Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Seattle Washington
250 PM PDT sun Apr 20 2014

Synopsis...an upper level ridge of high pressure aloft and surface
high pressure over Washington will give mainly dry weather to the
area through tonight. However...a stationary front over the coastal
waters will bring a chance of light showers to the coast 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 weekend.

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Short term...a weak upper level ridge over eastern Washington this
afternoon will continue to amplify as it shifts eastward into
western Montana. An upper level trough over the eastern Pacific
along 145w is also amplifying. The combination of these larger scale
features is resulting in a frontal system in the offshore waters
stalling off the coast and becoming more north-south oriented with
time. This front will remain offshore through Monday.

Radars do show rain offshore...and a few showers have popped up
over the olympic peninsula this afternoon. The showers on the coast
are moving northward and should pretty much clear out of the
area...with the exception of the immediate coastline...this evening.
A batch of mainly high clouds splitting eastward from the offshore
front will push east out of the area this evening. Some brief
clearing may allow temperatures to drop into the upper 30s in some
normally colder locations in the interior tonight. Swell on the
coast has started to drop off...it is now 16-18 feet. The high surf
advisory on the coast will be maintained until 9 PM this evening.

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 the development of a diffluent south-southwest flow
aloft will allow some showers to form on Monday...mainly in the
afternoon. 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.

The large scale models have converged on a solution for Tuesday
night into Wednesday. They generally show the upper low now near 50n
148w shifting east-southeast into the area late Tuesday night as a 538-540 dam 500
mb low and resulting in rather high probability of precipitation across the area. Wednesday looks
like a mixed bag with weak ridging and onshore flow giving some sun
breaks to the north but with a continued chance of showers. A warm
front will approach the area from the SW late in the day Wednesday for
increasing precipitation over southern portions of the forecast
area.

Temperatures across the region through Wednesday will average a few
degrees below normal. Albrecht

Long term...large scale models bring a wetter and more organized
frontal system across the area Wednesday night through Thursday or Thursday night.
Per conversations with NESDIS...this system...currently near the
international dateline...has some tap to subtropical moisture from
the western Pacific and may result in some moderate precipitation
amounts. This has been advertised by all of the global models for
several days now. Behind that system...mean upper troughing over the
eastern Pacific will maintain cooler and wetter than average
conditions for late April. Albrecht

&&

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 230 PM...the North Fork of the stillaguamish near Arlington is
at 5.5 feet...or 4195 cfs. The river will recede to around 4.5 feet
late Monday afternoon. The North Fork of the stillaguamish river
near Oso is at 217.0 feet or 2100 cfs. The river will recede through
late Monday. The slide pool east of the landslide is at 284.2 feet.
It will also 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 remain dry into tonight...although moisture aloft will still be
present. Lower levels will begin to moisten up tomorrow morning in
advance of weak front expected to move through tomorrow. Ceilings
expected to remain VFR for much of western Washington into the day tomorrow.

Ksea...ceilings expected to remain above 5k feet tonight and into
tomorrow. Southerly wind to 10 knots...shifting to the northwest tomorrow
afternoon. Smr

&&

Marine...a weakening cold front will move over the coastal waters
this afternoon and tonight...dissipating as it pushes onshore. South
to southeasterly small craft winds are expected over the coastal
waters/West Entrance to the Strait ahead of the front this evening.

Large swell of 17 to 20 feet will impact the coast late this
afternoon...then slowly subside tonight. Swells look to still exceed
Small Craft Advisory for hazardous seas criteria well into
tomorrow...so will issue an advisory to follow up the one already in
effect for winds for the afternoon forecast package. A high surf
advisory remains in effect for the coast. Observation indicate that high
surf might last a little longer than originally thought. Thus...will
extend advisory a few hours. If for no other reason...will give next
shift a chance to look things over and evaluate the situation. 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 evening.

Pz...Small Craft Advisory coast and west Strait of Juan Delaware fuca
through Monday evening.
Small Craft Advisory for rough Grays Harbor bar through noon
Monday.

&&

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