Area forecast discussion...updated 
National Weather Service Pendleton or 
400 PM PST Friday Nov 27 2015 

Updated aviation discussion 

Short through Monday...a Rex block with high pressure 
aloft over the Pacific northwest and an upper low over the Great 
Basin will continue through Sunday. This will keep cold northeast 
flow aloft over the area. With light winds and mainly clear to 
partly cloudy skies strong inversions with little mixing will form 
over valley locations. However dry cold northerly flow aloft will 
inhibit radiation fog formation by weakening the inversions. Patchy 
fog developed this morning near the Columbia and John Day rivers. 
Expect patchy freezing fog to expand each night as condensation 
nuclei trapped under the inversions increase. Fog will dissipate in 
the afternoon because the northerly flow will keep the inversions 
shallow. Sunday night and Monday the ridge will slowly move east 
over eastern Washington and Oregon. This will allow the stratus and 
fog to persist during the day Monday. A front will stretch out near 
the coast Monday as it runs into the departing ridge. This will keep 
any precipitation west of the Cascades during the day Monday. 

Long term...Monday night through Friday night...high pressure will 
remain over forecast area through Tuesday night, except for a chance 
of snow along the east slopes of the Cascades overnight in Wednesday 
morning. In addition, patchy freezing fog and widespread low 
clouds will continue Monday night through Tuesday night. The next 
Pacific weather system spreads moisture across the interior Pacific 
northwest on Wednesday through Friday. There will be a mix of 
wintery precipitation associated with this system, including a 
slight chance of freezing rain along with a chance of snow over 
the lower Columbia Basin, the Yakima Valley and north central Oregon. 
The Kittitas Valley will receive snow. The Blue Mountains, as well 
as the mountains in central and east central Oregon and the east 
slopes of the Washington and Oregon Cascades will have snow. Polan 


Aviation...00z tafs...VFR conditions over most of the area until a 
few hours after sunset. Increasing low level moisture in the 
boundary layer will continue through tonight and Saturday, too. This 
combined with near calm winds in the boundary layer - and thus none 
mixing in the boundary layer - and a strengthening surface-based 
inversion will allow low stratus clouds and areas of fog to develop 
just a few hours after sunset. As the inversion gets stronger 
through tonight low stratus and fog will continue to develop and 
spread out across the lower Columbia Basin and along the foothills 
of the Blue Mountains and impact most taf sites with MVFR/IFR or 
worse conditions tonight, except at kykm which may continue under 
only a weak inversion with some mixing preventing increasing 
moisture thus preventing the formation of low stratus clouds. The 
effects of a stronger inversion will finally be felt at kykm 
Saturday afternoon as IFR conditions begin around 28th/22z due to 
low ceilings. The inversion will strengthen at kdls late tonight with 
MVFR conditions developing due to low stratus clouds, which will 
persist through Saturday afternoon. Winds will remain light and 
mainly terrain driven at or below 8 kts through the period. 

We will need to be closely monitoring surface metar observations for 
necessary amendments to the taf forecasts throughout the forecast 
period. Polan 


Preliminary point temps/pops... 
PDT 11 26 13 30 / 0 0 0 0 
alw 17 28 18 31 / 0 0 0 0 
psc 17 29 19 31 / 0 0 0 0 
ykm 15 35 16 36 / 0 0 0 0 
hri 16 28 17 32 / 0 0 0 0 
eln 13 31 15 31 / 0 0 0 0 
rdm 2 32 6 38 / 0 0 0 0 
lgd 13 29 13 31 / 0 0 0 0 
gcd 8 30 10 32 / 0 0 0 0 
dls 22 37 23 38 / 0 0 0 0 


PDT watches/warnings/advisories... 

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