Mid-Winter Dry Spells
Jan Null, CCM
updated February 2018
|On almost as regular basis as the
swallows returning to San Juan Capistrano an extended period of dry
weather likes to return to Northern and Central California in the middle
Based on San Francisco's daily rainfall, over the past 67 rainfall seasons (i.e., July 1 to June 30) there has been a "dry" period in December or January averaging 20 days. For practical purposes a "dry period" is defined here as consecutive dry days with no rain, or consecutive days broken by no more than two non-consecutive intervening days of very light (* i.e., ≤ 0.08 inches) rain].
The shortest dry spell was 8 days which occurred twice, in 1957-58 and again in 1994-95. Both of these seasons were during El Niño events. The longest dry spell was 56 days in 2014-2015, when there was 18.19 inches. All of these dry periods began in December or January with the exception of the 1964-65 period which was 19 days but did not begin until February 6th.
Even the very wet El Niño seasons of 1982-83 and 1997-98 had intervening dry spells and 22 and 17 days respectively.
The bottom line is that an extended period of dry weather is the rule and not the exception in the middle of almost every winter as the atmosphere settles into a two to three week period of equilibrium dominated by high pressure.
|Season||Consecutive *Dry Days||Begin Date||*Intervening Days/Amount|
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