358 FXUS66 KMFR 151059 AFDMFR Area Forecast Discussion National Weather Service Medford OR 259 AM PST Fri Dec 15 2023 .SHORT TERM...An upper level ridge will remain over the region resulting in dry weather through Saturday evening. High pressure over the area will result in some valley impacts, including air stagnation conditions as well as areas of dense overnight/morning fog. Radiational cooling and boundary layer moisture has led to fog forming in many west side valleys overnight, including the Umpqua Basin, interior coastal river valleys, Illinois Valley and lower Rogue Valley near Grants Pass and the Scott Valley. Fog has been dense (with visibilities of 1/4 mile or less) for portions of the Umpqua Valley, Illinois Valley and the lower Rogue Valley near Grants Pass. A dense fog advisory (NPWMFR) is in effect for these areas. Conditions are likely to improve in the late morning. Elsewhere, east of the Cascades, patchy fog and low clouds are also present, including in the Klamath Falls area. Fog and low clouds are expected to gradually lift and clear in the afternoon for most areas but will return again Friday evening and night. High pressure over the area will continue to bring strong inversions and light winds across most inland valleys, resulting in limited mixing and air stagnation conditions. These conditions may allow pollutants to increase and lead to deteriorating air quality. An air stagnation advisory (NPWMFR) remains in effect through Saturday morning. Conditions will begin to change later this weekend as an upper low off the coast of California approaches the region. Models and ensembles show good agreement that as the low approaches the area, precipitation will gradually spread inland along the coast and into southwest portions of the area Sunday. The National Blend of Models shows a high 60-80 percent chance of precipitation along the coast and into the coast ranges and western Siskiyou County, with lesser chances (10-40%) further inland. As this low lifts north just off the Oregon coast, chances for precipitation increase across the area Sunday night and Monday but the main focus for periods of moderate to locally heavy precipitation will be along the coast, coast range, and into western and south- central Siskiyou County. Snow levels will remain high though, generally over 7000 feet for most the area (except lower for portions of Lake County). Ahead of this system, southeast winds will increase across the ridges Sunday night into Monday morning, especially for the Cascades, Siskiyous and mountains in Siskiyou County. Gusty winds are also expected to develop in the southern Shasta and southern Rogue Valley (near Ashland) during this period. Then, breezy to gusty southerly winds will develop for many areas during the day Monday, especially over the mountains, along the coast, in Shasta Valley and east of the Cascades. Precipitation chances continue Monday night into Tuesday as this low moves north and another low pressure system moves out of the Gulf of Alaska and becomes positioned off the Oregon and northern California coast. && .AVIATION...15/06Z TAFs...Valley fog, which is dense in some areas, has developed in the Umpqua Basin, including Roseburg (KRBG), in the Grants Pass area, and in the Illinois Valley. Localized fog and low clouds are also present in other valley areas, including near Klamath Falls (KLMT), portions of the Rogue Valley northwest of Medford (KMFR) and in the Scott Valley in California. Fog and low clouds will gradually lift and clear late this morning into this afternoon. Elsewhere, expect VFR conditions through the TAF period. -CC .MARINE...Updated 800 PM Thursday December 13, 2023...Seas will slowly decrease tonight through tomorrow morning, and then remain steady through Saturday with relatively light wind speeds over the waters. By Sunday morning, seas will be dominated by a southerly swell, and will remain steep through Monday morning with wind speeds only increasing over the southern waters. By Monday afternoon, southerly wind-driven seas will combine with a strong southerly swell to produce very steep seas through at least Tuesday morning. At this time, hazards are likely across the waters to start off next week. Hazards could start as early as Sunday for small craft over the outer waters, but hazardous seas or gales are going to be at the forefront of next week. -Guerrero/Hermansen && .PREV LONG TERM DISCUSSION... /Issued 243 PM PST Thu Dec 14 2023/ LONG TERM...Monday Dec 18 to Wednesday Dec 20...The extended forecast begins with the large closed low offshore of the CA Bay Area starting to open up and rejoin the larger scale synoptic flow as it sweeps northwest. This will strengthen precipitation Monday morning and afternoon, mostly in northern CA, as the low pulls moisture northwards. Strong 50-60 kt mid-level (700 mb) winds will lead to continued enhanced precipitation in the Mt. Shasta area, the south and west Siskiyou County mountains, and the coastal Curry County mountains as the terrain enhances precipitation over the southern slopes. Other areas on the northern slopes of the mountains, such as most of south Oregon, will see much lighter precipitation due to downslope drying. Gusty winds in the Shasta Valley are also possible again Monday. The movement of the system from the south will also bring warmer (relatively) air to the area and lead to snow levels rising to 7,000- 8,000 feet Monday. Heavier precipitation, as is expected in the Mt. Shasta area, may lower snow levels locally by evaporative cooling, but little impacts from snow are expected besides occasionally heavy snow over Mt. Shasta itself. Intermittent periods of precipitation will continue into Tuesday. Meanwhile, another low develops in the Gulf of Alaska and dives southward, re-strengthening precipitation late Tuesday into Wednesday. This is another system where the mid-level flow is out of the south, with more precipitation again expected for the areas mentioned above and much lighter precipitation elsewhere. The air mass, meanwhile, is moving in from the northwest and this relatively colder air mass will lead to snow levels lowering. Recent runs of the National Blend of Models suggest that these will decrease less than previously thought, to 6,500-7,000 feet Tuesday morning and further to 6,000-6,500 feet by Wednesday morning. The heaviest precipitation will have tapered off and retreated south into northern CA by early Wednesday, though periods of light precipitation in south Siskiyou and Curry counties are possible into late Wednesday as the low meanders south and then east into southern CA. Ridging starts to build in Thursday, with the majority of ensemble members suggesting dry weather resuming region-wide, though a minority have showery precipitation lingering over the coastal and Cascade mountains. There's a good deal of spread in ensemble members into the end of the week, with about half of the ensemble members suggesting precipitation over the weekend. Christmas Eve has a better chance for ridging and dry weather, though chances for precipitation increase again into late Christmas with some ensemble models bringing a front into the region. With the uncertainty in the forecast into the holiday, keep an eye on the forecast into next weekend, especially if you're travelling. -CSP && .MFR WATCHES/WARNINGS/ADVISORIES... OR...Air Stagnation Advisory until 10 AM PST Saturday for ORZ021>026- 028>031. Dense Fog Advisory until 10 AM PST this morning for ORZ023. Dense Fog Advisory until 10 AM PST this morning for ORZ024. CA...Air Stagnation Advisory until 10 AM PST Saturday for CAZ080- 083>085. PACIFIC COASTAL WATERS...None. && $$ CC