When I see severe weather bulletins issued, it always says that the particular watch/warning is in effect until a chosen time. Severe storms watches/warnings that are issued in he evening are sometimes in effect until midnight, while other storm watches/warnings that are issues early in the day are in effect until a pre determined time in the evening. Why is this? Do meteorologists just choose a time so people will still be on the lookout for severe weather, or is there some way that meteorologists know how long a storm will last?