Area forecast discussion...updated 
National Weather Service Pendleton or 
404 PM PST Tuesday Feb 9 2016 

Updated aviation discussion 

Short term...tonight through Friday...the upper level ridge over 
the region is beginning to weaken as weather systems off the coast 
bump up against it. The ridge will slowly flatten over the next 
couple of days allowing a southwest flow aloft to bring some 
moisture into the region starting as clouds overnight and Wednesday. 
There will be some light precipitation spreading into western 
Washington and Oregon on Wednesday eventually reaching the 
Washington Cascades Wednesday night and Thursday. The upper level 
ridge will finally break down on Friday allowing a cold front to 
push through the forecast area Friday increasing the chance of 
precipitation for the entire area. Snow levels will remain high so 
expecting mainly rain through Friday. 

Long term...Friday night through Tuesday...pressure pattern aloft 
across northeast Pacific and the Pacific northwest becomes very 
progressive with a series of Pacific weather systems tracking 
eastward through the region through the period. As such snow levels 
will be unseasonably high resulting in precipitation being in the 
form of rain each day. The first upper level trough approaches the 
Cascades Friday night and then sweeps a strong cold front through 
the forecast area on Saturday. Tightening surface pressure gradients 
accompanying the cold front will produce breezy conditions beginning 
soon after daybreak on Saturday morning with breezy winds continuing 
through Saturday afternoon. The upper level trough exits into 
Idaho/Montana Saturday afternoon followed by a flat upper level 
ridge of high pressure building over the Pacific northwest Saturday 
night, which will usher in a warm front and attendant warm air 
overrunning rain for Saturday night, which then persists through 
Sunday afternoon. As the ridge amplifies Sunday night and Monday... 
George Washington's birthday...sinking air associated with the ridge 
will yield dry conditions at lower elevations and a low chance of 
rain over the higher mountains. Monday night and Tuesday the ridge 
will still be over the interior Pacific northwest for dry conditions 
in central Oregon and east central Oregon, but with a low chance of 
rain in the mountains on Tuesday. Polan 


Aviation...00z tafs...VFR conditions will continue through this 
evening. Then a strengthening inversion will trap and increase 
moisture in the boundary layer producing low ceilings and fog due to 
increasing areal coverage of low stratus and fog in the lower 
Columbia Basin and adjacent valleys beginning around midnight and 
continuing well past daybreak on Wednesday. As such conditions are 
expected to deteriorate at kykm, kpsc, krdm and kbdn becoming vlifr 
at krdm and kbdn, with kpsc and kykm becoming LIFR. 

Conditions at kykm will improve to IFR by around 20z-21z tomorrow. 
Conditions at kpsc will improve to VFR by 21z. 
Conditions at krdm and kbdn should improve to VFR by around 16z-17z. 

I kept kdls, kpdt and kalw at VFR for the next 24 hours, as guidance 
showed downslope drainage winds and/or mixing in the boundary layer 
in combination with a drier boundary layer initially, so these 
factors should work against formation of fog and low stratus clouds 
at those taf sites tonight. If visibility does lower to 5-6sm with 
localized MVFR conditions at kdls, kpdt and kalw, these sites will 
be amended or their forecast updated based in new scheduled taf 
issuances later this evening if new model forecast guidance changes. 

Surface winds less than 10 kts for the next 24 hours. Polan 


Preliminary point temps/pops... 
PDT 30 46 34 50 / 0 0 0 10 
alw 31 46 35 50 / 0 0 0 10 
psc 28 47 32 47 / 0 0 0 10 
ykm 29 50 32 48 / 0 0 10 20 
hri 28 46 33 49 / 0 0 0 10 
eln 20 34 27 37 / 0 0 10 20 
rdm 28 60 32 59 / 0 0 0 10 
lgd 34 46 34 47 / 0 0 0 0 
gcd 33 56 33 52 / 0 0 0 10 
dls 32 50 37 50 / 0 0 10 20 


PDT watches/warnings/advisories... 
or...air stagnation advisory until 10 am PST Saturday for orz041-044- 

Washington...air stagnation advisory until 10 am PST Saturday for waz024- 


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