Ernesto vs. Upper Level Low

This is the story that everyone in the Caribbean and along the Gulf Coast needs to be following right now. Just in front of Ernesto, and moving westward like he is, is an upper level low. That's the coil of orange wind arrows south of Cuba in this shear diagram. Note the pockets of strong (30-40 mph) shear surrounding it. Both Ernesto and the low are moving west, the critical question is which will move faster. If they move at about the same speed, Ernesto will remain under 10-20 mph shear, and probably continue to intensify slowly, like he has been doing so far. Under this scenario, it could take a few days for Ernesto to reach hurricane strength. If the low is moving faster, then shear will decrease over Ernesto, and he could reach Jamaica as a Category 1 hurricane, and be a major Category 3 hurricane by the time he reaches the Gulf of Mexico. On the other hand, if Ernesto is moving faster than the low, he'll reach that zone of 30+ mph shear and be ripped apart. Dr. Jeff Masters notes that predicting the motion of upper level lows in the tropics is still a weakness in the major computer models. It's almost a guessing game to see which scenario would play out.

The NHC is sticking with the low moving faster. Right now their predicted track forecast is looking pretty bad for the areas of Louisiana and Mississipi hit by Katrina almost exactly a year ago. This is only one possibility, though. The "spaghetti diagram" shows that there is quite a range of variation in where the computers think the storm will go. One possibility is that Ernesto can slip northward through a gap in the ridge to his north. This would send him recurving across Florida. Or he could just slide up the ridge towards New Orleans. The ridge could also strengthen and force Ernesto towards Texas.