Forecasting Fluctuations in Hurricane Intensity
Within 24 Hours of Landfall


As excessive heat builds in the tropical regions and excessive cold builds in the polar regions, nature has a very interesting way of maintaining a balance. Nature will find a way to resolve this thermal disparity between the two regions by creating one of the most deadly storms known to mankind. This storm is known as a hurricane. Forecasting the intensity fluctuations of a hurricane just twenty-four hours prior to landfall is one of the most difficult forecasting dilemmas facing forecasters today along the Mississippi Gulf Coast.

A hurricane is an intense tropical cyclone that forms over warm tropical waters from June 1st to November 30th in the Northern Hemisphere. It is classified based on a sustained wind speed threshold of 64 knots (74 mph). Hurricanes occur in all ocean basins around the globe with the exception of the South Atlantic. These intense cyclones have different names around the world. In the Western North Pacific Ocean west of 180 degrees longitude, they are known as typhoons. In the North Indian Ocean, they are known as cyclones. Despite the difference in names, these cyclones operate in the same deadly fashion leaving devastating destruction and affecting a countless number of lives in their wake.

A hurricane starts as a "spin off" type of disturbance from a cluster of thunderstorms that is typically located just north of the geographical equator during the warm, summer season. This cluster of thunderstorms is produced by the convergence of the Northeast tradewinds from the Northern Hemisphere and the Southeast tradewinds from the Southern Hemisphere. Air vigorously rises as these two belts of wind converge towards each other. This area of convergence is known as the Intertropical Convergence Zone (ITCZ). If a thunderstorm cluster persists long enough, the cluster may be classified as an Easterly Wave. An Easterly Wave is an area of disturbed weather within the tropics and is considered a "seedling" for future hurricanes. The Easterly Wave is watched closely by meteorologists at the Tropical Prediction Center near Miami, Florida for future development.

There are four general requirements in order for a hurricane to develop. The first requirement is for water temperatures to be at or above 79 F (26 C). A hurricane feeds off this warm, moist water surface and requires it for development and to sustain itself. The second requirement for hurricane development is for the system to be at least five degrees from the Equator in order for the Coriolis force to be a factor. Coriolis force is an apparent force that deflects the wind and all objects above the earth to the right of their intended path of motion in the Northern Hemisphere. It is caused by the rotation of the earth. The coriolis force generates the "spin off" motion needed for the development of a hurricane. The coriolis force is negligible at the equator itself thus if any hurricane tried to develop right on the equator, there would be no initial spinning motion to the storm. The third requirement is for a moist air column to be present within a deep layer of the troposphere. This moist air column will allow for a parcel of air to rise at the moist adiabatic rate and to release tremendous amounts of heat energy to the surrounding air in the form of latent heat. A phase change from a vapor to a liquid results in the addition of latent heat to the surrounding air. This important process is known as the latent heat of condensation. This added heat energy will increase the instability of the atmosphere further. As a result, there will be an increase in the upward vertical motion component of the storm. Lastly, there must be very little to no vertical wind shear. Wind shear is a change of wind direction and/or speed with height. The ideal situation is for the wind to be from the same direction at a constant speed, generally from the east in the tropics, from the surface through a deep layer of the atmosphere. If all of these requirements are met, a tropical system will reach hurricane strength over time.

There are many known factors on hurricane intensification just prior to landfall. One factor is the eyewall replacement cycle. This cycle typically takes place in a stronger Category 3 or more intense hurricane. As a hurricane reaches the Category 3 threshold, it usually has a fairly well developed eyewall and a radius of maximum sustained winds in a more compact area surrounding the eye of the storm (less than 20 miles). At a certain point during an intense hurricane, some of the outer rainbands may organize into an outer ring of thunderstorms that slowly moves closer toward the eye. These outer rainbands moving inward toward the eye begin to take away the moisture from the pre-existing inner eyewall and act to weaken this eyewall with the winds decreasing slightly and the pressure actually rising. With time, the outer eyewall replaces the inner eyewall. After the replacement cycle is complete, the intensity of the hurricane may actually be stronger than when the cycle began. The timing of when the hurricane makes landfall and the stage in the eyewall replacement cycle will determine the final intensity of the storm at landfall. If the replacement cycle has just been completed, a more intense hurricane will make landfall. If the replacement cycle is still ongoing, the storm may actually weaken just prior to making landfall. Another factor of hurricane intensification just prior to landfall is above normal sea surface temperatures. This played a key role in the monstrous storms that formed over the Gulf of Mexico during the 2005 hurricane season as the sea surface temperatures in the Gulf of Mexico were two to four degrees Fahrenheit above climatological norms. These pockets, or warm eddies, of water provide a tremendous source of energy for hurricanes that pass over them. Just as there are intensification factors for hurricanes just prior to landfall, there are also weakening factors. One weakening factor is when a hurricane passes over an area of colder sea surface temperatures. This process typically occurs when a hurricane moves over an area that a previous hurricane had recently passed over. When a hurricane moves over an area, it mixes the water to great vertical depths. Cold, subsurface water will come up from below to replace the warm water near the top of the water column resulting in a temporary time period when the surface water is colder than normal. The colder area of water will decrease the hurricane's intensity just prior to landfall. Another weakening factor is increased vertical wind shear. Wind shear will rip apart the building updrafts of thunderstorms within a hurricane resulting in a weakening intensity trend. This increased wind shear typically takes place out ahead of an approaching upper level trough.

Hurricanes often fluctuate in their intensity quite rapidly within a twenty-four hour period leading up to landfall. A great example of this was along the Mississippi Gulf Coast with Hurricane Katrina in August 2005. Meteorologists forecasted Katrina to make landfall as a devastating Category 5 initially, but she weakened down to a strong Category 3 prior to moving ashore over the Louisiana/Mississippi border. There is a major difference in the effects of a landfalling Category 3 storm versus a landfalling Category 5 storm as evidenced by the Saffir-Simpson Intensity Scale. For example, a Category 3 storm has sustained winds of 111-130 mph and a storm surge of 9 to 12 feet above normal, while a Category 5 storm has sustained winds greater than 155 mph and a storm surge generally greater than 18 feet above normal. The consequences can range from considerable to catastrophic damage, especially with the storm surge and destruction from the wind. We have seen from past hurricanes how quickly the intensity of these systems can change and the implications on life and property. Hurricane Opal in 1995 and Hurricane Katrina in 2005 gave forecasters quite a headache with the final intensity aspect of the storm prior to landfall. Determining whether you will have a Category 3 storm or a Category 5 storm will have obvious implications on the local economy as far as damage and the rebuilding process goes. The cost of unnecessary evacuations alone is mind-boggling in the millions and billions of dollars. In fact, Hurricane Floyd in 1999 was a Category 4 storm over the open Atlantic as it approached the Florida coastline. As the storm changed direction and intensity, the largest peacetime evacuation in U.S. history ensued with over 2.6 million people evacuating from Coastal Florida, Georgia, and the Carolinas. The storm eventually made landfall in North Carolina as only a Category 2 hurricane.

Continued research is our only means of understanding the intensity fluctuations prior to a hurricane making landfall. Much of the research is conducted by sending out aircraft to examine the hurricanes. The Hurricane Research Division sends out a Gulf IV jet aircraft at high altitudes to study a hurricane more closely. In addition, the Hurricane Research Division is currently investigating intensity fluctuations through the use of real-time doppler winds, experimentation with higher resolution weather models, and looking back at past hurricanes. Only time will tell if this research will pay off in the long run.


-Meteorologist Jeff Haby:

-Mr. Stan Goldenberg, Hurricane Research Division: