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fxus66 ksew 190416 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
915 PM PDT Fri Aug 18 2017 


Synopsis...a few light showers were falling around western 
Washington this evening. Clouds will decrease on Saturday. Another 
weak system will move by to the north of the area on Sunday. High 
pressure at the surface and aloft will give dry and warmer weather 
to the area Monday and Tuesday. Another weak system will move across 
mainly the northern portions of the area later Wednesday through 
Thursday night. 


&& 


Short term...onshore flow will continue as a weak system moves by 
to the north of the area and higher pressure builds in from the 
west. A few light showers were falling here and there around the 
Puget Sound area. Ridging offshore will deflect another weakening 
frontal system to the north of the area Sunday. Some light showers 
could fall on the northwestern olympic peninsula but mainly there 
will be more clouds in the north and on the coast. Sunday high 
temperatures will likely be near or slightly higher than Saturday. 


Ridging aloft will build offshore Sunday night and will move east 
across the area later Monday and Monday night. The ridging will 
result in clearing late Sunday night into Monday morning and a 
warming trend. With the ridging and warming aloft, highs on Monday 
may reach 80 in the interior from Seattle southward. On Tuesday, we 
will see the offshore ridge shift eastward into western Montana by 
late in the afternoon. The eastward movement of the upper ridge will 
induce an increase in onshore flow, so it will be cooler on the 
coast. 


Long term...the European model (ecmwf) is showing better run to run consistency 
than the GFS, and the latest GFS solutions are trending toward the 
European model (ecmwf). 


The extended period will feature the transit of an upper 
level low or trough to the north of the area during the latter 
half of next week. This will result in an increase in onshore flow 
and clouds and will give cooler daytime high temperatures. Even 
the weaker European model (ecmwf) shows a possibility of showers across the area 
later Wednesday through Thursday night, so they were included in 
today's extended forecast. The GFS appears to be backing away from 
its solution of a deep upper low moving across the region mid to 
late week. 


Some ridging is possible beginning next Friday or Saturday, for 
dry and warmer weather. The GFS is more extreme with the ridging; 
a model blend about 2/3 toward the European model (ecmwf) was used for the forecast 
fields for next Friday and beyond. Albrecht 


&& 


Aviation...west to northwest flow aloft over western Washington 
will continue tonight and Saturday, as the upper trough over 
British Columbia moves east and an upper ridge builds offshore. 
Surface high pressure remains offshore with lower pressure east of 
the Cascades, and moderately strong onshore flow will prevail. 


Ceilings below 1000 feet this evening along the coast. Over the 
interior scattered to broken layer near 3500 feet with a broken 
cloud layer near 6000 feet. Little change for the next few hours. 
Low level onshore flow combined with some convergence will lower 
ceilings in the central Puget Sound area down to 1000-2000 feet 
after 11z Saturday morning. Ceilings will improve all areas 
Saturday afternoon. 


Ksea...ceilings near 6000 feet with scattered layer near 4000 
feet going broken at times through early Saturday morning. Lower 
stratus moving into the terminal around 11z with ceilings 
lowering down to near 1000 feet. Ceilings improving late morning 
with clouds scattering out by noon. Northerly wind 6 to 10 knots. 
Felton 


&& 


Marine...kuil-kbli gradient near plus 3 mb this evening. Winds 
have not hit gale yet in the central and eastern Strait of Juan de 
Fuca but with the gradient this high will keep the gales going 
into the early morning hours. Small craft advisories for 
Admiralty Inlet and the northern inland waters will also remain 
in effect. 


After tonight, a typical August pattern will prevail through the 
middle of next week; onshore flow of varying strength will persist 
due to high pressure offshore and lower pressure east of the 
Cascades. Small Craft Advisory westerlies are possible in the 
central and eastern Strait of Juan de Fuca in the evening and 
early morning hours beginning Saturday evening and continuing into 
the middle of next week. Felton 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory until noon PDT Saturday for coastal waters 
from Cape Flattery to James Island 10 to 60 nm-coastal 
waters from James Island to Point Grenville 10 to 60 nm- 
coastal waters from Point Grenville to Cape Shoalwater 10 
to 60 nm. 


Gale Warning until 3 am PDT Saturday for central U.S. Waters 
Strait of Juan de Fuca-East Entrance U.S. Waters Strait of 
Juan de Fuca. 


Small Craft Advisory until 3 am PDT Saturday for Admiralty Inlet- 
northern inland waters including the San Juan Islands. 


&& 


$$ 



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