From Longman Dictionary of Contemporary Englishwhen/if the need ariseswhen/if the need arises (also should the need arise formal)HAPPEN when or if it is necessary Should the need arise for extra staff, we will contact you. → arise
Examples from the Corpus
when/if the need arises• They are ready to fight if the need arises.• Alterations to your flight details sometimes occur for operational reasons and we reserve the right to make these if the need arises.• Families, too, are a great source of help and are roped in when the need arises.• Her powers seem curiously independent of age, and she can call upon extraordinary sources of energy when the need arises.• In fact, they could prop up the Conservative Government for a fifth term, if the need arises!• The other side of this coin is an impressive surge capability on hand when the need arises.• They remain like this motionless with the woman stemming any premature ejaculatory urges by squeeze control, if the need arises.• As and when the need arises, sub-committees will be established to consider specific environmental issues. 