The strength categories are not officially defined (unlike the thresholds for when we declare a La Nina or El Nino.) But the informal thresholds we use for calling an event weak, moderate or strong are as follows:

Anomalies:

0.5 - 1.0: Weak

1.0 - 1.5: Moderate

> 1.5: Strong

(make these anomalies negative for La Nina)