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November 19, 2018 – A major change in the weather pattern is coming to NorCal this week which will impact holiday travel. A series of winter like storms will plow through NorCal later this week bringing rain and mountain snow to the region. The first system is set to arrive late Tuesday into Wednesday. Roadways will likely be slick due to previous stretches of drier weather, so motorists should be mindful when traveling especially Wednesday into Thursday morning. As rainfall accumulates over the holiday weekend, ponding water will be possible where storm drains have been clogged by leaves and other debris. Debris flows over any of the recent fires will also become a major concern.

Up in the mountains, accumulating snow is forecast with travel difficulties likely starting Wednesday afternoon and continuing through Saturday. Gusty winds and accumulating snow will make for treacherous conditions over the mountains with whiteout conditions being possible at the passes Thursday into Friday.

Dry with near to above normal daytime temperatures and cool nights through Tuesday with smoke in the valley due to the Camp Wildfire. Cooler and wetter weather is expected by midweek and for the holiday weekend. Snow over the higher elevations of the Sierra for possible travel difficulties during Thanksgiving travel weekend.

Discussion

Another quiet day is expected as high pressure holds over the region. This will lead to another day of hazy and smokey skies as winds will remain light. The good news is that these conditions will begin to mix out on Tuesday as high pressure shifts to the east due to a trough digging towards the West Coast. Conditions will remain mostly dry throughout the day on Tuesday, with rain arriving to the coastal mountains Tuesday night. Rain will then spread eastwards arriving to the Valley and Sierra by Wednesday morning. Snow levels will gradually be decreasing during this time frame and will fall to pass level by late afternoon or early evening hours. For those commuting over the Sierra Wednesday evening, accumulating snow is looking likely for the passes.

Thanksgiving Day will be another wet day, as Wednesday’s system exits the region and a second trough pushes into the region. Models show a lull between each system; however, the chance for rain and accumulating snow at high elevation passes will be possible throughout the day. Winds with the second system will stronger than the first, especially over the ridges. This combined with snow, could lead to white out conditions at times.

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Thinking about fire weather concerns, the good news with this event is that it will have enough rain to end new wildfire concerns. The bad news is it will be the start of watching for debris flows over the recent fires.

Extended Discussion (Friday through Monday)

Potentially stronger system moves through the region Friday. Periods of moderate to heavy precipitation are expected. This will likely cause local ponding on roads with clogged storm drains. Commuters or Thanksgiving holiday travelers should allow extra time for driving. Less unsettled weather is expected over the weekend into early next week.

Newly burned areas will likely see ash flow. The major question is whether rain intensity could reach levels sufficient to cause more hazardous debris flows. While rain amounts should be significant, this system may lack enough convective aspect to bring heavy enough rain. Uncertainty is high now, but the potential should become as we get closer to the event.

Storm total rainfall amounts Friday thru early Saturday could range up to 1 inch in the Valley with 1-2 inches in the foothills and mountains. Accumulating snowfall could be significant across the higher elevations with 12 to 18 inches possible across the northern Sierra passes, so plan ahead for wintry driving conditions in the mountains and possible travel delays.