November 21, 2018 – Cooler and wetter weather spreads into the area by today and continues into early Saturday. Newly burned areas could experience ash/debris flows. Snow over the higher elevations of the Sierra will make travel difficult for the Thanksgiving travel period.
Satellite imagery this morning shows the first of a series of storm systems pushing towards the West Coast with the radar showing some light rain over the coastal range. This will continue to advance eastwards and eventually bring widespread rain to the area later this afternoon and evening. Snow levels will be falling during this time frame as well, with the Sierra seeing accumulating snow around 6000 feet. These conditions will make driving conditions hazardous so motorists should take extra precautions if they must go out.
As we head into Thanksgiving, the first storm system will be exiting the region with a second and more potent system arriving late in afternoon and into the evening hours. Periods of heavy rain and mountain snow are expected with this system in addition to gusty southerly winds. These conditions will continue as we head into Friday, so travelers will once again have to take extra precautions as these conditions will make driving difficult. By late Friday night and into Saturday morning, the low will be exiting the region. Showers will likely linger over the Sierra and its associated foothills, but elsewhere conditions should be dry. Finally, we`re still concerned about the newly burned areas as they will likely see debris/ash flow from moderate rain. The major question is whether rain intensity could reach sufficient levels to cause more hazardous debris/ash flows. 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.
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Extended Discussion (Sunday through Wednesday)
Drier weather is expected into early next week as high pressure builds into California. A weak system could bring a few showers over the northern mountains on Sunday, but guidance has trended drier with this feature, so only light precipitation amounts are expected. A return of precipitation is likely during the second half of next week across interior NorCal. However, model uncertainty is evident in terms of precipitation amounts, and timing, with the latest ECMWF run being quicker than the GFS. Regardless, widespread precipitation, locally gusty winds, and cooler temperatures could be possible. Seasonal temperatures expected across the area during the extended period, with Valley highs in the mid 50s to low 60s, and 40s to 50s over the mountains.