Hurricane Humberto strengthened into a powerful Category 5 storm Saturday, a remarkably quick intensification for a system that was a tropical storm Friday morning.

But it's another storm -- known currently as Tropical Depression Nine and likely to become Tropical Storm Imelda this weekend -- that has prompted concern across the southeastern United States. That storm has been lashing the Bahamas and Cuba this weekend.

Because the two systems are so close together, meteorologists have struggled to know exactly what they will do as their energy affects each other. Significant rain is expected across the Caribbean through the weekend, and in the United States, officials from Florida to the Carolinas have begun making preparations for storm surge and strong winds.

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