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fxus66 ksew 272219 

Area forecast discussion
National Weather Service Seattle Washington
319 PM PDT Thu Apr 27 2017

Synopsis...a weak low pressure system over the region will exit
late tonight, and will be followed by high pressure. Another low
pressure system will bring more precipitation to the area on
Saturday. Expect a chance of Post-frontal showers on Sunday.


Short term...
there were scattered showers over the area this afternoon. A few of
the showers contained small hail per an observation from a cocorahs
observer in Lake Stevens.

Based on satellite and radar data, it appeared that the Puget Sound
convergence zone (pscz) has developed along a line roughly from near
Hoodsport to Auburn and then east into the Cascades. Expect the pscz
to strengthen some this afternoon in response to increased westerly
flow through the Strait of Juan de Fuca. The pscz might briefly shift
slightly north of its current position this afternoon, impacting
places such as sea-tac, Burien, and Bremerton. The pscz will drift
into Pierce County this evening before dissipating overnight.

Meanwhile, an upper level trough of low pressure presently over the
region will move into the intermountain region late tonight. Expect
a ridge of high pressure aloft to build into the area from the west
during the day Friday. There will probably be a few instability
showers over the mountains during the day; otherwise, expect dry
weather with near or slightly below normal Max temps.

High pressure aloft will prevail over the region Friday night before
shifting east during the day Saturday. Expect a low pressure system
to approach the Pacific northwest on Saturday for a threat of
precipitation. The latest GFS was a little faster than its previous
solutions. Chose to compromise between the faster and slower model
solutions. At any rate, it now looks like there will be a threat of
precip over the entire County Warning Area Saturday afternoon.

Saturday night still appeared wet for the County Warning Area. The cold or occluded
front is anticipated to sweep across the area by daybreak Sunday.
The upper trough will move across the region on Sunday for Post-
frontal showers.

Long term...
confidence in this period was not very high, especially concerning
the details. It looks like the overall trend in the models was to
gradually build an upper level ridge over the Pacific northwest. A
warm front may bring some precipitation to the County Warning Area early in the
period; otherwise, look for warmer and drier conditions the middle
of next week. Day 7 was a question mark. It might end up being a
transition day from warm, dry conditions back to cool and unsettled


Aviation...moderate northwest flow aloft will weaken tonight and
become more northerly Friday. Moderate to strong low level
onshore flow will decrease overnight. The air mass is moist and
weakly unstable with scattered showers. A Puget Sound convergence
zone is likely to form early this evening and drift south.
Ceilings will remain VFR into the evening, then subside into 2-4k
ft ceilings late tonight. Friday will be mostly VFR.

Ksea...discussion above applies. Southwesterly wind 10-14 kt
will switch to northerly by 00z. Winds should Switch Back to
southwesterly by late evening. Puget Sound convergence zone may
produce showers at the terminal this afternoon and evening. Chb

Marine...onshore flow will increase late this afternoon and
evening and will probably produce gale westerlies in the Strait.
The Gale Warning is already in effect. There is a Small Craft
Advisory in effect everywhere else except Puget Sound. A pscz will
switch the winds in Puget Sound from southerly to northerly this
afternoon, at least over the northern part.

High pressure over the area will give light winds Friday into
Saturday morning. A front will bring increasing winds to the coastal
waters Saturday afternoon and to to the inland waters Saturday
night. Strong onshore flow Sunday will ease Monday. Chb


Sew watches/warnings/advisories...
Pz...Gale Warning until midnight PDT tonight for central U.S. Waters
Strait of Juan de Fuca-East Entrance U.S. Waters Strait of
Juan de Fuca.

Small craft advisories are in effect for the remaining waters
except the Puget Sound/Hood Canal.



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