Tropical Storm Debbie

Debbie seems to confirm my suspicion that it would not be making landfall west of New Orleans. The forecast track has changed significantly and is now projected to make landfall along the Florida peninsula. I am actually revising my initial forecast that Debbie will make it to a 70mph tropical storm before landfall. The low level center of circulation is tilted. A tilted circulation displaces the mid-level warm core of the center and can dry out the center of circulation if the shear is strong enough. This gives a tropical cyclone a very asymmetrical appearance.

Structure of a tropical cyclone, from Britannica.

CIMSS animation of water vapor and infrared imagery.

Unisys upper level wind (and low)

 There is wind shear over the center, which keeps displacing the core of convection and tilting the circulation. If you look at the surface, the low level center isn't even underneath the upper level center.

The convection associated with Debbie is far displaced from the center and the upper level circulation is no longer directly over the low level circulation.

And there is another problem with Debbie, which I will show in the next two images.

Twisterdata 500mb relative humidity

Twisterdata 300mb relative humidity

A warm core tropical system derives its strength from the condensation of water vapor, which with condensation, releases heat. The mid-level (below 500mb) is healthy and shows an effective supply of latent heat. But at and above the 500mb level, dry air intrusion is choking off thunderstorm activity. When the rising motion around the center of circulation starts ingesting less buoyant air around it (dry air), it in turn becomes less buoyant and continued rising motion will decrease or cease altogether. We see a healthy environment for tall thunderstorms both along the Florida coast and the Gulf Coast, where the convective activity is. We do not see this over the center, and thus, another reason why we are not seeing convective growth over the center. Debbie is a shallow top system partially because of this.

Verdict - Wind shear will shift to a southerly direction which will not help Debbie strengthen. I have a hard time seeing Debbie get past 70mph, and even that number is optimistic. That would be a scenario if the upper level center were to become more co-located with the low level center. If the core structure does not have a significant change, Debbie will struggle to get past the current strength of 60mph.