Mid-Winter Dry Spells
Jan Null, CCM
updated February 2006
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 55 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 "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 20 days during "ENSO neutral" years and 21 during La Niņa.

Even the shortest dry spell was 8 days which occurred twice, in 1957-58 and again 1994-95.  Both of these seasons were during moderate to strong El Niņo events.  The longest dry spell was 42 days in 1962-63 which still ended up with 22.15 inches, very near the 53-year normal of 21.73 inches.  All of the 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.

 

San Francisco City Rainfall   (see "Climate of San Francisco" for daily amounts)
Season Consecutive* Dry Days Begin Date *Intervening Days/Amount Season Total ENSO Class
1950-51 13 17-Dec   24.00 La Niņa
1951-52 12 6-Dec 1/.08 32.56  
1952-53 13 21-Jan   21.10  
1953-54 13 21-Dec 1/.01 14.27  
1954-55 16 14-Dec   15.74  
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 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  
1963-64 19 25-Dec 2/.08 12.32  
1964-65 19 6-Feb   22.29 La Niņa
1965-66 21 7-Jan 1/.02 16.33 El Niņo
1966-67 30 11-Dec 1/.01 29.41  
1967-68 23 19-Jan   14.46  
1968-69 13 24-Dec   25.09  
1969-70 12 26-Dec   20.80  
1970-71 29 17-Jan   18.79 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  
1977-78 16 20-Jan 1/.02 26.87 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  
1985-86 21 8-Dec   28.68  
1986-87 15 7-Jan   13.86  
1987-88 28 30-Jan   17.74 El Niņo
1988-89 11 11-Jan   17.43 Strong La Niņa
1989-90 36 26-Nov   14.32  
1990-91 17