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Area forecast discussion 
National Weather Service Seattle Washington 
840 PM PDT Fri Sep 22 2017 


Synopsis...a weak north to south moving weather system will brush 
the coast Saturday with clouds and a threat of light rain for the 
coast and Olympics. Another weak system Sunday night and Monday 
moving into British Columbia should bring more clouds and a chance 
of light rain to mainly northern portions of the area. Then, higher 
pressure aloft is expected to build offshore Tuesday into mid week 
for a return to sunshine and warmer temperatures. 


&& 


Short term...the positive tilt upper ridge offshore along about 
135w has a lot of moisture moving through it. One weak shortwave 
along the central b.C. Coast this evening is forecast to track south 
just off the coast through Saturday with mainly mid and higher level 
clouds and a threat of light rain to the coast and Olympics. There 
should be enough passing mid level clouds overnight to limit any 
areas of fog to primarily the south sound/SW interior Saturday 
morning. 


The upper ridge should keep the weather dry Saturday night and 
Sunday. Anticipate a bit more in areas of fog in the usual fall 
prone areas Sunday morning. Then the next shortwave moving over the 
ridge is expected to track onshore into b.C. Sunday night and Monday 
for additional cloud cover and a threat of light rain mainly across 
the northern part of western Washington. This system should weaken 
further as it moves onshore and inland as heights aloft rise. 


High temperatures this weekend should remain close to or a little 
below late September averages. Buehner 


Long term...from the previous edition... 
Tuesday will be kind of a transition day as the upper level ridge 
builds offshore. There still appears to be a lot of residual low 
level moisture but this may be clearing out in the afternoon. The 
models have been generally consistent with the building ridge for 
mid week. It now looks strong enough to keep skies mostly clear and 
warm daytime temperatures into the lower 70s in the warmest spots. 
The models differ a bit on just how quickly the ridge will shift east 
of the area, but for now the forecast remains dry through Friday. Kam 


&& 


Aviation...western Washington lies in between a trough over the Great 
Basin and a ridge offshore. The flow aloft is northerly. The air 
mass is dry and stable with mainly mid-high level clouds. Patchy fog 
may form early Saturday morning, mainly in the valleys. Light rain 
is possible along the coast where there is weak warm air advection 
early Saturday morning and afternoon. The interior will remain dry. 
The ridge will shift inland Saturday night for dry conditions. 33 


Ksea...light south winds and VFR this evening. Patchy low clouds or 
stratus may form early Saturday morning for brief MVFR conditions. 
Otherwise dry and VFR through Sat night. 33 


&& 


Marine...high pressure over the NE Pacific will maintain light 
onshore flow this weekend. A warm front will clip the area on Monday 
with possible Small Craft Advisory winds over the coastal waters. 
The flow will turn offshore by midweek as a thermal trough forms 
along the coast. 33 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...none. 
&& 


$$ 






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