FXUS66 KPDT 111806 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1006 AM PST Mon Dec 11 2023 Updated Aviation Discussion .UPDATE...Condition trend calmer today into tonight as a ridge of high pressure begins to build over the Pacific Northwest by Tuesday. Headed through the remainder of the day, areas of dense fog will be the primary weather related impact with current traffic cameras and surface observations indicating heavily degraded visibility along the foothills of the Blue Mountains and in the Kittitas Valley - all of these areas are under a Dense Fog Advisory until noon. It remains to be seen how quickly conditions will improve in this regard, as high resolution guidance like the HRRR and HREF don't depict too much improvement for the Blue Mountain foothills, especially the mid to upper slopes between roughly 1500-2500ft due to ample surface moisture and light northerly upslope flow. It's possible the aforementioned Dense Fog Advisory may need to be extended. Otherwise, a weak passing upper-level shortwave produces light shower chances(15-45%) over the eastern Mountains into portions north-central Oregon today headed into the early evening hours before dry weather finally returns overnight as the influence of the ridge takes hold. As a result, overnight fog and low stratus formation will be the mainly weather feature through Wednesday. Schuldt/99 .AVIATION...18Z TAFs...Variable conditions across the sites this morning, with conditions as low as VLIFR expected through the period. At site ALW, fog 1/2SM or lower with stratus less than 500ft AGL is expected to persist through the period, with low confidence (20%) in prevailing improvements. Sites DLS/PDT/PSC will continue to see IFR to LIFR CIG conditions, with vsby improving to 3SM-6SM by this afternoon; periodic light misting will be possible through the remainder of the morning at sites DLS/PDT as well. Sites RDM/BDN/YKM will see some clearing CIGS within their respective regions today, however, bkn-ovc CIGS AOA 1kft to 3kft AGL are expected to prevail through the afternoon, then drop to less than 1kft AGL overnight. Vsby is also forecast to drop to 3SM or less overnight at sites RDM/BDN/YKM. Winds will remain light, less than 12kts, through the period. Lawhorn/82 PREV DISCUSSION... /issued 341 AM PST Mon Dec 11 2023/ .SHORT TERM...Today through Wednesday night...Current radar and infrared satellite imagery showing light returns over the Blue Mountains and foothills as upslope snow showers continue at elevation over the Blues under mostly cloudy skies. This is in response to a weak upper level shortwave that is approaching the coast before slowly passing through the Pacific Northwest later today. This will keep light snowfall occurring over the Blue Mountains this morning before focusing more along the Southern Blue Mountains through the late morning and early afternoon. Minimal snow amounts are expected as moisture is lacking with this passing shortwave. The main weather concern this morning will be associated with areas of fog that developed overnight, which is a result from the presence of low level moisture and a weak ridge that is being eroded by the passing shortwave. A Dense Fog Advisory has been issued for the Kittitas Valley and along the Northern Blue Mountain foothills until noon as visibilities of one quarter mile or less are occurring and will persist through the morning hours. Confidence in these decreased visibilities continuing through the morning is moderate to high (70-80%), as the HREF highlights a 50-70% chance of visibilities of 1/2 mile or less through 11AM Monday over the aforementioned areas. Morning low temperatures should only drop into the upper 30s to low 40 along the Blue Mountain foothills, Eastern Gorge, Central Oregon, and at lower elevations of the Basin. However, low temperatures across the Kittitas Valley will drop to between 30-32 degrees so there may be areas of freezing fog and slick roadways - so use extra caution if traveling this morning. Conditions will continue to dry through the day today as an upper level ridge begins to build in from the west in the wake of the passing system. This will again allow for areas of fog to develop into Tuesday morning along the Northern Blue Mountain foothills and the Kittitas Valley, as the HREF shows a 30-50% chance of 1/2 mile visibilities at 8AM Tuesday. Skies will slowly clear through the day as the upper level ridge builds in from the west- southwest. This will promote west-northwest flow aloft with high temperatures warming 1-3 degrees from Monday, which is 2-5 degrees above normal for this time of year. Clear skies overnight will attribute to much cooler temperatures Wednesday morning with morning values in the upper 20s to low 30s across Central Oregon, the Eastern Gorge, Lower Columbia Basin, Blue Mountain foothills, and the Yakima/Kittitas Valleys. These cooler temperatures, coupled with the possibility of developing overnight fog along the foothills and east slopes of the Washington Cascades, will lead to the potential for freezing fog to develop. This will need to be closely monitored as slick roadways and surfaces may result. Afternoon temperatures will be similar to Tuesday, if not a degree or two cooler due to the presence of the infiltrating upper level ridge. This ridge does begin to erode along the coast and broaden to our east as a weak shortwave begins to ride the ridge and approach the coast late Wednesday. The presence of the ridge and its increasing influence across the area on Wednesday will attribute to a pressure gradient developing across the Grande Ronde Valley, allowing for winds to increase through the morning and extend through the remainder of the day Wednesday. The GFS and SREF are advertising a pressure gradient of 4-5mb between Baker City and Meacham, which correlates to wind gusts of up to 45 mph out of the south. Currently, these values are below wind advisory criteria, but this will need to be monitored as wind highlights may need to be considered. 75 .LONG TERM...Thursday through Sunday...Rather quiet weather is in store during the extended period. On Thursday, a weak system will bring precipitation mainly to the Washington Cascades. Models agree decently on that. All of the deterministic models have backed off even low pops further east, except for the NBM during the day on Thursday. The rest of the extended forecast is dry, with a ridge establishing itself over the western CONUS. By Sunday, the axis of the ridge will move over the Pacific Northwest. Under this pattern, there will likely be fog and/or stratus that will begin to affect portions of the Columbia Basin and nearby areas. Overall, the deterministic models are in excellent agreement through the extended portion of the forecast. High temperatures on Thursday will range from the low 40s to around 50 degrees in central Oregon. Highs on Friday will be a few degrees cooler than Thursday. High temperatures on Saturday will range from the low 40s to the low 50s in central Oregon. Highs on Sunday will range from the upper 30s to the upper 40s. Overnight lows will mainly be in the 20s. && .PRELIMINARY POINT TEMPS/POPS... PDT 43 35 42 27 / 30 0 0 0 ALW 41 36 42 31 / 30 0 0 0 PSC 43 37 43 33 / 10 0 0 0 YKM 43 29 42 30 / 0 0 0 0 HRI 43 36 44 31 / 20 0 0 0 ELN 41 29 39 30 / 0 0 0 0 RDM 47 33 45 28 / 20 10 0 0 LGD 41 30 41 28 / 40 0 0 0 GCD 45 32 45 27 / 40 10 0 0 DLS 48 40 46 38 / 20 10 10 10 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Dense Fog Advisory until noon PST today for ORZ507-508. WA...Dense Fog Advisory until noon PST today for WAZ026-029. && $$ SHORT TERM...75 LONG TERM....77 AVIATION...82