Predicting Hurricanes: A Not So Exact Science

Written by Aubrey Samost

Predicting the weather has come a long way in just the last century. Today’s meteorologist no longer looks into his crystal ball. He has far more sophisticated tools available to him, from satellite images to Doppler radar. He can make a fairly accurate prediction for the weather up to a week in advance, and yet, with all of this early warning, the coast still sustains a lot of damage whenever a hurricane comes through because there is simply no time to fully prepare. A meteorologist can only make a guess, and a guess can always be wrong.

How do meteorologists predict hurricanes?

Hurricane predictions can fall into two categories: seasonal probabilities and the track of a current hurricane. These two fields are very different in their methods and approaches.

Predicting Hurricane Activity in a Season

Every year around April the meteorologist on the news starts talking about how many named storms are predicted for the season and how many hurricanes are expected to make landfall. Scientists can predict the number of named storms and their breakdown by intensity (i.e. the number of hurricanes, tropical storms, intense hurricanes, etc.). They can also predict approximate wind speeds and intensity for sustained winds. These can be easily calculated using elementary statistics. Compared to past seasons, the sustained wind speed follows the Poisson Distribution with fairly consistent accuracy. Named storms are typically predicted based on past occurrences and current measures of factors in the climate. At the beginning of the season these are only labeled as probabilities (Gray, 2006). Scientists cannot say that the third named storm of the season will hit Florida on June 30^th. They can only say that there is a five percent chance of a major hurricane hitting the coast from April to November.

Forecasting Hurricane Routes

Once a hurricane has formed, it can be tracked. Scientists can usually predict its path for 3-5 days in advance. A hurricane’s possible trajectory is usually represented as a cone, which shrinks over time as the error in the prediction decreases. To predict the path of these storms, meteorologists can use many different models. The original best model was CLIPER (Climate and Persistence). It is designed as a statistical regression equation based on past data and current climatological data. This was the major forecasting model used up until the 1980’s. Today it is used primarily for testing and comparing new models. NHC90 and BAM (Beta and Advection Model) are two models based on data gathered by planes. They use measurements taken multiple times in a day, and the models themselves are updated every couple of years. The National Hurricane Center relies heavily on two different international forecasting systems, the United Kingdom Meteorological Office’s global model and the United States Navy Operational Global Atmospheric Predictions Systems (NOAA, 2004). There are many more models used. This list includes only several of the major, most common models used to forecast the movement of storm systems.

The above models are all designed to track the path of a hurricane. Unfortunately, there are far fewer models around that can be used to track the intensity changes of hurricanes in the Atlantic. Intensity models are essential to understanding how dangerous a hurricane will be when it makes landfall. An accurate assessment of storm intensity is necessary to allow people to take the appropriate actions, like boarding up windows and evacuating. The Statistical Hurricane Intensity Forecast Model (SHIFOR) is analogous to CLIPER from above. It employs the same mathematical techniques as CLIPER but it predicts intensity instead of trajectory. The NHC also uses SHIPS (Statistical Hurricane Intensity Prediction Scheme), which uses data from the surface of the ocean to predict any changes in intensity. The RI scheme is one of the newest models, which uses data obtained by SHIPS to calculate the chance of rapid intensification of the hurricane. Only one common system in use today predicts both trajectory and intensity. It is the Geophysical Fluid Dynamics Laboratory Model designed in the early 1990s. The GFLD model uses a moveable equation to make predictions (NOAA, 2004). As with the trajectory models, these are only some of the most common models available. This list is by no means exclusive. However, there are far fewer good options available to predict the intensity of hurricanes because the reasons behind intensity changes are not fully understood and there are many factors involved.

What are the problems with the current hurricane predictions?

There have been great strides forward made in the science of forecasting hurricanes, but there is still a lot to do. One major problem is accuracy. The National Hurricane Center has been forecasting the paths of hurricanes since the early 1950’s. They issue 120 hour, 96 hour, 72 hour, 48 hour, 24 hour, and 12 hour forecasts. (The 120 hour and 96 hour forecasts were introduced in 2003.) The error decreases as the time before landfall decreases. The error has also decreased over the years as models become more accurate (NOAA, 2004). Despite becoming more accurate, the error is still relatively large.

