November 20, 2018 – Dry with near to above normal daytime temperatures today with smoke in the valley due to the Camp Wildfire. Cooler and wetter weather starts tomorrow and will continue through the holiday weekend. Snow over the higher elevations of the Sierra will make travel difficult for the Thanksgiving travel weekend. Winter Storm Watch from Wednesday afternoon through Friday morning.
Satellite imagery this morning shows a Pacific trough digging into Baja California which will help nudge the persistent West Coast ridge eastwards. Behind this ridge is another area of low pressure which will slowly be advancing towards the region. Rain is forecast to spread into the Coastal Range late tonight and into tomorrow morning and eventually encompass all of NorCal by tomorrow afternoon and evening. With that being said, commuters and holiday travels are urged to allow extra time for travel since this is the first widespread rain of the season and roads will be particularly slippery due to an accumulation of oil through the summer and early fall.
In addition to our wet roads in the valley, those that need to travel over the Sierra tomorrow should be mindful of the snow levels since they are forecast to fall to around 6000 feet in the evening hours. Accumulating snow is forecast at these levels, thus the Winter Storm Watch remains in effect from tomorrow afternoon through Friday morning.
Thanksgiving Day we will see our first system exiting, with another and more potent storm arriving in the afternoon and evening. There may be a brief lull in the intensity of rain during the day, but it will remain possible through the day with some light snow continuing through the mountains. Come Thursday afternoon and evening, southerly winds will be ramping up and become quite breezy which will then last into the overnight hours.
Periods of moderate to heavy rain are expected, with the heaviest expected Thursday night into early Friday for areas north of I80. Snow impacts will be a problem for those needing to travel, thus we extended the Winter Storm Watch through Friday morning.
Conditions will gradually improve on Friday as the low exits the region and snow levels start to rise.
Newly burned areas will likely see ash flow from moderate rain. The major question is whether rain intensity could reach levels sufficient to cause more hazardous debris flows as a cold front moves inland. There is still quite a bit uncertainty, but there is enough potential to keep the Flash Flood Watch in effect for debris flows at the Mendocino Complex, the Carr, Delta, Hirz fires and the Camp Fire. Flooding outside of the burn areas is not expected, except for local ponding on roads with clogged drains.
Extended Discussion (Saturday through Tuesday)
Drier weather is expected by Saturday afternoon as the trough shifts eastward into the Great Basin. Then, ridge of high pressure is expected to build into CA, bringing dry conditions into early next week. A weak system moves north of the forecast area late Saturday into early Sunday, and might bring some light precipitation mainly over the northern mountains. Model guidance indicates the possibility of additional precipitation moving across interior NorCal by the middle of next week, but model differences exist in terms of timing and precipitation amounts. Forecast highs will be in the mid 50s to low 60s across the Central Valley, and 40s to 50s over the mountains.