I mentioned something below about “eyewall replacement cycles,” and I wanted to attempt an explanation of what I’m talking about. First, take a look at the infrared satellite images below of Hurricane Frances, which were taken today at 4:15 PM EDT and 9:45 PM EDT, respectively:

First, a basic orientation: the darker-colored cloud tops are colder, which means they’re higher in the atmosphere, which means the thunderstorms there are more intense. The colors, of course, are artificially added to make the contrasts easier to see. The eye itself is an area of cloudless calm; the surrounding eyewall is the region where the strongest winds and the most intense rains occur.

Now then, you’ll notice that in the second (later) image, the eye is substantially larger than it was in the first image. Normally, if a hurricane’s eye is getting bigger, that means it’s weakening; the pressure is rising, and the storm become less “tightly wrapped.” (Occasionally, intense hurricanes have huge eyes, but as a rule, a small eye indicates a strong storm.)

In this case, however, what we’re seeing is not a sudden weakening, but an eyewall replacement cycle — quite common in Category 3, 4 and 5 hurricanes — wherein the “inner eyewall” basically collapses in on itself, briefly producing the extremely tiny eye of the first image, which then falls apart and is replaced by the larger eye of the second image (previously the “outer eyewall”; you can actually see a hint of it in the lighter-colored cloud tops north of the eye in the first image).

Eyewall replacement cycles can produce temporary weakening, though the extent to which that occurs varies from storm to storm; in this storm, it seems to have been rather mild so far. Sometimes a temporary eyewall-replacement-triggered weakening trend can become more permanent, especially if it occurs concurrently with other unfavorable conditions (e.g., if dry air is sucked into the storm’s inner core). This is one reason why forecasters have so much trouble predicting the intensity of storms like these even a few hours into the future, let alone days.

On the other hand, once a replacement cycle is completed and the new eye and eyewall are fully formed, an intensification phase — sometimes a rapid one — can occur. A storm such as Frances can potentially replace its eyewall, drop its central pressure, and gain 20 mph in wind speed within a few hours. I’ve seen it happen before. (Remember how Charley strengthened from a Category 2 to a Category 4 in a matter of hours with no warning at all? I’m not sure if that’s what happened — I was on a desert island at the time, not watching TV or the Internet — but I’m guessing probably so.)

But again, whether such intensification occurs after an eyewall replacement cycle varies from storm to storm, and from cycle to cycle.

Mind you, I’m a layman, not a meteorologist, so I may be wrong on some of the details here. But this is my understanding, based on what I’m picked up from watching many, many Tropical Updates on The Weather Channel over the years. :)

I know this for sure: the processes that govern both eyewall replacement cycles and rapid intensification phases are very poorly understood by meteorologists, and so are almost impossible to predict. Monster storms like this really create their own environments, and are unlike any other weather phenomena we encounter on Earth. They’re remarkable, beautiful, awe-inspiring — and terrifying, if you’re in their path.

Say a prayer tonight for the people of the Bahamas and the southeastern United States, especially Florida.