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".
(July 1 through June 30) percent of normal precipitation for each
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.
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.