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Area forecast discussion 
National Weather Service Seattle Washington
846 PM PDT Monday Apr 20 2015

Synopsis...an upper level ridge will keeps skies mostly clear
tonight. Weak low level onshore flow overnight will allow a few
areas of low clouds to form over western Washington Tuesday morning.
A weak front will bring clouds and a few showers to the coast late
Tuesday morning and then over Puget Sound sound late in the day. An upper
level trough will follow Tuesday night and Wednesday morning.
Another weak front will bring a few showers on Thursday. Moist
southwest flow aloft will bring periods of rain next weekend.

&&

Short term...the upper level ridge over West Washington this evening will
shift over north Idaho Tuesday morning...allowing the weak front just
offshore near 130w to move toward the Washington coast. Onshore flow is
increasing this evening ahead of the front...which is very
reminiscent of a Summer-time pattern. There was just a little ribbon
of stratus along the central coastline late this afternoon and
onshore pressure gradients have only made it up to 1.0 to 1.5
mb...so it does not look like there will be a whole lot of stratus
forming in place over the interior Tuesday morning. Best guess is
areas of stratus through the Strait and over the lower Chehalis
valley mainly west of Shelton.

The weak front along 130w this evening will reach the area on
Tuesday. Models have the leading edge of the frontal clouds reaching
the coast around late Tuesday morning...and Puget Sound sound late in the
afternoon. The front should be over the Cascades by late evening.
Frontal precipitation will be pretty skimpy...with light
accumulations on the coast mainly upstream of the Olympics. Over the
interior it looks like most of the precipitation will be around central
Puget Sound sound where a convergence zone is expected to form...and over
the Cascades where low level west winds will generate upslope flow.

An upper level trough will be right behind the front...crossing
Puget Sound sound early Wednesday morning...then moving over East Washington
Wednesday afternoon. NAM high res soundings indicate the air mass
will remain stable with both the weak front and the associated upper
level trough. Precipitation on Wednesday behind the upper trough should be
mainly associated with the convergence zone...and those showers
should be diminishing by afternoon.

It looks like Wednesday afternoon and evening will be mostly dry. A
small weak upper level shortwave ridge will move across the
area...but the break will be mostly due to The Gap in-between
weather systems.

GFS and European model (ecmwf) agree that a weakening cold front will reach West Washington on
Thursday. This would be the system currently far offshore near 170w.
This system could bring a little more widespread precipitation than
tuesdays system...but still on the light side. Kam

Long term...models agree that a weak upper level trough will move
across the area Friday...followed by a weak ridge Friday night.
Scattered showers associated with the trough should diminish Friday
night.

A large cold upper level longwave low will set up over the NE
Pacific this weekend with strong flow aloft around the low. Moist
west-southwest flow aloft over the NE Pacific east of the low will be aimed at the
pacnw. This looks like potentially the wettest period for West Washington since
middle March. Kam

&&

Aviation...the upper level ridge over the region will weaken as the
ridge axis shifts east of the Cascades overnight. A weak cold front
is on track to move through the forecast area Tuesday
afternoon...followed by an upper level trough Tuesday night. This
will result in a short period of rain followed by showers Tuesday
night. Northwest flow aloft will become southwest late tonight as
the upper level trough approaches from the west. The air mass will
become moist late tonight and Tuesday as low level onshore flow
increases.

Marine stratus along the coast will deepen and push inland tonight.
The push continues to look weak...the current forecast reflects
stratus reaching Olympia late tonight with any low clouds reaching
the central sound remaining thin and relatively short lived Tuesday
morning.

Ksea...north wind to 10 knots...becoming southerly to 8 knots late...then
southwest 6-12 knots Tuesday afternoon. Thin marine stratus will likely
reach the terminal around 13z with several hours of scattered-bkn005
Tuesday morning.

&&

Marine...onshore flow will strengthen tonight as low pressure over
western Washington moves east of the Cascades. Small Craft Advisory
winds remain forecast for the coastal waters...western and central
Strait of Juan Delaware fuca...Admiralty Inlet...and northern inland
waters. The strongest winds should occur in the central and eastern
Strait.

Onshore flow will continue to increase Tuesday and Tuesday night as
a weak cold front moves inland. Gale westerlies remain likely for
the central and eastern Strait. A gale watch remains in place for
the potential for marginal gales over the northwest coastal waters.
Small Craft Advisory winds will continue for all other marine zones
except Puget Sound sound and Hood Canal.

A second weak cold front will cross the forecast area Thursday and
Thursday night...with a stronger frontal system expected to reach
the area Saturday.

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory coastal waters...central and eastern
Strait of Juan Delaware fuca...Admiralty Inlet...northern inland
waters.
.Gale Warning for Tuesday night central and eastern Strait.
.gale watch for Tuesday night coastal waters zones 150 170 173.

&&

$$

You can see an illustrated version of the forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html

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