When must an alternate airport be specified in an IFR flight plan?

An alternate airport must be specified in an IFR flight plan when forecasted weather at the destination airport is below minimums. This is crucial for ensuring safety during flight operations; if weather conditions at the destination airport are not suitable for landing, having an alternate allows the pilot to divert and land safely at a different airport. FAA regulations typically require that if the weather at the destination is not forecasted to meet VFR (Visual Flight Rules) minimums at the time of arrival, a suitable alternate airport must be designated on the flight plan. This helps to mitigate risks associated with unexpected weather changes and ensures that pilots have a plan in place for safe continuation of the flight.

While other options might suggest conditions under which an alternate is useful or advisable, they do not align with the specific regulatory requirement tied to weather conditions at the destination airport.