1 It is necessary to bring an umbrella when ...
2 It will be necessary to bring an umbrella when ...
From my interpretation, #1 is a definite requirement. The speaker is probably some authoritative person i.e. experienced or has some knowledge of the situation to offer that advice.
In #2, the speaker is trying to be tactful when imparting the advice. Rather than forcing the issue; the speaker leaves it up to the recipient to decide whether or not to bring an umbrella.
Your comments on my comments ?
H5N1 virus carrier