Latest reporting on Hurricane Rita has its internal air pressure at 898 millibars. That's the third lowest on record, and it's only assumed since the measuring instrument dropped into Rita doesn't measure lower!
So what? Air pressure is one measure of a hurricane's power. A thermometer-like device uses a column of mercury (in some cases) to measure the weight of the column of air against it. Hurricanes are just enormous low pressure systems, like tornadoes and the low-pressure systems you see on weather reports, usually riding along cold fronts. Hurricanes are so powerful they can move independently of fronts.
Normal air pressure usually runs around 1000 to 1020 millibars of pressure, so Rita's internal air pressure is 10% lower than that. That may not sound like a lot, but imagine a volume of air with a diameter of several miles and a mile high. Then subtract ten percent of that. Wow.
Rita's a Cat4 storm right now and the National Hurricane Center thinks it may strengthen overnight. Its movement has been slightly to the east of late over earlier landstrike predictions. It's not the sprawling monster of Katrina, but it packs a severe central punch. So? Katrina went just slightly to the east of Memphis when it moved inland, so we got the less-stormy western side of the storm; lots of soaking rains but no tornadoes or windstorms. Rita is looking to pass us just to the west, meaning we may see those tornadoes this time, as the eastern side of the hurricane is where the most violent interactions between the cooler, lower pressure air of the storm meets the warmer, more energetic air its moving into. That's where the internal air pressure measurement of Rita comes in. The larger the difference in pressure, the stronger the gradient measured over distance, the more violent the storm's potential.
In Memphis' case, we've been seeing record warmth all week. That's a recipe for juicy, high-energy air with lots of moisture. Have you noticed all that haze the past few nights? The red moon? That's dust being carried by moisture in the air. That's how much humidity we have right now. If Rita aims right, if she retains her power far enough inland, we can see some nasty storms over the weekend.
Please be careful.