The featured chart presents monthly average temperature departures for Ames with the monthly Niño 3.4 index values since the start of 2018. The value of this index drives the designation of either La Niña (negative value) or El Niño (positive value) state with values between -1 and 1 considered as "weak". So we have spent much of 2025 in a weak La Niña state with the present forecast not calling any additional strengthening, so remaining weak. Much is made of using this index to make monthly/seasonal scale forecasts for weather over the globe due to the importance of the equatorial Pacific Ocean basin for global atmospheric circulations. As the chart shows, it is certainly not a simple relationship between this index and monthly temperature departures for a location in the center of Iowa!

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