Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Seattle Washington
855 am PDT sun Apr 20 2014 upper level ridge of high pressure aloft and surface
high pressure over western Washington will give mainly dry weather
to the area through tonight. However...a stationary front over the
coastal waters will bring a chance of precipitation to the coast
through tonight. There will be a chance of showers on Monday as the
air mass becomes unstable across the area. The offshore front will
move inland Monday night. Additional weather systems will give cool
and unsettled weather through next week.


Short term...a ridge of high pressure aloft is centered over
central Washington this morning. This ridge is amplifying as it
shifts eastward toward western Montana. The amplification of the
flow aloft is forcing a vertically stacked low near 50n 140w to lift
northward and is slowing the eastward progress of a frontal system
now nearing 130w. This front will stall and become north south
oriented over the coastal waters later today through Monday.

It will remain mainly cloudy over the olympic peninsula and along
the coast well as over the north Cascades. However
moisture has decreased somewhat in the interior lowlands. While some
clouds will bubble up with afternoon heating...expect a bit less
cloud cover in the interior today. Radar shows some isolated showers
over the coastal waters this morning...these will mainly remain
offshore...but may clip the coastal beaches at times. Temperatures
today will remain in the middle to upper 50s along the coast and will
rise to around 60 in the interior lowlands. A 20 foot westerly swell
continues to decay onto the coast this morning. The swell will
slowly subside through Monday and a high surf advisory remains along
the coast for today.

The front will remain stalled off the coast tonight. Conditions will
be mostly cloudy across the region...with some clearing at times
over the interior south of Seattle contributing to temperatures in
the south interior locally falling into the upper 30s. Most lows
will be in the lower to middle 40s.

While the front is expected to remain offshore on Monday...the
combination of some destabilization of the lower and middle levels
during the day combined with diffluent south-southwest flow aloft will allow
some showers to form...mainly over the Cascades and during the
afternoon hours. The shower activity will become more widespread
Monday night as the front and its associated upper trough finally
drags inland. Cool onshore flow with the upper trough on Tuesday
will contribute to the formation of convergence zone activity in the
interior. With the shower activity and cooling aloft...temperatures
will remain in the 50s on Tuesday.

Will update the grids and zones to tweak cloud cover and temperature
forecasts today through Monday. Albrecht

Long term...from the previous long term discussion...00z models
runs showing the same inconsistencies as the previous runs for
Wednesday. The GFS solutions continue with the idea of a dirty ridge
and some drying while the Canadian and European model (ecmwf) runs are wetter. Good
agreement in the models Wednesday night into Thursday with a cold
front moving through the area for periods of rain. There is general
agreement in the models for Thursday night through Saturday with an
upper level trough in the area. The GFS and the Canadian are deeper
with the trough which could result in some drying especially on
Saturday. The European model (ecmwf) is not as deep with the trough keeping showers
over the area. Current broadbrush forecast with chance of showers
and slightly below normal temperatures for the most part will
continue through this forecast package. Felton


Hydrology...river flooding is unlikely for the next 7 days.

The exception is the North Fork of the stillaguamish river east of
the sunrise 530 landslide near Oso where a Flood Warning remains in
effect until further notice. Flooding continues for this river as a
result of water pooled behind the debris dam that partially blocks
the river.

At 830 am...the North Fork of the stillaguamish near Arlington is at
6.0 feet...or 5082 cfs. The river will recede to around 4.5 feet late
Monday afternoon. The North Fork of the stillaguamish river near Oso
is at 217.4 feet or 2706 cfs. The river crested at 6 am and will
recede through late Monday. The slide pool east of the landslide is
at 284.6 feet. It crested around 6 am and will slowly recede through
Monday. Albrecht


Aviation...upper level high pressure will traverse the Pacific northwest
today. A low pressure trough will develop offshore tonight and reach
western Washington on Monday. Light to moderate S-SW flow aloft. Lower levels
will continue to dry through the day...although moisture aloft will
remain. Ceilings mostly VFR...though some isolated locations may see
ceilings down to 2k feet this morning. Widespread VFR expected by
afternoon as conditions dry.

Ksea...ceilings expected to remain above 10k feet through tonight with
some scattered lower level clouds possible for the remainder of the
morning. Southerly wind to 10 knots. Dtm/smr


Marine...high pressure exiting the coastal waters this
morning...moving inland through the afternoon afternoon. A front
will move over the coastal waters starting by middle afternoon and is
expected to dissipate as it pushes onshore this evening. South to
southeasterly small craft winds are expected over the coastal
waters/West Entrance to the Strait ahead of the front today.

Large swell of 17 to 20 feet will impact the coast today...then
slowly subside tonight. A high surf advisory remains in effect for
the coast through this afternoon. This is a razor clam dig weekend
for Twin Harbors...copalis and Moclips/mocrocks beaches. Extreme
caution should be exercised as waves will likely race further up the
beach than normal. Dtm/smr


Sew watches/warnings/advisories...
Washington...Flood Warning remains in effect for the North Fork of the
stillaguamish river east of the sunrise 530 landslide.
High surf advisory north and central coast through
this afternoon.

Pz...Small Craft Advisory coast and west Strait of Juan Delaware
fuca through this afternoon.
Small Craft Advisory for rough Grays Harbor bar through this