Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 221025
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
325 AM PDT Sat Apr 22 2017

.SYNOPSIS...Increasing clouds this morning as an upper level low
pressure system brings rain back to the area by this afternoon. Wet
and cool weather will continue this weekend and into next week as a
steady stream of weather systems takes aim at the Pacific Northwest.

&&

.SHORT TERM...Current satellite imagery shows clouds starting to
fill in over W WA this early morning...but there still appear to be
some breaks here and there allowing for some peeks at the sky. This
is expected to fill in as the morning progresses. On the radar side
of things...generally no echoes over the area...although you can
perhaps catch a hint of things to come out over the farthest
southwestern reaches of the radar...out over the Pacific
waters...can see some faint echoes that likely herald the rain that
is to come.

Newton`s third law of motion states that for every action...there is
an equal and opposite reaction. Upon looking at the
forecast...having a hard time seeing where one nice
day...yesterday...merits the equal reaction of an entire week of
transiting weather systems and all the rain/showers that come with
them. As hinted at above...upcoming front associated with an upper
level trough over the Pacific will make its way over the area today
as associated showers reach the coast by mid to late morning and
moving inland by noon today. There does look to be a bit of
instability with this front...as such have left in the possibility
of thunder for inland locations this afternoon. Some breezy
conditions may also arise over portions of the area in advance of
the front today and continue into tonight. The front moves through
by evening although the trough remains off the coast and as
such...will transition to showers this evening and tonight. May see
some downtime during the overnight hours where pops look to relax a
bit but as the upper low remains relatively stationary...should not
come as any surprise that another disturbance associated with the
feature pushes through the area for Sunday bringing another round of
showers. Another front is expected to push into the area Sunday
evening. While heaviest rainfall with current models looks to fall
to the south of W WA...it appears there is plenty of precip to go
around overnight Sunday and into Monday. Upper level trough finally
starts to push eastward...very slowly...late Monday afternoon and
into Tuesday. As the trough axis plods across the area...rain looks
to transition to showers and very gradually taper off through the
overnight hours Monday and Tuesday morning.

Looking at temperatures...might be able to squeak out some low 60s
over the area today...but otherwise looks like temps will be unable
to claw their way out of the mid 50s in the lowlands for both the
near and long term.  SMR

.LONG TERM...As has been the story for what seems to be an
eternity...when one low leaves another is close behind.  Such is the
case as a front associated with another upper level low...this time
to the northwest off the Canadian coast...will bring another slug of
rain to the area Tuesday afternoon and evening. Thankfully neither
model projects that this feature will linger like the previous low
and has it swooping down into the area...although there is about a
12 hr difference between models as to when this occurs...the ECMWF
being much faster. Further disagreement arises when considering the
strength of the upper level ridge that moves in afterward...the
ECMWF solution favoring a dirty ridge that still allows for a chance
of showers Wed night while the GFS solution shows a weak ridge akin
to what has generally been part of the pattern over the area for a
while now...strong enough to allow for dry conditions Wed night and
into Thursday afternoon. This disagreement does not allow for much
confidence...but as this year has gone so far...if there is a way to
squeeze rain into the forecast...likely to be more right than wrong
if it is in there. As such...although they fall at the lower end of
chance range...pops remain in the forecast for the second half of
next week.  SMR

&&

.AVIATION...A negatively tilted upper trough will move northeast
through Western Washington today. It will be accompanied by a
northeast-moving cold front, which will cross the area late this
morning and early this afternoon. Light to moderate rain will reach
the coast around 15Z (8 am) then spread NE across the interior 18Z-
21Z (11 am-2 pm). In advance of the cold front, the air mass will be
dry and stable. Along and behind the front, the air mass will
quickly become moist and unstable. Owing to the unstable air mass,
there is a slight chance of afternoon and early evening
thunderstorms as this feature moves NNE across W WA.

KSEA...Increasing high clouds for the rest of this morning. E wind
with a slight southerly component (100-130 degrees) is expected
until a cold frontal passage around 17z (10 am). That will bring a
sudden gusty shift to S-SW wind. Rain and the sudden arrival of cigs
in the 030-045 range will arrive with the frontal passage, then some
vigorous shower activity is expected in the vicinity of the terminal
this afternoon. Shower activity will settle down tonight, but the
moist air mass will cause cigs to lower into the 020-030 range late
this evening. Haner

&&

.MARINE...Moderate east to southeast offshore flow will precede a
cold front, which will lift northeast through the waters late this
morning. Following the front, a sudden increase in southerly winds
is expected. A weak surface trough will move northeast through the
waters on Sunday. A low pressure center will rapidly weaken to about
1003 mb by the time it reaches the central or north Washington Coast
on Monday afternoon. Another frontal system will affect the area on
Tuesday.  Haner

&&

.CLIMATE...Precip at SeaTac looks to reach another record
soon...this time for most precip for a period starting in October
and running through April. Current October-April rainfall stands at
44.06 inches as of 3 AM PDT this morning. The record...set in 2015-
2016 (meaning Oct 2015 thru Apr 2016) stands at 44.52. Given the
amount of rain expected this weekend and into next week...would not
be surprised to see the standing record fall.  SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     Out 10 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory from 9 AM this morning to 9 PM PDT this
     evening for Admiralty Inlet-East Entrance U.S. Waters
     Strait Of Juan De Fuca-Northern Inland Waters Including The
     San Juan Islands-Puget Sound and Hood Canal.

     Small Craft Advisory until 9 AM PDT this morning for Central
     U.S. Waters Strait Of Juan De Fuca-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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