Northern California
Mid-Winter Dry Spells
Jan Null, CCM
updated December 2015
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 of winter.

Based on San Francisco's daily rainfall, over the past 65 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 average dry period during seasons with an El Niņo was 17 days, while the average rose to 19 days during "ENSO neutral" years and 21 days during La Niņa.

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 Niņo 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 Niņo 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 San Francisco % of Average ENSO Type
1950-51 13 17-Dec   111% Weak La Niņa
1951-52 12 6-Dec 1/.08 150% Weak El Niņo
1952-53 13 21-Jan   97% Weak El Niņo
1953-54 13 21-Dec 1/.01 66% Weak El Niņo
1954-55 16 14-Dec   73% Weak La Niņa
1955-56 20 28-Jan   126% Moderate La Niņa
1956-57 33 6-Dec 2/.03 69%  
1957-58 8 6-Dec   169% Strong El Niņo
1958-59 12 12-Jan   48% Weak El Niņo
1959-60 13 15-Dec   71%  
1960-61 36 19-Dec 1/.02 64%  
1961-62 21 22-Dec   82%  
1962-63 42 18-Dec   102%  
1963-64 19 25-Dec 2/.08 57% Moderate El Niņo
1964-65 19 6-Feb   103% Weak La Niņa
1965-66 21 7-Jan 1/.02 75% Strong El Niņo
1966-67 30 11-Dec 1/.01 136%  
1967-68 23 19-Jan   67% Weak La Niņa
1968-69 13 24-Dec   116% Weak El Niņo
1969-70 12 26-Dec   96% Weak El Niņo
1970-71 29 17-Jan   87% Moderate La Niņa
1971-72 21 30-Dec   51% Weak La Niņa
1972-73 15 24-Dec   159% Strong El Niņo
1973-74 11 20-Jan   128% Strong La Niņa
1974-75 22 5-Dec 2/.04 84% Weak La Niņa
1975-76 25 10-Jan   37% Strong La Niņa
1976-77 26 13-Jan   51% Weak El Niņo
1977-78 16 20-Jan 1/.02 124% Weak El Niņo
1978-79 15 19-Dec   87%  
1979-80 26 18-Jan   113% Weak El Niņo
1980-81 16 5-Dec   71%  
1981-82 12 6-Jan   171%  
1982-83 22 24-Dec   176% Very Strong El Niņo
1983-84 15 31-Dec   104% Weak La Niņa
1984-85 11 27-Dec   92% Weak La Niņa
1985-86 21 8-Dec   132%  
1986-87 15 7-Jan   64% Moderate El Niņo
1987-88 28 30-Jan   82% Moderate El Niņo
1988-89 11 11-Jan   81% Strong La Niņa
1989-90 36 26-Nov   66%  
1990-91 17 20-Dec   65%  
1991-92 17 8-Jan 1/.04 89% Moderate El Niņo
1992-93 14 22-Jan   123%  
1993-94 20 15-Dec 1/.03 70%  
1994-95 8 31-Jan   157% Weak El Niņo
1995-96 15 31-Dec 1/.02 115% Weak La Niņa
1996-97 20 27-Jan 2/.07 105%  
1997-98 17 15-Dec 2/.03 218% Very Strong El Niņo
1998-99 21 21-Dec 2/.02 109% Moderate La Niņa
1999-00 27 14-Dec 1/.03 115% Moderate La Niņa
2000-01 22 16-Dec 1/.03 90% Weak La Niņa
2001-02 18 3-Jan 1/.02 116%  
2002-03 19 24-Jan   110% Moderate El Niņo
2003-04 11 10-Dec 1/.04 95%  
2004-05 17 9-Dec   147% Weak El Niņo
2005-06 14 3-Dec 1/.03 159%  
2006-07 12 5-Jan   78% Weak El Niņo
2007-08 10 11-Jan   81% Moderate La Niņa
2008-09 18 3-Jan   84%  
2009-10 12 31-Dec 1/.05 111% Moderate El Niņo
2010-11 14 31-Dec   133% Moderate La Niņa
2011-12 49 1-Dec   72% Weak La Niņa
2012-13 16 7-Jan 1/.01 77%  
2013-14 36 12-Dec 2/.08 58%  
2014-15 56 21-Dec 1/.07 84%  

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