We’ve gotten so used to seeing storms quickly spin up in the Gulf that it’s easy to forget it doesn’t always happen.
Take this week’s disturbance we’ve been tracking, for example. It never had more than a 10% chance of developing into a tropical depression or storm. One of the biggest reasons? Wind shear.
What is wind shear?
In simple terms, wind shear is a change in wind speed or direction with height in the atmosphere. For tropical systems, that’s usually bad news.
To grow and organize, a tropical system needs calm, consistent winds from the surface up to around 5,000 feet. When winds at different levels blow at different speeds or in different directions, it disrupts the storm’s structure, essentially blowing the top off the storm and keeping it from strengthening.
In this case, the disturbance in the Gulf was moving east to west, but upper-level winds were blowing west to east. That strong wind shear made it nearly impossible for the system to come together.
This is a video I put together a couple of years ago about what it takes for a hurricane to form. I talk about wind shear at :58 seconds.
The system in the Gulf:
Since Tuesday night, this cluster of thunderstorms never had more than a 10% chance of developing. It was dealing with some dry air coming in from the north, but the bigger issue was strong wind shear over the Gulf, making it tough for the system to organize or strengthen.
What wind shear looks like:
At KPRC 2, we use state-of-the-art weather technology to track wind shear in real time. This allows us to see not only where wind shear is currently strongest, but also where it’s headed.
This week, wind shear over the Gulf was especially pronounced. In the image below, areas in red highlight zones of strong wind shear, exactly what kept this system from organizing into something stronger.
Exceptions to the rule:
While wind shear usually works against tropical development, there are a few exceptions.
In some cases, weak wind shear can actually help a storm. If the shear is blowing in the same direction as the storm is moving, it can help ventilate the system and even speed up its rotation, creating a more favorable environment for strengthening.
Also, light to moderate shear can help move a storm system into a region where conditions are better for development.
And once a hurricane is well-organized, it can sometimes hold its own against shear, but only up to a point. If wind shear is under 45 mph, the storm may survive with minimal disruption. But anything stronger can start to tear the storm apart.
