Ernesto was upgraded to a hurricane this morning at 5:04 AM EDT. At the time, NHC thought that Ernesto might be undergoing rapid intensification (defined as a greater than 30 kt increase in wind speed over 24 hours). It appears at the moment that Ernesto was not rapidly intensifying. However, the probability of Ernesto rapidly intensifying is relatively high.
The track forecast has shifted significantly since last night's post (Figure 1). The NHC "line," which they tell you not to look at, is now forecasting a US landfall just north of Tampa. However, there is large uncertainty in this forecast. People from Pensacola to Daytona need to be paying attention to this storm, as the models are still quite divergent (Figure 2). Interestingly, the GFS, which was the only model yesterday forecasting a Florida peninsula landfall has now moved back to the west and by 5 days has the storm lurking about 300 miles west of Tampa in the Gulf of Mexico. The track forecast is highly dependent on an upper-level disturbance making its way out of the Rockies. How far south that disturbance digs will play a major role in how Ernesto evolves.
The intensity forecast is equally as challenging. Again, there is much model spread (Figure 3). However, the models may prove to be a poor indicator of how fast Ernesto's winds may become. The models do not handle two things very well in their intensity forecasts: intensity change due to land interaction and intensity change due to rapid intensification. Ernesto's evolution is going to be highly dependent on both of these factors. The current NHC forecast has Ernesto making its US landfall as a Category 2 hurricane. As mentioned above, Ernesto has a relatively high probability of undergoing rapid intensification, and looking at Figure 2, there is not much agreement on how long Ernesto will remain over Cuba. Both of these factors will play a large role in determining the intensity forecast for Ernesto.
Currently, Ernesto is surrounded by very favorable environmental conditions. There is low shear (Figure 4) and the upper-level outflow is strengthening (Figure 5). Both of these factors are what is providing the environment for rapid intensification. In addition, these two parameters are forecast to only get better with time for Ernesto. Also, Ernesto is currently over extremely warm sea-surface temperatures and is forecast to continue over these warm waters, except for when the storm is over land. Any shift in the forecast track will have significant effects on the intensity forecast and for that reason, folks on the Florida coast need to realize that there is very real threat of a Category 3+ hurricane making landfall in the next 4 days. Stay tuned to the StormTrack for more.