Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Seattle Washington
930 PM PDT Tuesday Apr 15 2014

Synopsis...a warm front will move over the area tonight and then
stall over the region on Wednesday for rain at times. A cold front
will cross western Washington on Thursday. Drier weather is forecast
on Friday with weak high pressure overhead. The next front will
arrive later Saturday.


Short term...satellite imagery shows diffuse warm front just off
the coast this evening. Its been raining along the entire coast
since about 00z with a few observations in the interior starting to
report light rain at 04z. With the cloud cover temperatures are in a
narrow range...middle 40s to lower 50s.

No changes to the forecast of rain at times through Thursday this
evening with satellite imagery showing a long fetch of moisture off
the coast streaming towards western Washington in the westerly flow
aloft. Flow aloft turns a bit northwesterly right over western
Washington tonight into Wednesday which could rain shadow the
central sound for a little while but eventually by Wednesday morning
the moisture will fill in here too as well. Rainfall amounts are
picking up along the coast with both kuil and khqm reporting over a
tenth of an inch of rain in the last hour. These rainfall rates will
make their way inland for a few hours Wednesday morning.

Rain continuing at times on Wednesday with the models indicating the
best chance for a short break Wednesday afternoon into Wednesday
evening for the interior before the rainfall rates pick up again
with the approach of the cold front Wednesday night into Thursday
morning. 00z model runs are a little slower with the cold frontal
passage pushing the timing back into Thursday evening. Will wait for
another model run before making any changes to the forecast.

Models are still indicating cumulative rains over the next 48 hours
to be in the 0.75 to 1.50 inch range over the interior lowlands...
1.5 to 2.5 inches at the coast...and 1.5 to 3.0 inches for the
mountains. Felton

Long term...previous discussion follows. A frontal system will
reach the area later in the day Saturday with rain changing to
showers Saturday night. Sunday will probably be a mainly dry day in
between weather systems. A fairly weak front is forecast to bring a
little more rain Monday. After that...models diverge significantly.
The GFS build an upper ridge over the area on Tuesday for a dry day.
The European model (ecmwf) drops a cold upper trough overhead with a least some
showers. Schneider


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.

The North Fork of the stillaguamish near Arlington and the pool
upstream of the sunrise 530 slide are forecast to rise about a half foot
tonight...another foot or so on Wednesday...and then about two feet
on Thursday. The river and pool levels will begin falling later
Thursday night and continue falling Friday as high pressure moves
over the area.

Total rainfall for the slide area tonight through Thursday will be
on the order of two inches. A quarter to half inch is expected
tonight...another three quarters of an inch or so Wednesday...and
then about an inch on Thursday.

Additional fronts will affect the slide area later Saturday and
Monday. These systems are not expected to be especially wet or
hydrologically significant but they will keep conditions wet.


Aviation...moderate northwest flow aloft over the area through Wednesday
with upper level high pressure offshore. A warm front will lift
across the waters tonight into Wednesday morning. Moist at all
levels and stable. Rain at times.

MVFR conditions along the coast through Wednesday. VFR ceilings 3-5k feet
across the interior will deteriorate to mostly MVFR ceilings after
midnight. Visibility occasionally 3-5sm in steadier moderate rain.
After 15z Wednesday some ceilings will fall below 2k feet.

Ksea...southerly wind 8-12 knots tonight and Wednesday. Rain tonight
with VFR ceilings lowering to MVFR after about midnight. Rain at times
through Wednesday though some rain shadowing may reduce
precipitation Wednesday afternoon/evening.


Marine...a warm front will slowly lift across the waters tonight
and Wednesday morning. Periods of small craft winds expected over
the coastal waters and West Entrance to the Strait through Wednesday
morning. S/SW winds over the inland waters will push 20 knots at times
but should remain just below small craft. Will need to continue
monitoring observation.

High pressure and lighter flow on Wednesday afternoon. A stronger front
will bring Small Craft Advisory winds to most of the waters on
Thursday. Gale force westerlies are possible in the Strait Thursday
afternoon and evening but confidence is low. Winds ease Friday as
high pressure builds over the area. Dtm/chb


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 and West Entrance.




You can see an illustrated version of the forecast discussion at