Wrong: The 50-year return period wind speed of 55 m/s occurs once every 50 years.
Wrong: A wind speed of 60 m/s has been recorded recently in this area. Therefore the 55 m/s 50-year return period wind speed is now wrong.*
Right: The 50-year return period wind speed of 55 m/s has a 63% chance or probability of occuring or being exceeded at least once in the next 50 years.
A 50-year return period event (say wind storm or earthquake) is also called an event with a 50-year recurrence interval, or a 1-in-50 year event.
This all actually means it has a 1/50 (2%) probability or chance of occurring in 1 year.
But, as mentioned above, it only has a 63% chance of occurring in the next 50 years.
It actually has a 37% chance of NOT occuring in the next 50 years.
Therefore, no one can say it WILL happen ONCE in a span of 50 years.
It CAN happen ONCE in 50 years.
It also CAN not-happen ONCE in 50 years.
It also CAN happen MORE THAN ONCE in 50 years.
It CAN happen tomorrow.
It CAN not-happen at all.
The probability is based on STATISTICAL ANALYSIS, based on the available data and data quality, and based on the modelling approach used for analysis.
It is a PROBABILITY and not a PREDICTION.
*To be discussed further in the next Engineering Misconceptions