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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