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