Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Seattle Washington
830 PM PDT Thursday Apr 24 2014

Synopsis...isolated to scattered showers will continue tonight and
Friday with an upper trough over the area. A frontal system will
bring another round of rainy weather Saturday night into Sunday. An
upper ridge will bring warmer and drier weather Tuesday onwards.


Short term...showers have begun to taper this evening with the loss
of daytime heating. Isolated showers will continue tonight and
Friday morning...then there should be enough instability to cause an
uptick in shower activity again Friday afternoon with an upper
trough overhead. Once again...showers should decrease diurnally
Friday evening...and pretty much come to an end after midnight as
very weak high pressure aloft builds over the area and the air mass
becomes more stable. Models remain consistent showing the next
frontal system spreading rain to the coast Saturday afternoon and
into the interior Saturday evening. Rain will change to showers
Sunday morning as the front moves inland. Sunday will be another
cool and showery day with moist onshore flow behind the front.

Long term...previous discussion...a weak warm front may clip
western Washington on Monday for a chance of rain. Confidence is low
and any precipitation looks light at this point. Models then build a ridge
over the Pacific northwest Tuesday and Wednesday for a dry and warm period of weather.
The low level flow will flip offshore with highs in the 60s to
70s...along with mostly sunny skies. The current forecast is nudged
toward the European model (ecmwf) and Canadian models which both keep high pressure over
the region through next weekend. The GFS breaks down the ridge on
Thursday but this solution looks like an outlier for now. 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
5.7 feet...or 4560 cfs. The North Fork of the stillaguamish river near
Oso was at 217.5 feet at 2880 cfs. The pool east of the slide was
283.1 feet this evening. The river has crested from heavy rains last
night and will continue to recede.

Expect isolated to scattered showers in the region tonight and
Friday with rainfall amounts up to 0.25 inches through Friday
evening. A cold front Saturday night may produce another 0.50
inches. Otherwise dry weather is forecast throughout much of next


Aviation...moderate southerly flow aloft tonight will become light
Friday as an upper level trough moves over the area. The air mass is
moist and unstable with scattered showers. The air mass will
stabilize overnight and there will be some areas of clearing. Gusty
southwest winds will ease this evening. The presence of the upper
level trough will lead to some instability and shower activity again
on Friday afternoon. Ceilings will be mostly VFR but MVFR 2-3k feet
ceilings are possible in showers.

Ksea...mostly VFR ceilings overnight except brief MVFR around
showers. Gusty southwest wind 10g20kt easing tonight by 06z. South
to southwest wind 10 knots or less late tonight through Friday


Marine...small craft advisories remain in effect over all waters as
gusty southwest winds 15-30 knots continue. Winds will ease this evening
and most areas should be below 20 knots by 06z tonight.

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...Small Craft Advisory all 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)