From hurricanes to fire weather, it’s been a busy couple of weather weeks.  So far, Davis has been spared any real impact from the fires here in CA.  It’s been a little smoky, but that’s nothing.  There has been a lot of discussion recently about the weather situation here in CA.  I just wanted to talk a little bit about rainfall. It’s dry here in the Central Valley in the summer. This year has been no exception to that rule, but I have seen several media outlets implying that the current fire weather situation has something to do with unusual dryness this summer.  So, has it been unusually dry this summer?

I pulled 50 years of precipitation data from the weather station at UC Davis’ Campbell Tract. Panel a) in the figure below shows the mean and median rainfall from the Campbell Tract and our rainfall from 2017.  May and June 2017 saw above-median rainfall, but we’ve had no measurable rain since June which is in keeping with the median rainfall.  Panel b) shows the fraction of years with *any* measurable rainfall.  Over 90% of all Julys over the past 50 years have seen zero days with rainfall, and only about half of all Junes and Septembers see even one day with rainfall.  So our not having seen any rain since May is far from unusual even if the headline that we are tied for the driest June-September ever is technically true.  I added panels c) and d) just for fun.  Panel c) tries to show the impact of climate change (in an extremely unscientific way) by comparing median rainfall between 1968-1993 with median rainfall from 1994-2017.  One could conceivably infer that climate change has resulted in wetter springs and longer rainless summers.  Panel d) gets into some daily data.  It shows the percentage contribution to the 50-year total monthly rainfall by individual days.  The most extreme story comes from July where rain has fallen on just 11 July days over the past 50 years!  One day in July 1985 contributed 40% of all July rainfall over 50 years!

So long story short, rainfall here in the Central Valley (in Davis which has admittedly seen no fires) has not been unusual.