How a snowman can help you visualize the impacts of wind shear on Tropical Storm Elsa
Wind shear has been a big influence on the evolution of Tropical Storm Elsa over the last day, and a big reason why it was able to weaken from a hurricane back to a tropical storm earlier this weekend.
Something to keep in mind during storm season is that the strongest tropical storms and hurricanes that can develop are often well-organized vertically. By this, we mean that the center of circulation in the low levels of the atmosphere lines up fairly closely with the center of circulation in the mid and upper levels of the atmosphere. When storms organize and align vertically they can become more robust.
On the other hand, when a storm doesn’t have good vertical organization it can run into problems. If the low and mid-levels of circulation don’t line up fairly closely, it can prevent named storms from ever forming. And if an already-formed storm’s vertical alignment gets out of sorts, it can rapidly weaken.
What throws off the vertical alignment of tropical systems?
The biggest culprit is wind shear. This happens when winds in the atmosphere change in speed or direction with height. Oftentimes, wind shear becomes a limiting factor on tropical systems when they encounter an area of faster-moving winds in the mid and upper levels of the atmosphere. When this happens, the integrity of the storm’s vertical organization becomes flawed, and the storm tilts with the mid and low levels of circulation becoming de-coupled.
A way to visualize the concept of wind shear is by picturing a snowman. A snowman has its best chance of standing upright and lasting for a long period of time if it’s built vertically with the low level lining up with the mid-level while aligned with the upper level. The same thing applies to tropical systems; the better they are aligned vertically, the better chance they have of becoming strong systems.
On the other hand, if a snowman is built poorly and the middle and upper parts don’t line up with the bottom, that snowman will start to tilt and ultimately crumble. A similar outcome can occur with tropical systems. When they’re not aligned from the bottom up, they’ll have difficulty staying strong.
In the case of Tropical Storm Elsa, this storm has had difficulty staying aligned because of the fast forward motion that it had earlier in the weekend. At one point Elsa’s forward motion was around 30 mph. That’s a blisteringly fast forward speed for a system in the tropical Atlantic. The forward speed was so fast that the lower parts of the storm were racing out and ahead of the middle parts of the storm, and eventually to such a degree that the system’s intensity started to weaken.
In the last 12 hours though, Elsa has slowed down a lot. As of the latest advisory from the National Hurricane Center the forward speed is down now to 13 mph. If the system had lots of open water ahead of it, this slowing motion could allow it to try to regain strength. Instead, Elsa will be dealing with land today and land weakens tropical systems by removing their fuel source: warm open water.
The eventual impacts we see in Southwest Florida are greatly dependent on how much Elsa is weakened by land interaction with Cuba. This will be an important piece of the story for our local concerns to monitor on Monday and Monday night when the system pops out into the Florida Straits.
Stay up on the latest advisory with Tropical Storm Elsa here.
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