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Area forecast discussion 
National Weather Service Seattle Washington
905 PM PDT Tuesday Jul 22 2014 upper level low pressure system is just offshore of
the Pacific northwest tonight. The low will move inland over
Washington late Wednesday. The low will move to southern Alberta
Thursday. An upper ridge of high pressure will build for the weekend.


Short upper low will bring increasing shower activity to
western Washington through Wednesday. Much of the western Washington lowlands will
receive more than a half inch rain. There will be isolated
thunderstorms as the air becomes increasingly unstable in fairly
strong southerly diffluent flow aloft. So far the thunderstorms have
been over Oregon and southeast Washington this evening...but the
instability should work north over Washington through Wednesday. The upper low
will move to southern Alberta on Thursday...putting western Washington in drying
northwest flow aloft with showers mainly in the Cascades then ending by
afternoon. Most of The Lowlands should be dry by Thursday afternoon
with partly sunny skies. An ridge will build north from The Four
Corners high for the end of the week...with dry warm weather and
sunshine for the weekend.

Long term...the upper ridge will continue to build with its axis
along the Rocky Mountains over the weekend. By Sunday...the 500 mb
heights over western Washington will be around 5850 meters...and
highs over much of the western Washington lowlands should be well
into the 80s. Onshore flow will be weak...and the night and morning
marine layer clouds should be limited mostly to the coast. The GFS
starts to weaken the ridge with the arrival of an upper trough along
the British Columbia coast around Tuesday...implying a marine push
for Tuesday night. The European model (ecmwf) looks at least a day later with that.


Climate...there have been 23 July days with 0.50 inches of rain or
more since records started at sea-tac in 1945. Only 3 of those days
have been since 2000...0.65 inches on July 16th 2001....0.60 inches
on July 20th 2012 and 0.58 inches on July 7th 2002. The record for
the wettest July day at sea-tac is 0.85 inches on July 13th 1981.
The wettest July day on record in Seattle is 0.98 inches on July 5th
1897 when records were kept at the federal building downtown. Felton


Aviation...upper level low offshore with southwesterly flow
aloft tonight. Flow aloft becoming southerly on Wednesday as the low
approaches the area. The low will move over western Washington
Wednesday evening. Air mass stable becoming somewhat unstable on

Short break in the cloud cover over the area this evening. 7000-8000
foot deck moving up from northwestern Oregon will be over the area
later tonight. Ceilings lowering down into the 1000-2000 foot range
Wednesday morning with precipitation increasing as the low
approaches the area. Conditions slowly improving Wednesday night.

Ksea...7000 foot deck arriving before midnight. Ceilings lowering
down to 1000-2000 feet by late Wednesday morning. Some improvement
Wednesday evening. Light winds through Wednesday morning. Felton


Marine...a weak 1015 mb surface low will move into the coastal
waters tonight and remain through Wednesday morning. This pattern
will create light pressure gradients over the waters through early
Wednesday. Pressure gradients increasing middle day Wednesday into
Wednesday night with Small Craft Advisory winds developing over most
of the waters Wednesday night or Thursday then easing Thursday
night. The onshore flow pattern will return Friday and into the
weekend with Small Craft Advisory westerlies in the central and
eastern Strait late in the day. Felton


Sew watches/warnings/advisories...



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