Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Seattle Washington
400 am PDT Wednesday Apr 23 2014

Synopsis...showers will persist this morning before a warm front
begins to spread rain into western Washington from the south this
afternoon and tonight. A cold front will move inland Thursday
morning. An upper level trough digging into the Pacific northwest on
Friday will keep scattered showers in the forecast. A third frontal
system will bring another round of more organized rainfall to the
area Saturday afternoon into Sunday morning.


Short term...showers associated with the upper level short wave
trough that has been working through the Pacific northwest overnight
have tapered off somewhat over the last few hours. Expect the
showers to continue to decrease as the upper level trough lifts into
British Columbia this morning. The break will be shortlived as rain
will increase from the southwest during the afternoon as a warm
front moves into the area. The warm front will bring more organized
rainfall to the area...followed by the cold front Thursday morning.
Rain will transition to showers Thursday afternoon with moist and
somewhat unstable southwest flow aloft behind the front. Expect 1 to
2 inches of rainfall Wednesday evening through Thursday afternoon
over the Olympics and the central Washington Cascades near Mount
Rainier. The coast will see 0.5 to 0.75 inches and the interior up
to 0.5 inches over the same period. Decent southerly surface
gradients behind the front will produce breezy winds.

An upper trough will dig into the Pacific northwest on Friday
keeping a chance of showers over the region. The good news is
showers will be spotty and not organized with a decent amount of
sunshine in between showers.

Long term...the active weather pattern will continue into early
next week. The digging upper level over the West Coast will keep
showers across western Washington on Saturday. A frontal system will
move inland Saturday evening bringing another round of organized
rainfall. Onshore flow and zonal flow aloft on Sunday will keep
showers mainly over the coast and the mountains. Another frontal
system will affect the region Monday or Monday night with medium range
models hinting at an upper level ridge building over the region on
Tuesday for drier and warmer weather.


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

The exception remains 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 315 am...the North Fork of the stillaguamish near Arlington was
at 4.65 feet...or 2889 cfs. The North Fork of the stillaguamish river
near Oso was at 216.4 feet at 1841 cfs. The pool east of the slide was
282.5 feet at 315 am this morning.

A warm front will spread rain into the area tonight...followed by a
cold front Thursday morning. This system will bring around 1.00 inch
of rainfall to the area through Thursday afternoon. This is expected
to produce rises on the stillaguamish river beginning Wednesday
evening through Thursday...cresting Thursday evening around 10 PM at
just over 5 feet.


Aviation...a trough will move east of western Washington today with a break
between weather systems. Another stronger low pressure system will
affect the region tonight into Thursday. Moderate SW flow
aloft...becoming strong tonight. Moist and somewhat unstable
becoming stable tonight. Decreasing showers this morning...then
increasing rain late this afternoon and evening. A mix of VFR and
2-3k feet MVFR ceilings this morning...then VFR above 4k feet. Warm
frontal rain tonight should produce widespread MVFR ceilings.

Ksea...variable ceilings between MVFR and VFR due to showers through
16z...then mostly VFR. MVFR ceilings and visible should dominate tonight
with steady rain. S wind 4-8 knots...becoming SW 10-15 west/ gusts to 25
knots later tonight.


Marine...there will be a lull in winds today ahead of the next
stronger low pressure system. Southerly winds over the coastal waters will
vary in strength through the day topping 20 knots at times west/ swell
around 10 feet. Therefore...a Small Craft Advisory remain up for the
coast. Winds elsewhere below 20 knots today. A 990 mb low will track NE
outside the outer coastal waters into Vancouver Island tonight. The
track is close enough so that high end small craft winds over the
coast and West Entrance are likely...possibly reaching gale force at
times from this evening through Thursday morning. A gale watch
is in effect tonight and Thursday morning for the coast/West Entrance
but kept winds to 30 knots in the forecast for now given the
uncertainty. Small craft winds should develop over the remaining
waters tonight into Thursday...then ease.

A lull in winds expected Friday...then more widespread small craft
winds possible with the next system on Saturday.


Sew watches/warnings/advisories...
Washington...Flood Warning remains in effect for the North Fork of the
stillaguamish river east of the sunrise 530 landslide.

Pz...Small Craft Advisory coast through this afternoon.
Gale watch coast and West Entrance to the Strait of Juan Delaware
fuca from 5 PM PDT through Thursday morning.




For an illustrated version of the forecast discussion please see
www.Weather.Gov/Seattle/gafd/latest_webafd.Html (all lowercase)