The limiting factors on Helene are starting to fade away and the storm is developing “that look” for major hurricanes. What is that look? Let me describe it for you. The upper-level low-pressure system that was producing strong shear over the low-pressure system has now shifted far enough to the west to instead enhance the northern outflow upper-level circulation and enhance divergence over the whole environment. So vertical wind shear is no longer a factor. Meanwhile, the whole structure has become vertically stacked and maturing rapidly. The upper-level circulation is maturing rapidly and enhancing latent heat release. We can see this with the feathery look of the cirrus on all sides of the low-pressure system. Meanwhile, deep convection and heat cores are forming around the low-level center. The close position of the low-pressure system to the Yucatan Peninsula will briefly slow but not stop the rate of intensification, which should allow Halene to become a hurricane by this afternoon.
Helene will fall under the influence of a developing upper-level low-pressure system in the Mississippi River Valley and an upper-level ridge over the Southeastern United States, forcing Helene to quickly move to the north-northeast towards the Florida Panhandle by tomorrow night. Between the upper-level low-pressure system and the upper-level ridge, the atmosphere will support an enhanced divergence at 250 MB, enhancing the rising motion of the warm-core low-pressure system. This environment, combined with very warm waters in the eastern Gulf of Mexico, no wind shear, and very little dry air will increase the potential for rapid intensification of Helene into a major hurricane. Aside from an eyewall replacement cycle or some mid-level dry air that is currently not seen in the guidance, there isn’t much to stop Helene from becoming a major hurricane with significant impacts on the Florida Panhandle. In addition to the intensification, the speed of the storm will enhance sustained winds on the eastern side of the storm, possibly adding 10 to 20 mph winds on top of the observed sustained winds in the eyewall.
The interaction of the upper-level low-pressure system will create another unique dynamic to this storm threat for the rest of the Southeastern United States from the Gulf Coast to the Tennessee River Valley. When an upper-level low-pressure system (cold core) interacts and absorbs a hurricane, the surface low-pressure system rapidly transitions into an extratropical storm with the rain shield expanding and the wind field expanding while core winds subside. So, instead of 115 mph wind, the storm features 50 mph sustained wind but over a 40 mile radius rather than a 10 mile radius. Any way you slice it, this storm will have significant and lasting impacts on the Southeast.