Mid-Winter Dry Spells
 Jan Null, CCM
 updated January 2011
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 60 rainfall seasons (i.e., July 1 to June 30) there has been a "dry" period in December or January averaging 19 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 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 moderate to strong El Niņo events.  The longest dry spell was 42 days in 1962-63, though that season still ended up with 22.15 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 Season Total ENSO Type
1950-51 13 17-Dec   24.00 Weak La Niņa
1951-52 12 6-Dec 1/.08 32.56 Weak El Niņo
1952-53 13 21-Jan   21.10  
1953-54 13 21-Dec 1/.01 14.27  
1954-55 16 14-Dec   15.74 Moderate La Niņa
1955-56 20 28-Jan   27.17 Strong La Niņa
1956-57 33 6-Dec 2/.03 15.04 Weak La Niņa
1957-58 8 6-Dec   36.48 Strong El Niņo
1958-59 12 12-Jan   10.46  
1959-60 13 15-Dec   15.47  
1960-61 36 19-Dec 1/.02 13.87  
1961-62 21 22-Dec   17.65  
1962-63 42 18-Dec   22.15 Weak La Niņa
1963-64 19 25-Dec 2/.08 12.32 Weak El Niņo
1964-65 19 6-Feb   22.29 Moderate La Niņa
1965-66 21 7-Jan 1/.02 16.33 Strong El Niņo
1966-67 30 11-Dec 1/.01 29.41  
1967-68 23 19-Jan   14.46 Weak La Niņa
1968-69 13 24-Dec   25.09 Weak El Niņo
1969-70 12 26-Dec   20.80 Weak El Niņo
1970-71 29 17-Jan   18.79 Moderate La Niņa
1971-72 21 30-Dec   11.06 Weak La Niņa
1972-73 15 24-Dec   34.36 Strong El Niņo
1973-74 11 20-Jan   27.76 Strong La Niņa
1974-75 22 5-Dec 2/.04 18.26 Weak La Niņa
1975-76 25 10-Jan   7.95 Strong La Niņa
1976-77 26 13-Jan   11.06 Weak El Niņo
1977-78 16 20-Jan 1/.02 26.87 Weak El Niņo
1978-79 15 19-Dec   18.74  
1979-80 26 18-Jan   24.49  
1980-81 16 5-Dec   15.39  
1981-82 12 6-Jan   37.10  
1982-83 22 24-Dec   38.17 Strong El Niņo
1983-84 15 31-Dec   22.47  
1984-85 11 27-Dec   20.01 Weak La Niņa
1985-86 21 8-Dec   28.68  
1986-87 15 7-Jan   13.86 Moderate El Niņo
1987-88 28 30-Jan   17.74 Moderate El Niņo
1988-89 11 11-Jan   17.43 Strong La Niņa
1989-90 36 26-Nov   14.32  
1990-91 17 20-Dec   14.08  
1991-92 17 8-Jan 1/.04 19.20 Strong El Niņo
1992-93 14 22-Jan   26.66  
1993-94 20 15-Dec 1/.03 15.22  
1994-95 8 31-Jan   34.02 Moderate El Niņo
1995-96 15 31-Dec 1/.02 24.89 Weak La Niņa
1996-97 20 27-Jan 2/.07 22.63  
1997-98 17 15-Dec 2/.03 47.22 Strong El Niņo
1998-99 21 21-Dec 2/.02 23.49 Moderate La Niņa
1999-00 27 14-Dec 1/.03 24.89 Moderate La Niņa
2000-01 22 16-Dec 1/.03 19.47 Weak La Niņa
2001-02 18 3-Jan 1/.02 25.03  
2002-03 19 24-Jan   23.87 Moderate El Niņo
2003-04 11 10-Dec 1/.04 20.54  
2004-05 17 9-Dec   31.86 Weak El Niņo
2005-06 14 3-Dec 1/.03 34.43  
2006-07 12 5-Jan   16.89 Weak El Niņo
2007-08 10 11-Jan   17.50 Moderate La Niņa
2008-09 18 3-Jan   18.11  
2009-10 12 31-Dec 1/.05 24.09 Strong El Niņo

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