Ernesto headed for Florida

Ernesto continues its trek across Cuba, and has dumped over 8 inches so far at Guantanamo. No reports of injuries or damage have come out of Cuba thus far, and Ernesto's top winds have only been 40 mph for the duration of its trek. The storm has maintained a large envelope of spinning clouds and moisture around its center, and is already starting to fire up some heavy thunderstorms north of Cuba as its center prepares to emerge into the Florida Straits. However, the system is very disorganized at present and will not be able to strengthen quickly once it moves over water.

The 12Z (8am EDT) model runs are all in, and they don't show any significant changes to the track or expected intensity of Ernesto for its Florida landfall. Ernesto is expected to pop off the Cuban coast tonight, cross the Florida Straits, and make landfall somewhere between Miami and Naples. The SHIPS intensity model is calling for Ernesto to be a tropical storm with 55 mph winds at landfall, while the GFDL model thinks Ernesto will be a Category 1 hurricane with 75-80 mph winds. The GFDL is probably too strong, since its 6 hour forecast verifying at 2pm today predicted 60 mph winds, and the real winds were only 40 mph. With only 18-36 hours over water, Ernesto's winds will most likely be in the 50-75 mph range at landfall. My best guess is for a very wet tropical storm with maximum winds of 60 mph hitting the Everglades.

The models' expectations of what might happen to the Carolinas later in the week are very different, since small changes in the forecast track put the storm over land where it weakens, or over the warm Gulf Stream, where it can intensify dramatically. The worst case model is the GFDL, which brings a strong Category 2 hurricane to the South Carolina coast Thursday night. The best case scenario is a total miss, with the storm going out to sea, as depicted by the Canadian model (although it's unclear if the storm might circle back and strike the coast next week).

Finally, my summary of the computer models and where to find them
I've put together a detailed desciption of the computer models we provide on the wunderground.com computer model plot for each tropical storm. A brief description of the important models is given, along with a web site to get the data from. I've linked this description page under the "My links" section at the right of my blog, and it is also available on the tropical page. Here's a condensed version:

Types of hurricane forecasting models
The best hurricane forecasting models we have are "global" models, that solve the mathematical equations governing the behavior of the atmosphere at every point on the globe. Models that solve these equations are called "dynamical" models. The four best hurricane forecast models--GFDL, GFS, UKMET, and NOGAPS--are all global dynamical models. These models take several hours to run on the world's most advanced supercomputers.

Non-global models
The BAMM model (Beta and advection model, medium layer) is included on wunderground.com's computer model page. The BAMM is a simple trajectory model that is very fast to run, and did the best of any individual model at 3-5 day track forecasts in 2005. Since this model is always available, we have included it along with the "big four". In general, one should not trust the BAMM model for the 1-2 day time period when output from "the big four" are available. "The big four" are generally not available for tropical disturbances, and for these situations we post plots of a number of other non-global models such as the LBAR, A98E, etc. All of these models are described in detail on NHC's web site.

Model performance in 2005
The National Hurricane Center issues annual verification reports comparing model performance to the official NHC forecast. The 2005 report found that the official NHC forecast was usually the best forecast, but was closely matched by taking an average of the "big four" models to come up with a consensus forecast. There are several techniques used to come up with these consensus model forecasts. The three best techniques are called the GUNA, CONU, and Florida State University Superensemble (FSSE). The FSSE model was developed by FSU with funding from the private company Weather Predict, and is not available to the public. The performance of the "big four", official forecast, and consensus models are plotted below.

Among individual track models, the GFDL did the best at 1-2 day forecasts, and the UKMET and BAMM (not shown in the plot) did very well at 4-5 day forecasts. For intensity, the SHIPS model (which we post in the lower right corner of the wunderground.com computer model forecast image) was the best performer. The SHIPS model is run using input from the GFS model.


Figure 1. Forecast performance in 2005, compared to a simple "Climatological and Persistence' (CLIPER) model. OFCL=Official NHC forecast. The "big four" global dynamical models are the GFDL, GFS, NOGAPS, and UKM (UKMET). Three methods of averaging the "big four" and coming up with a consensus model forecast are CONU, GUNA, and FSSE (Florida State Super Ensemble). The Official forecast and the three consensus forecasts did the best at all time periods. Among individual models, the BAMM model(not shown) did the best at the 3-5 day range, followed closely by the UKMET. The GFDL did the best in the 1-2 day range. Image credit: National Hurricane Center.

Next update
I'll be live on Internet Partnership Radio (www.ipr365.com), formerly Radio NHCWX, tonight at 8:30 pm EDT. I'll be talking with host Mike Watkins about Ernesto. My next blog will be Tuesday morning.

Jeff Masters