2 0 1 2 – 2 0 1 6 , ▁a ▁blocking ▁region ▁of ▁atmos pher ic ▁high ▁pressure ▁over ▁the ▁eastern ▁Pacific ▁Ocean , ▁between ▁Al aska ▁and ▁the ▁Pacific ▁North west , ▁diver ts ▁the ▁jet ▁stream ▁and ▁its ▁flow ▁of ▁cold ▁front s ▁far ▁to ▁the ▁north ▁through ▁Canada ▁and ▁down ▁into ▁the ▁U . S . ▁Mid west , ▁leaving ▁the ▁U . S . ▁west ▁coast ▁and ▁especially ▁Los ▁Angeles ▁under ▁warm ▁and ▁dry ▁conditions ▁for ▁weeks ▁to ▁months ▁at ▁a ▁time . ▁ ▁Because ▁the ▁ra iny ▁season ▁begins ▁in ▁late ▁fall ▁and ▁ends ▁in ▁early ▁to ▁mid ▁spring , ▁precip itation ▁is ▁measured ▁using ▁the ▁water ▁year ▁instead ▁of ▁the ▁calendar ▁year , ▁to ▁give ▁an ▁accurate ▁picture ▁of ▁each ▁ra iny ▁season ' s ▁precip itation ▁amounts . ▁Each ▁water ▁year ▁begins ▁October ▁ 1 ▁and ▁ends ▁the ▁following ▁September ▁ 3 0 , ▁during ▁the ▁dri est ▁part ▁of ▁the ▁year . ▁ ▁Sometimes ▁ra inf all ▁can ▁occur ▁in ▁the ▁summer ▁from ▁west ward - st ray ing ▁mon so on ▁th under st orm s , ▁but ▁this ▁is ▁unusual . ▁Even ▁less ▁common ▁is ▁rain ▁from ▁rem n ants ▁of ▁diss ip ating ▁eastern ▁Pacific ▁h urr ican es . ▁Los ▁Angeles ▁aver ages ▁only ▁ ▁of ▁precip itation ▁per ▁year , ▁and ▁this ▁is ▁lower ▁at ▁the ▁coast ▁and ▁higher ▁in ▁the ▁mountains ▁and ▁fo oth ill ▁cities . ▁Rain ▁is ▁typically ▁brought ▁with ▁when ▁High ▁Press ure ▁systems ▁in