west winds will continue in the Strait...with a
chance for gale force winds at times. Kam

&&

Fire weather...updated...red flag warnings have expired.
However...it just means that weather will be less of an aggravating
factor in the fire environment. Background fire danger will still be
elevated due to dry fuels.

The new wrinkle in the fire weather forecast is the introduction of
isolated dry lightning to the Cascades late tonight through Monday
afternoon.
As the upper ridge axis moves east...southerly flow aloft will
develop and bring north the needed moisture and instability. Today's
models came around to showing the Cascades at the northwest edge of
a larger-scale lightning risk area. The first threat over the
Cascades will be from high-based nocturnal convection that could
bring isolated dry lightning to the Cascades from Mount Rainier on
south in the hours around sunrise on Monday. The lightning threat
will reach the north Cascades on Monday afternoon...then end on
Monday evening. A dry sub-cloud layer and the steady forward
movement of the storms is a recipe for sub-wetting rains with any
storms. Haner

&&

Sew watches/warnings/advisories...
Washington...heat advisory remains in effect until 8 PM this evening for parts
of the Puget Sound sound region and southwest interior.

Pz...Small Craft Advisory outer coastal waters and two southern
inner coastal waters zones...central Strait of Juan Delaware
fuca...East Entrance Strait.

&&

$$

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