La Niña likely before end of the year, this could mean dry winter for Arizona

Forecast models show a La Niña forming by fall of this year as Sea Surface Temperatures (SSTs) in the East Pacific Ocean continue to decline. These SSTs define El Niño and La Niña. The warmer the water; the stronger the El Niño. La Niña defined by cooler-than-average water temperatures. Generally, a La Niña during the cooler months of the year lead to a dry winter in Arizona.

The below image shows the latest SST forecast from the International Research Institute for Climate and Society's (IRI). The '0.0' line indicates average SST with the El Niño threshold marked at '0.5' or above. The current El Niño index, which is indicated by the black dot, shows the SST is sitting well above that threshold. The current index is calculated from the previous three months of SST observations. The thinner, rainbow colored lines represent different computer models that forecast El Niño and La Niña trends. The thicker yellow line is the average of all the dynamical model data. The greenish line is the average of all the statistical model data. The letters at the bottom represent the months. For example, 'MAM' represents March, April, and May. In the forecast, you can see that the yellow line drops through spring and into summer. This trend indicates the current El Niño is forecast to rapidly weaken through the year, with a La Niña in the forecast by fall.

The current El Niño is already among one of the strongest on record. Will it take over the top spot currently held by the 1997/98 El Niño? Only time will tell. The numbers are measured through spring of this year and it is the accumulation of those numbers that will determine if a new record is set. The table below shows the current El Niño index measurements compared to those of 1997/98. The measurements are averaged over three months. For example, FMA stands for February, March, and April. An El Niño occurs when the index numbers are 0.5 or higher. You can see this El Niño started earlier than the one in 1997/98. Plus, SSTs earlier in 2015 were slightly higher than the measurements for the same time in 1997. However, measurements during the summer, while strong, did not remain stronger than the 1997/98 event. As we head to towards the end of this El Niño, the numbers once again remain slightly stronger than the 1997/98 event.

Measurement Months	1997/98 El Niño	This Year's El Niño
FMA	-0.2	0.5
MAM	0.1	0.7
AMJ	0.6	0.9
MMJ	1.0	1.0
JJA	1.4	1.2
JAS	1.7	1.5
ASO	2.0	1.7
SON	2.2	2.0
OND	2.3	2.3
NDJ	2.3	2.3
DJF	2.1	2.2
JFM	1.8	2.0

The graph below shows the measurements of El Niño and La Niña events since 1950. La Niña is defined by cooler than average SSTs. You can see the most powerful El Niño events were in 1972/73, 1982/83, and 1997/98. The most powerful La Niña occurred in 1973/74.