Elevated Fire Danger

Tuesday night into Wednesday we have an elevated risk of fires for parts of the Bay Area.  Fires have not been a major concern across California this year due to last winter and spring being very wet, with generally cooler than average temperatures this summer.  But in autumn, while much of the country is cooling down, the wildfire risk can reach its peak in California.  In many years, the wet season does not begin until November, meaning the ground and vegetation have been dry for several months.  Also in autumn the marine influence weakens, with less fog and lower relative humidities.  The fire danger is highest when winds turn offshore (usually they are onshore, hence, the persistent overnight/morning fog).

Tuesday night and Wednesday, the weather pattern will lead to offshore flow, due to a strong surface high over Idaho and the Great Basin with lower pressure over coastal California.  This is indicated on the model forecast for early Wednesday morning.  The yellow lines are isobars, or lines of equal pressure.  When isobars are packed closely together, such as over northern California, winds are stronger.  In this case, winds will be quite gusty from the northeast, especially in the hills of the North and East Bay.  Air originating from the Great Basin is already quite dry, and becomes even more dry as the air descends the Sierra Nevada.  By the time that air arrives here, relative humidities are very low - perhaps down to just 10%.  The combination of these gusty, dry winds combined with sunny, warm weather will result in the enhanced fire danger.  Fortunately, the rains we received earlier this month have resulted in some greening of the vegetation, which will somewhat mitigate the fire threat.  Also, temperatures will not be excessively warm.  This weather pattern also leads to Santa Ana winds in southern California.  Santa Ana winds are also warm and extremely dry, and are accelerated through the valleys connecting the California deserts with the coast.

Otherwise, the longer range models are keeping the storm track well to our north into next week, which will keep us dry with slightly above normal temperatures.