The tropical disturbance just south of Jamaica has strengthened into a tropical depression on Thursday morning (and could be a storm by Friday morning). The track that had shows a swath that included northern Mexico to central Louisiana (with the Houston area square in the middle) shifted significantly over the last 24 hours, now putting the bullseye on central or eastern Louisiana even though we are not entirely out of the woods just yet. Let's discuss.
Where is it and why has that changed?
The center of circulation, which had not formed Wednesday and is critical to determining the potential track and intensity of a storm, finally formed Thursday morning south of Jamaica. Forecast models understandably have a very difficult time forecasting storms that have no center. Had the center formed several hundred miles south, for example, the track would have been quite a bit farther south. That didn't happen giving a fairly dramatic easterly shift in the forecast guidance.
Where is it going and why?
The storm is going to be steered around the western periphery of high pressure currently sitting over the Carolinas. All the forecast models are in good agreement on a gradual northwesterly movement to the storm with an eventual slight northerly track late in the forecast. That puts a landfall most likely somewhere between central Louisiana and Mississippi, but we still have at least another 24 hours before we can know for sure.
How strong will it get?
There are several factors that will influence development. The first is another storm in the eastern Pacific that will be sending a push of disturbed upper-level air into the Gulf. The hope is that will impart some wind shear on the system and limit its development. On the other hand, the Gulf is plenty warm and the National Hurricane Center is still predicting a strong category 2 hurricane, borderline major storm, at landfall. That is not only because of the warm waters of the Gulf, but because of the loop eddy that sits south of Louisiana most summers. It is a very deep, very warm area of water that spawned the rapid intensification of hurricanes like Katrina and Rita in 2005. Any interaction with that eddy could cause this storm to expand rapidly.
When will it make landfall?
The good news is that we won't have to wait long. Landfall is predicted sometime Sunday, as early as around sunrise. Because the storm will be moving relatively swiftly, it won't have days and days over water to strengthen and, even though there will be tons of rainfall, this won't be a Harvey situation where it sits over an area for a long period of time.
When are we out of the woods?
For Houston and Galveston, we should know by Friday afternoon if the storm is a direct threat to us. If you haven't already, this would be a good time to get your stuff together just in case. We still have a month of peak hurricane season ahead, so even if this one misses us, there will be more to come.