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 Niño was 17 days, while the average rose to 20 days during "ENSO neutral" years and 21 during La Niña.

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 Niño 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 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.