Days Before Landfall	Error in miles
5	100
4	160
3	230
2	290
1	350

*Data in chart from the National Oceanic and Atmospheric Administration in 2004.

These errors still have a substantial effect on the damage done to a certain area. A difference of one hundred miles could determine whether or not people are forced to evacuate. These are large distances for errors in forecasting landfall.

Another major issue is trying to predict intensity. In some cases, the calculations are very straight forward and the hurricane strengthens according to a nice equation as it approaches land. Other times, there are unforeseen factors that greatly increase or decrease a hurricane’s intensity. One of the most common causes of a sudden intensity increase in the Gulf of Mexico is the Loop Current, a stream of deep warm water that provides a lot of fuel to a hurricane. Instead of just having a thin surface layer of warm water, the Loop Current has deep warm water, so when the hurricane churns the ocean, it only stirs up more warm water. Usually, a hurricane stirs up the water, cooling the overall temperature of the ocean surface and weakening the storm. The Loop Current changes position, depth, and strength over the years, so it can make predicting hurricanes really hard (Gyory, Mariano, and Ryan, 2005). A hurricane that is relatively small that hits the Loop Current can suddenly strengthen to a Category 4 or 5 storm, which spells disaster for a place like New Orleans.

Finally, there is the time component. Scientists simply cannot predict hurricanes early enough for cities to be completely prepared for it to make landfall. There is no certainty in the position of a hurricane until it is too late to respond. It will not be certain that a hurricane will hit a city until only hours before landfall, which leaves almost no time for people to secure their property and evacuate safely. Hurricane predictions in the future need to be more accurate earlier on in the forecasting process.

Flood Risk

Hurricane predictions and the probability that a hurricane will hit a specific area has a great deal of relevance to the flood risk of an area. Flooding from a hurricane can be caused by excessive quantities of rain, broken and breached levees, and storm surges from the ocean or a major lake. A lot of research has been put into flood risk by the National Flood Insurance Program (NFIP). They created different classifications of flood zones. In New Orleans, the common flood zones are A, V, and B zones. Any areas of the city in the A zones from 0-30 (0 is higher in elevation and lower in risk than A30), the land is below the base flood elevation, which puts them at higher risk than other areas of the city. The B zone is above the base flood elevation. They might still flood, but they are less likely. For this reason, people living in the B zones are not required to purchase flood insurance like the people in the A zones (City of New Orleans). The V zones are also below the base flood elevation, but they are at an even more increased risk because they are located in areas in danger of storm surges. These are the areas that will receive the most damage should a hurricane come through the city. Residents in a V zone are also required by law to carry flood insurance (FEMA, 2006). The V zones in New Orleans tend to be along the coast of Lake Ponchartrain, in areas like Lakeview and Gentilly. The rest of the city is mostly low A zones and high B zones (City of New Orleans). Flood risk is directly proportional to the probability of being hit by a hurricane, the elevation of the land, and the proximity of an area to a major body of water.

City of New Orleans. What Does the FEMA Flood Zone Mean for Me. Retrieved November 17, 2006, from http://www.cityofno.com/portal.aspx?portal=1&tabid=56

FEMA. (2006, August 21) Frequently Asked Questions. Retrieved November 17, 2006, from http://www.fema.gov/plan/prevent/fhm/fq_term.shtm#3

Gray, William. (2006) Landfalling Tropical Cyclone Webpage Questions and Answers. Retrieved November 17, 2006, from http://www.e-transit.org/hurricane/faq.html

Gyory, Joanna, Arthur J, Mariano, and Edward H. Ryan. (2005) The Loop Current. Retrieved November 17, 2006, from http://oceancurrents.rsmas.miami.edu/atlantic/loop-current.html

National Oceanic and Atmospheric Administration. Frequently Asked Questions. Retrieved November 17, 2006, from http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html