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.1 It is necessary to bring an umbrella when ...
2 It will be necessary to bring an umbrella when ...
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
2 could have various uses, including the one you suggest. It could be used for emphasis or to denote a change in circumstance.