Area forecast discussion...updated National Weather Service Pendleton or 400 PM PST Friday Nov 27 2015 Updated aviation discussion Short term...today 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. Coonfield 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... or...none. Washington...none. && For additional weather information, check our web site at... www.Weather.Gov/Pendleton $$ 76/99/99 !--not sent--!
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