Marco is not longer a tropical system, defeated by strong southwesterly shear that literally tore the system apart. With Laura moving into the Gulf as well, what makes the situation with Marco different from what we are expecting with Laura?
Marco dealt with wind shear for most of its life.
Wind shear is simply a change in speed or direction of wind with height.
We have to remember that the atmosphere is three dimensional, and as such, storms are also three dimensional.
Storms will feel the wind at different levels, and if they are not from the same direction, it causes the storm to be pushed in different directions at different heights, which is detrimental to weak and organizing storms.
For Marco, winds at the surface were out of the southeast while winds were much stronger at the middle portion of the atmosphere (about 25 to 40,000 feet, where airplanes fly) from the southwest, causing winds to compete against steering the storm.
The winds were strong enough that the center of Marco actually broke in two, a mid-level center and the surface center.
The mid-level center followed the southwesterly flow and moved northeast. The surface center followed the southeasterly flow and moved to the west-northwest. The result was a decoupled storm that quickly weakened.
Now the question is why won’t the same thing happen with Laura? Laura will encounter a completely different environment as it moves through the Gulf. An area of high pressure will set up directly over the center of the Gulf, where Laura will be moving. Tropical systems love to be underneath areas of high pressure, and actually create one on their own when they are strong enough, as this allows for strengthening. So with little to no shear for Laura to worry about, it will be in a prime environment to strengthen.