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Area forecast discussion 
National Weather Service Seattle Washington
900 am PDT Friday Jul 31 2015

Synopsis...a strong upper level ridge bringing hot temperatures
will remain over the Pacific northwest through early next
week...when it will weaken and shift eastward. An upper level trough
will move over the area by the middle of next week. Temperatures
will moderate to near normal with a chance of a few showers
Wednesday through the end of next week.

&&

Short term...with a strong upper level ridge axis almost directly
overhead and 500 mb heights around 590 dm over western Washington...expect
temperatures today to be the warmest of the bunch...with a large swath of
highs in the 90s Seattle southward. Highs around the rest of the
area will also be well above normal. The surface flow is light
northerly and onshore.

On Saturday and Sunday the upper level heights will slowly fall and
the ridge will begin to weaken and move slowly eastward. Upper level
heights will drop a few dms each day. Temperatures will likely drop
a couple degrees each day as well...but still remain very warm. Lows
early Saturday and Sunday mornings in the Puget Sound sound area will
remain quite warm too...in the middle 60s. Surface flow will remain
light to moderate northerly and onshore through early Sunday.

Recent models have been indicating that a marine push will develop
Sunday evening...around the time that the heat advisory expires.
Gales are possible in the Strait and marine air will infiltrate the
interior lowlands. This will start a trend of moderating
temperatures. Jsmith

Long term...the ridge will weaken more and shift eastward over The
Rockies by Monday as an upper trough drops southeast toward the Pacific northwest.
Lower heights and onshore flow mean highs will struggle to reach the
low 80s around Puget Sound sound...with 70s across the north interior and
Strait. Highs at the coast may struggle to reach 70.

Models take the upper low southward off the Pacific northwest and southern b.C. Coasts on
Tuesday. Onshore flow will be stronger and high temperatures will
finally fall to the 70s over all The Lowlands...near or even
slightly below normal. The low could move close enough to the coast
to bring a few showers to the coast and mountains by Wednesday. By
Thursday and Friday there may be a better chance of showers if the
low moves closer to the area as some models indicate. Highs will be
near or even below normal toward the end of next week. Mercer/jsmith

&&

Aviation...a broad upper ridge will remain over the region
through tonight with light westerly flow aloft. The air mass will be
dry and stable.

There is a thin line of shallow stratus along the Washington coast
and in the western Strait of Juan Delaware fuca this morning...bringing
local IFR conditions. This should retreat off the coast by late
morning...then return tonight. Otherwise clear skies will
continue.

Ksea...north to northwest wind mostly 4-10 knots.

&&

Marine...persistent high pressure offshore and lower pressure east
of the Cascades will maintain light northwest pressure gradients
across the forecast area. Small Craft Advisory westerlies are likely
in the central and eastern Strait of Juan Delaware fuca this evening.
Onshore flow will increase this weekend...and gale force westerlies
are possible Saturday night and Sunday night.

&&

Fire weather...a critical period for fire weather continues over
all of western Washington...with very dry air through most of the
depth of the troposphere through Saturday afternoon. In
addition...elevated mountain locations are feeling the added effect
of middle-level instability...with both middle-level and high-level Haines
5 conditions continuing into Saturday evening. Modis imagery and
visible satellite imagery both indicate a pickup in the activity
level of ongoing fires in both olympic and north Cascades National
Parks since yesterday. Will keep the red flag warning going for
elevations above 2000 feet until Saturday evening considering both
the dryness of air mass and the effects of middle-level instability.
This is the kind of weather setup that helps small fires grow to
become large fires.Haner

&&

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

Red flag warning in effect until 10 PM Saturday night for the
Cascades and Olympics above 2000 feet.

Pz...Small Craft Advisory coastal waters except zone 150...central
Strait of Juan Delaware fuca...East Entrance Strait.

&&

$$

An illustrated version of this forecast discussion can be seen at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html

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