Sunday, February 12, 2006

Column 1: Rita (originally posted September 21, 2005)

Hurricanes are a part of life on the Gulf of Mexico coast. We heard some stories about how some people didn't want to evacuate for Katrina--or how they couldn't afford to, on account of their already having had to evacuate this year. Now they're taking no chances. Evacuation orders are pending, but I have no doubt mandatory evacuations will occur for the area around Houston-Galveston. Houston? Isn't that where many of the evacuees from the Superdome went? I kind of think it is. Geez, these people can't seem to catch a break.

Forecasting tropical storms is different from forecasting other weather, yet in some ways it's so similar it's spooky.

When we're forecasting the weather, we always think process. What's going on physically, what's going to change, and how will that affect the outcome of what we're forecasting? This is known in some circles as ingredients-based forecasting.

In the case of hurricanes, there are a two main ingredients we search for. The first is high oceanic heat content. Tropical storms thrive over waters with sea surface temperatures (SSTs) of 26.5 degrees Celsius or warmer. The second ingredient needed is light tropospheric winds, or low wind shear. The lower, the better. Low-shear environments are characterized by a lack of fronts and the presence of upper-level ridges.

So let's examine the ingredients present in Rita's environment.

SST - Rita is travelling over waters with SSTs in the 30 to 31 degree range. These temperatures are similar to those present when Katrina slammed into Louisiana and Mississippi.

Shear - Rita is on the southern periphery of a fairly strong deep-layer ridge. The shear is minimal.

So put this all together, and what do we get? Intensification. Sort of.

Hurricanes intensify based on these things, but a couple more, less well-understood things can affect the intensity. The main one is called the eyewall replacement cycles. When a hurricane reaches category 4 or higher, the eye sometimes cuts itself off from the rest of the hurricane--you'll see 2 concentric eyewalls. What this does is it cuts off the main updrafts of the hurricane from feeding into the hurricane as a whole. As a result of this phenomenon, the hurricane weakens ever so slightly. Forecasting these eyewall replacement cycles is next to impossible; even the NHC doesn't try. But when all the other ingredients are in place, the exact intensity at landfall isn't that important.

It's going to be very powerful.


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