FXUS66 KSEW 301025

Area Forecast Discussion
National Weather Service Seattle WA
325 AM PDT Sun Apr 30 2017

.SYNOPSIS...Scattered showers remain possible throughout the day
today over the area in the wake of a passing cold front. A weak
weather system passing to the south of the area may result in light
showers Monday. Upper level ridging will bring mostly dry conditions
Tuesday through Thursday although some locations may see brief light
rain. The leading edge of an upper level trough will bring a return
to rain starting Thursday night with the trough expected to pass
through the area for next weekend.


.SHORT TERM...Radar this morning is pretty quiet with possibly some
light showers along the coast but otherwise activity seems to be
relegated elsewhere. Not unexpectedly...clouds remain in place over

A weak shortwave will linger over the area today allowing for the
chance of showers to persist. On top of that...models continue to
suggest the development of a Puget Sound Convergence Zone this
afternoon. However...models do not really agree on where to place
it...the NAM has it over central King Co...the HRRR has it over the
King/Snohomish Co line...and the GFS puts it over central Snohomish.
Thus...have opted to paint forecast in broad strokes to cover this.
Some weak upper level ridging looks to taper activity off tonight
and into Monday.

Would like to advertise Monday as a dry day...and for some locations
it may well be...but models over the past 24 hours have shown a
weakening shortwave within the ridge that looks to bring a chance
for showers before dipping southward and taking any activity along
with it. By Monday night...said ridging will continue to intensify
but do not get too excited. GFS and ECMWF have completely flip-
flopped on their solutions for Tuesday into Wednesday. Previous GFS
runs showed a system approaching but moving just to the north of the
area while previous ECMWF solutions showed the system failing to
pull up in time and as such...bringing some rain to the area. Now
the GFS shows a wet solution and the ECMWF is showing drier.
Apparently the motto for this spring is that we simply cannot have
nice things. With more than just a little feeling of defeat...have
opted to up pops slightly for the Tues/Weds time frame to allow for
slight chance to low end chance pops to enter into the forecast. SMR

.LONG TERM...Maintaining the aforementioned reversal...GFS now wants
to go with a dirty ridge over W WA for Weds that would still allow
for showers while the ECMWF remains dry. On the bright side...the
two converge on the solution that the ridging will continue to
intensify Thursday with dry conditions from both models. This ridge
intensification could also lead to many interior locations seeing
their first 70 degree day either Wed or Thu. As the end of the
upcoming week approaches...models agree an upper level low over the
Pacific will begin to impact the area starting Friday and into the
weekend. The ECMWF follows a pretty rational...standard approach of
an upper low that W WA has seen many...many times as of late. The
GFS however throws in a line of precip well ahead of the leading
edge of this trough with seemingly only a minor shortwave to justify
it. Cannot really see biting on to that solution at this time...but
did nudge pops up slightly in this time period. Would hope models
get their bits together for future runs because due to current
waffling nature...have a hard time putting a great deal of
confidence in any current solutions. SMR


.AVIATION...Moderate westerly flow aloft becoming light
northwesterly this afternoon through tonight. Generally moist air
mass and weakly unstable in the lower levels. showers generally
decreasing today, except for a Puget Sound convergence zone from
KPAE to just north of KSEA/KBFI, possibly reaching the KBFI terminal
by early this evening before dissipating. Mostly MVFR stratus with
local IFR and light showers or drizzle. Cigs generally improving to
VFR by late morning with scattering clouds some places, except MVFR
still possible where any convergence zone develops.

KSEA...southwesterly 12-15 kt w/ gusts 24 kt, becoming more westerly
and easing to 8-12 knots by late this afternoon and evening. The
convergence zone may approach the terminal from the north by early
evening. There is a 25 percent chance a northerly wind shift at the
KSEA terminal this evening, with a better chance of north wind to 10
kt at KBFI. Opposing flow at KSEA/KBFI is possible in this
scenario, with best chance between 23z-04z this evening. dtm


.MARINE...Moderate post frontal onshore flow will prevail today,
gradually easing tonight. Residual small craft winds over the
coastal waters, western Strait, and Puget Sound will ease this
morning. Stronger westerly winds to 30 kt will continue in the
central/eastern Strait, easing after midnight. Small craft
advisories remain up through tonight for the central/eastern Strait
and also for portions of the north inland waters and Admiralty Inlet
near the east entrance to the Strait. While winds will ease in Puget
Sound, a switch from south-southwesterly to a northerly direction is
expected across the north Sound later this morning, reaching close
to around seattle this afternoon and evening. dtm


PZ...Small Craft Advisory all waters.




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