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Area forecast discussion 
National Weather Service Seattle Washington
930 am PDT Thursday Oct 23 2014

Synopsis...an upper trough will maintain showery weather over
western Washington today. The air mass will be unstable enough for a
chance of thunderstorms. Showers will gradually decrease later
tonight. Friday will be mostly dry between weather systems. A warm
front will bring some rain back to the area Friday night and
Saturday. Low pressure will bring more rain Saturday night and
Sunday with windy conditions possible.

&&

Short term...a strong west-southwest 140-150 knots jet at 300 mb is aimed at northwest
Oregon and SW Washington today. This will keep the area in a
favored region of upper level lift. Widespread showers are seen over
the area this morning with some thunderstorms seen from around Vancouver British Columbia
SW to Forks and some more thunderstorm activity from the SW Olympics to off
the mouth of the Columbia River in the north Oregon coastal waters. The
air mass will remain unstable enough today to produce a chance of
thunderstorm activity through the area today.

A short wave trough extending from the main Gulf of Alaska low
centered near 53n 142w southwestward to 44n 165w is dropping
southeastward. This trough is expected to phase with some
subtropical moisture and energy from the central Pacific to cause
rapid development of a system around 40n 132w later Friday. Downstream
from this system models are consistent in forming weak short wave
ridging over the Pacific northwest and strengthening surface high
pressure over NE Washington. The end result for our area will be a decrease in
shower activity Thursday night and a rather dry day on Friday in low
level offshore flow...although there will be quite a bit of middle and
high clouds around.

The 12z nam12 and GFS both show the low becoming vertically stacked
as it reaches its maximum depth around the northwest Oregon offshore waters
Sat morning. This will increase the low level offshore flow over the
area...but with 700 mb flow turning to the S and 850 mb flow from
the east with a gradually lowering frontal inversion...may
result in some mountain wave activity near the foothills of the
Cascades. Some areas east of Interstate 405 in the central Puget Sound
sound area may get rather windy Friday night into early Saturday
morning. While the GFS shows 0.25 to 0.5 inches of rain Friday night or
early Sat as a strong warm front lifts north across the
area...precipitation amounts will likely get cut back near the
Cascades as a result of the gusty easterly winds.

The forecast for Saturday into Saturday evening will be tricky and
will depend on the track and depth of the offshore low as it moves
NE across the Pacific northwest. Models have generally come into a
consensus that the low will deepen to 977-990 mb off the northwest Oregon
offshore waters then slowly fill to 985-995 mb as it lifts NE to
a track from about Hoquiam to Seattle then into the north Cascades.
Kpdx-kbli pressure gradients range from 10 mb in the weaker and somewhat
less developed 12z GFS...to up to 15 mb in the 12z nam12 and 00z
Canadian solutions and 12-14 mb in the 00z European model (ecmwf). The GFS tends to
be a bit underdeveloped with these systems. This would hint that the
coast and the interior from Seattle southward could see some winds
touch the high wind criteria --35-40 mb with gusts to 50-60 miles per hour--
for a few hours late Sat afternoon or evening. Will likely issue a
Special Weather Statement early this afternoon after looking at the
wrfgfs...12z Canadian and European model (ecmwf) solutions. Albrecht

Long term...showers are expected on sun with onshore flow behind
the low that tracks across the area late Sat or Sat night and they
are expected to persist into Sun night. Monday will likely be a lull
between systems.

The GFS brings the next system containing the remnants of Ana into
the area Monday night and Tuesday. Most models rapidly fill the low
rapidly as it approaches so wind may not be an issue but it also
bares watching. It should bring rain back into the area but at this
point it looks to be a progressive system which will limit the flood
potential. Additional low pressure system may bring rain into the
area through middle week. Mercer/albrecht

&&

Hydrology...showers will persist today but additional rain amounts
will not be hydrologically significant. The Skokomish River will
remain above flood stage but gradually recede today...falling below
flood stage this evening. Refer to the latest flood statement
bulletin for the most up to date information.

Wet weather is expected to continue through the upcoming week. But
rainfall amounts are not expected to drive any rivers above flood
stage. Mercer/albrecht

&&

Aviation...air mass unstable over western Washington today and
tonight with southwesterly flow aloft.

Nothing really organized moving through the area today but satellite
imagery shows plenty of showers embedded in the southwesterly flow
aloft offshore extending back to near 160w. In general ceilings in
the 4000 to 6000 foot range lowering down to near 2000 feet with
visibilities 3-5sm briefly in the heavier showers. The air mass is
unstable enough for the possibility of isolated thunderstorms. The
chance for thunderstorms is not high enough to include in the tafs.

Ksea...ceilings in the 4000-6000 foot range dropping down to near
2000 feet in the heavier showers late this morning into the
afternoon hours. Felton

&&

Marine...
lower pressure over British Columbia and higher pressure over California
will result in southerly flow today. The flow will become easterly
on Friday due to a surface low approaching from the SW. This low will
deepen to 987 mb off the Oregon coast. Expect it to weaken to 990 mb
as it moves across the Washington coastal waters Saturday night.

The numerical weather models were still not in good agreement with
the track...timing...and strength of the low that is expected the
end of this week. Depending on which model is correct...this system
could produce anywhere from high end gale or low end storm force
winds over parts of the area or inconsequential wind speeds. The
forecast reflects the consensus.



&&

Sew watches/warnings/advisories...

Washington...Flood Warning in effect for the Skokomish River in Mason
County.

Pz...small craft advisories are in effect for much of the
coastal waters...central/eastern Strait of Juan Delaware
fuca...northern inland waters...Admiralty Inlet...and Puget Sound
sound.

Small Craft Advisory for rough bar until 8 PM PDT this
evening for the Grays Harbor bar.

$$

Www.Weather.Gov/Seattle

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www.Weather.Gov/Seattle/gafd/latest_webafd.Html

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