Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Seattle Washington
830 PM PDT Wednesday Apr 23 2014

Synopsis...a warm front will bring rain 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 night into Sunday morning.


Short term...rain has spread across the area from the southwest
this evening as a warm front moves through. A trailing cold front
will move inland Thursday morning with rain changing to showers. An
upper trough will settle over the region Friday. This will keep a
chance of showers in the forecast but there should also be sunshine
at times. Another frontal system will approach from the west on
Saturday...with rain reaching the coast in the afternoon and
spreading into the interior by evening. Schneider

Long term...previous discussion...moist westerly flow will keep a
chance of showers in the forecast on Sunday. Models show a weak
upper level disturbance tracking through western Washington for scattered
activity. Showers will decrease Sun night as a ridge start to build
over the Pacific northwest.

A weak warm front may clip western Washington on Monday for a chance
of rain. The models then strengthen the ridge over the Pacific northwest Tuesday
and Wednesday for warmer and drier weather. The forecast leans toward the
European model (ecmwf) solution which shows an 500 mb 580 ridge over Washington. The low level
flow is also turning offshore and we should see highs in the upper
60s to 70s if this pans out. 33


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 8 PM...the North Fork of the stillaguamish near Arlington was
at 4.6 feet...or 2810 cfs. The North Fork of the stillaguamish river
near Oso was at 216.4 feet at 1800 cfs. The pool east of the slide was
282.2 feet this evening.

A warm front will bring rain 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 this evening
through Thursday. The latest hydrograph shows the stillaguamish near
Arlington cresting late Thursday morning around 5 feet.


Avalanche...previous discussion...further heavy snow and then
potential rain should be seen in the Montana Rainier to Montana Hood area
Wednesday night and Thursday morning. This is very likely to cause
wet loose...storm slab and wind slab avalanche conditions in this
area. An avalanche warning will be issued for the Montana Rainier to Montana
Hood area for Wednesday night and Thursday morning. Back country
travel in avalanche terrain is this area is not recommended
Wednesday night and Thursday morning.

Significant snow and avalanche conditions are also expected in other
areas but that are slightly less than warning criteria. Avalanches
are also likely in these other areas as well.

Please visit nwac.US for details. Gkf


Aviation...a warm front will lift northward through western
Washington late tonight followed by a cold front late morning
Thursday. Upper flow will turn southwesterly. Widespread rain with
low MVFR and areas of IFR can be expected with the warm front. The
rain will turn to showers midday Thursday with ceilings lifting to
mostly VFR. Surface winds will become gusty from the south and
southwest Thursday morning as an associated surface low moves into
Vancouver Island.

Ksea...MVFR ceilings 2-3k feet overnight. Rain will reduce visibility
to 3sm at times tonight. Southeast wind 7 to 12 knots...shifting to
southerly after 09z. Winds will rise from the S-SW after
12z...gusting to 20-25 kts at times into Thursday afternoon.


Marine...winds over the coastal waters are starting to rise over
the past hour as a warm front edges closer. Gale warnings remain in
effect for the coastal waters overnight and Thursday...with small
craft advisories all other waters. Strong pressure rises will follow
a surface low as it lifts into Vancouver Island early Thursday. This
will lead to some gusty south or southwest winds over the inland
waters much of Thursday. Winds could briefly approach gale force in
parts of Puget Sound sound Thursday morning.

After a lull in the action Friday...another front will spread Small
Craft Advisory strength winds over the area Saturday night. 27


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...Gale Warning coastal waters.
Small Craft Advisory all other waters.
Small Craft Advisory for rough Grays Harbor bar.




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