The ubiquitous "forecast cone" maps contain a huge amount of information, but they're often misused. And I'm not just talking about sharpies.
Here's the current map for Marco.
People in the news media, who really ought to know better, often describe this as showing that the eye will make landfall just west of Houston on Tuesday, and that the shaded areas will be hit by the tropical storm.
This is wrong.
The small circles are the central point of a range of possible eye positions, with 2:1 odds that it will be somewhere within the "cone zone". That means there's a 1 in 3 chance that the eye itself will make landfall south of Brownsville (tip of Texas) or eastern Louisiana. If that happens, the storm impacts will reach well down the Mexican coast or east to the Florida panhandle.
When you look at this cone, the easiest interpretation is that the eye will probably hit somewhere inside the cone, or close to it, probably within a day of the estimated time.
Formally, each point on the map is the center of a circle that indicates 67% probability that the storm will be in that circle, and the cone is created by blending the circles together. On a five day forecast, odds are about 60% (based on recent years' forecast accuracy) that the eye will stay in the cone for the entire track.
The small circle's color and letter convey more information. Black circles indicate a tropical storm (This doesn't mean it's in the tropics, it means it's getting energy from rising humid air in the center, rather than a clash of weather fronts in an extra-tropical storm.) The letters are intensity, for Depression, Storm, Hurricane or M for Major Hurricane.
So if this is just predicting the eye, where will the storm conditions hit? Next slide...
This shows the odds that each point will experience sustained winds over 39 mph, aka tropical storm force. Dallas is around 10%, which is pretty high for us. Houston is between 30 and 40%. Notice that areas to the right of the storm track (if you're facing the direction the storm is moving) get higher wind probabilities. This is because they're getting the force of the storm's rotation, plus the speed of the storm's motion. On the left side, the storm's motion subtracts from the rotation wind speed.
This map is a bit unusual because it shows the composite risk from two separate storms, so the bulk of Louisiana's risk is from Laura following behind Marco.
I think this format is much better for risk assessment.
This is Laura's latest forecast cone. Unlike Marco, Laura is expected to reach hurricane strength after it leaves Cuba. Note that part of the Florida keys are inside the cone, it's entirely possible the eye will hit keys and southern Florida will take a hard hit (being on the right hand side of the storm). I'm seeing lots of reports that Laura is going to "miss Florida", and that is NOT what this map says.
The eye could also hit Houston...or make landfall in Alabama, and a 1 in 3 chance that it will hit west of Houston or in the Florida panhandle.
This map is useful for planning evacuations. It shows the earliest reasonable arrival time of 39 mph+ winds, which make evacuation difficult or impossible due to fallen trees or flooding.
This is for Marco alone, and the range is from Brownsville to Alabama.
Laura is likely to make landfall a day or two after Marco, and its track is getting more similar to Marco's.
Tropical storms can interact if they get close enough, especially if they're slow moving and in tropical latitudes. Marco and Laura aren't expected to be terribly powerful, and they'll be out of the tropics by the time they're near each other, interacting with continental weather systems as well.
In most cases, the interaction would involve the two storms rotating counter-clockwise around a common center. It's possible for storms to merge, but it doesn't form a "megacane", there's a lot of destructive feedback as the storms' winds are in opposite directions as they approach.
In summary, expect a pair of tropical storms and weak hurricanes, nothing extreme like Maria or Katrina - but their proximity could be interesting to watch.