Tropical Storm Humberto could tangle with another developing storm, with high stakes for the US East Coast

By CNN Meteorologists Briana Waxman and Chris Dolce

(CNN) — Tropical Storm Humberto is spinning in the west-central Atlantic, but the bigger question for the United States is what’s brewing behind it.

Humberto is not a direct threat to the US, but it could rapidly intensify and grow into a major hurricane this weekend as it tracks northwest, most likely staying west of Bermuda by early next week.

All attention then turns to a mess of thunderstorms moving through the northern Caribbean, known as Invest 94L. It’s already drenching Puerto Rico, the Virgin Islands and Hispaniola, and it has a high chance of organizing into Tropical Storm Imelda.

The track of this potential storm and its level of impact on the Southeast coast is more uncertain than usual because of how many different factors are involved, including its potential interaction with Humberto and the jet stream pattern over the US East Coast.

We won’t be confident in the exact track of what’s likely to be Imelda until sometime Friday or Saturday, leaving little time to prepare for a possible impact from a tropical storm or a hurricane on the Southeast coast, which could happen as soon as Monday.

Two storms, two very different scenarios

Invest 94L, the likely storm, is moving through Hispaniola, Puerto Rico and the Virgin Islands Thursday where it could produce flooding and landslides, particularly in areas with steep terrain.

It will then move into an increasingly favorable environment for development as it nears the Bahamas by this weekend, where it’s expected to become a tropical depression or Tropical Storm Imelda.

That sets up a complex scenario where the size, strength, proximity and timing of both it and Humberto will decide whether the Southeast coast of the US gets a brush, a direct landfall or a miss from what will likely be Imelda.

Humberto could tug a weaker, slower-moving storm harmlessly out to sea, keeping any serious impacts offshore. But a stronger, faster-moving storm could resist Humberto’s pull and take a path that threatens the Southeast.

There’s also the chance the two storms get close enough to interact. When that happens, they can do-si-do around a common point in what’s known as the Fujiwhara effect. Time will tell if Humberto will be swinging its partner round and round, flinging it harmlessly out to sea or closer to land.

As if all these variables weren’t enough, the broader weather pattern over the US adds another layer of uncertainty. A dip in the jet stream over the eastern US will play a major role in steering any tropical systems that enter its sphere of influence. If Humberto drifts east and the jet stream’s influence dominates, the future Imelda could be drawn into the Southeast coast.

Monday into Tuesday would be the mostly likely timing for any impacts in the Southeast. The Carolinas have the highest odds for possible impacts right now.

Even if the next storm remains offshore, its tropical moisture could funnel into the Southeast and fuel heavy rain and a flood threat there and in the mid-Atlantic early next week.

For now, the outcome of this story is far from settled. The only thing forecasters can promise this week is that the forecast itself will keep changing.

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