Overview:  The criteria for a La Niña event is when the Oceanic Niño Index (ONI) cools to  -0.5 or less for five consecutive overlapping 3-month "seasons". For this analysis I have limited the period used to the rainiest months in California (i.e., Sept-Oct-Nov through Jan-Feb-Mar). There were 8 seasons that were in the -0.2 to -0.4 ONI "cool" category and 10 seasons in which the ONI ranged from -0.5 to -0.7 as weak "La Niñas". Seasonal (July 1 through June 30) percent of normal precipitation for each category, was collated for a number of cities and geographic areas. These include the seven NCDC climate divisions (map) plus the entire state of California (CA), the three major watershed areas [i.e., the Northern Sierra Nevada Eight Station Index (8SI, map), the Southern Sierra Nevada Five Station Index (5SI, map) and the Tulare Basin Six Station Index (6SI, map)] plus five major cities.   Observations:  The average "signals" associated with both the cool neutral seasons and weak La Niña events are wetter in the Northern and Central California. And it should be noted that the middle third of the state sees a larger ranger between the cool neutral season and weak La Niña seasons.  Central California also showed a broader range between the wettest and driest seasons within each category.   Very Important Caveat:  The data presented here is a climatology of past events and is made up of a broad range of events from a small relatively sample that are averaged together.  Consequently, like any climatology, this type of data should not be used as a forecast but rather a context for possible future events.

Percent of Normal Precipiation

Table 1.  Average Percent of Normal Seasonal Precipitation Ranked by ONI

ONI		Div. 1	Div. 2	Div. 3	Div. 4	Div. 5	Div. 6	Div. 7	CA	8SI	5SI	6SI	SF	EKA	SAC	FAT	LAX
-0.2 to -0.4	Avg.	97%	98%	108%	95%	99%	83%	72%	95%	99%	99%	101%	92%	95%	104%	92%	83%
	Min	55%	57%	65%	44%	47%	35%	32%	54%	56%	46%	48%	53%	52%	53%	42%	30%
	Max	118%	125%	138%	134%	136%	128%	110%	121%	130%	136%	161%	124%	125%	142%	130%	147%
-0.5 to -0.7	Avg.	95%	91%	94%	82%	88%	75%	76%	89%	93%	93%	87%	85%	96%	85%	80%	82%
	Min	59%	66%	58%	48%	56%	50%	49%	69%	62%	57%	53%	47%	57%	50%	46%	48%
	Max	138%	140%	133%	120%	125%	99%	95%	127%	148%	132%	117%	146%	146%	127%	127%	120%

Table 2.  Percent of Normal Precipitation Ranked by ONI  (-0.2 through -0.7)

	Normal (in.)	49.96	36.98	20.75	22.21	20.99	18.03	6.37	23.18	54.52	42.57	30.50	23.65	40.33	18.52	11.50	14.93
Avg. ONI	Season	1	2	3	4	5	6	7	CAL	8SI	5SI	6SI	SF	EKA	SAC	FAT	LAX
-0.2	1961-1962	81%	85%	98%	96%	100%	119%	89%	91%	83%	95%	95%	75%	72%	94%	99%	126%
-0.2	2001-2002	93%	86%	75%	72%	76%	35%	32%	77%	86%	79%	82%	106%	101%	97%	61%	30%
-0.3	1966-1967	109%	122%	138%	134%	136%	128%	89%	120%	115%	132%	161%	124%	110%	142%	130%	147%
-0.3	1962-1963	116%	115%	129%	109%	103%	61%	58%	105%	117%	103%	97%	94%	109%	136%	101%	56%
-0.3	1956-1957	89%	81%	91%	74%	78%	76%	65%	81%	82%	76%	74%	64%	91%	74%	71%	64%
-0.3	1985-1986	114%	125%	137%	121%	131%	115%	110%	121%	130%	136%	140%	121%	97%	128%	129%	120%
-0.4	2013-2014	55%	57%	65%	44%	47%	42%	63%	54%	56%	46%	48%	53%	52%	53%	42%	41%
-0.4	1996-1997	118%	115%	133%	111%	118%	86%	66%	111%	125%	127%	112%	96%	125%	107%	104%	83%
-0.5	2005-2006	138%	140%	133%	120%	125%	92%	83%	127%	148%	132%	117%	146%	146%	127%	127%	88%
-0.6	1967-1968	79%	76%	77%	62%	66%	76%	95%	76%	70%	57%	59%	61%	70%	64%	63%	111%
-0.6	1954-1955	68%	68%	61%	79%	76%	78%	71%	71%	67%	66%	76%	67%	83%	81%	89%	80%
-0.6	1974-1975	103%	92%	100%	91%	93%	92%	74%	94%	92%	103%	94%	77%	99%	86%	67%	96%
-0.6	1983-1984	111%	107%	118%	77%	93%	63%	88%	99%	109%	107%	94%	95%	120%	78%	66%	70%
-0.6	2008-2009	69%	77%	75%	70%	75%	67%	78%	74%	86%	92%	84%	77%	74%	88%	68%	61%
-0.6	1964-1965	116%	112%	118%	96%	107%	81%	69%	106%	116%	121%	102%	94%	101%	99%	97%	92%
-0.7	1950-1951	111%	108%	116%	86%	105%	56%	62%	100%	117%	116%	117%	101%	115%	95%	90%	55%
-0.7	1971-1972	92%	67%	81%	48%	56%	50%	49%	69%	65%	66%	53%	47%	98%	50%	46%	48%
-0.7	2000-2001	59%	66%	58%	89%	80%	99%	93%	72%	62%	69%	74%	82%	57%	81%	92%	120%