Storm Season 2009

Tropical Storm Fay became the first Atlantic tropical cyclone to make landfall in the same U.S. state on four separate occasions when it soaked Florida. Fay also produced a two-foot rise on Lake Okeechobee in one week, the largest rise since record keeping began in 1931. Fay produced significant flooding across much of the state of Florida, southern Georgia, and also portions of Alabama. A maximum of just over 27 inches of rain fell near Melbourne, Fla. In addition to the widespread flooding, Fay also spawned at least 80 tornadoes.

After wreaking havoc across the Caribbean, Gustav made landfall as a Category 2 hurricane in southeast Louisiana. Gustav produced widespread wind damage, at least 40 tornadoes, and flooding across Louisiana, including a significant storm surge that topped levees.

Finally, Hurricane Ike became the most significant hurricane to affect the U.S. since Hurricane Katrina (2005). The U.S. experienced its third-costliest season primarily due to the damage produced by Hurricane Ike. When all losses are totaled, Ike is expected to become the third costliest U.S. tropical cyclone in history, behind only Hurricanes Katrina and Andrew.

Nothing to Like About Ike

Hurricane Ike was a long-lived tropical cyclone that originated from a well-defined tropical wave that moved off of the western African coast on Aug. 28, 2008. It was not estimated to be a tropical depression until Sept. 1, 2008, when it was 775 miles west of the Cape Verde islands. The depression continued to become better organized and quickly strengthened to become Tropical Storm Ike just six hours after becoming a depression. Moving west-northwest around the southern periphery of a strong subtropical high, Ike continued to strengthen and became a hurricane early on the afternoon of Sept. 3, 2008, when an eye became apparent on visible and microwave satellite imagery. Almost a week later and just prior to crossing the northwest tip of Cuba as a Category 1 hurricane, Ike began producing tropical storm-force winds across portions of the Florida Keys. Fortunately for the Keys, Ike only delivered a glancing blow, as the hurricane continued to move west-northwest toward the U.S. Gulf Coast.

Although Ike's interaction with Cuba disrupted the inner core of the hurricane and prevented rapid strengthening over the warm waters of the Gulf of Mexico, it quickly grew in size. For example, tropical storm-force winds reached out 275 miles and hurricane-force winds stretched 115 miles from the center of the tropical cyclone.

Ike slowly intensified to Category 2 strength with maximum winds of 100 mph by Sept. 10, 2008. It made landfall on Sept. 13, 2009, near Galveston, Texas, with maximum sustained winds of 110 mph. Once inland, Ike moved north-northwest just east of Interstate 45 and brought hurricane force winds to the eastern two-thirds of southeast Texas. It even extended its havoc across the Midwest, affecting states like Ohio, Illinois, Pennsylvania, Kentucky, and Missouri.

For residents of southeast Texas, though, Ike will forever be remembered for its large size and significant storm surge. Although just under the wind speed criteria for a major hurricane (115 mph or greater), Ike produced the greatest storm surge across the upper southeast Texas coast since Hurricane Carla made landfall near Port Lavaca in 1961 as a Category 4 storm. Interestingly, although Ike was considerably weaker than Carla as far as maximum wind speeds are concerned, it had a larger area of hurricane force winds at landfall and a similar tropical storm-force wind field. Therefore, from a total energy standpoint, Ike was very similar to Carla, which explains why the magnitude of these surge events was similar.

Ike's highest storm surge occurred on the Bolivar Peninsula and in parts of Chambers County (including the east side of Galveston Bay), roughly between the Galveston Bay entrance and just northeast of High Island. Complete tide gauge records for this area are unavailable since many of the sensors failed from salt water intrusion and large wave action; although ground assessment teams determined that the surge was generally between 15 and 20 feet. The highest water mark was 17.5 feet, located about 10 miles inland in Chambers County. Much of the southern part of Chambers County was also inundated by at least 10 feet of water. Storm-surge levels on Galveston Island and on the west side of Galveston Bay are estimated to be between 10 and 15 feet. The highest inundation, of at least 10 feet, occurred on the bay side of Galveston Island, the coast of mainland Galveston County, as well as over Apffel Park at the northern tip of Galveston Island where Ike made landfall.

Looking Ahead to 2009

The National Oceanic and Atmospheric Administration's (NOAA) 2009 seasonal forecast will be issued during the last week of May. NOAA's lead scientist is Dr. Gerry Bell from the Climate Prediction Center. He has a team of researchers that includes members of the National Hurricane Center and the Hurricane Research Division. Although NOAA began issuing seasonal forecasts in 1999, ranges of named storms, hurricanes, and probabilities did not appear in the outlook until 2001.

In addition to these ranges, NOAA now uses the Accumulated Cyclone Energy (ACE) index to categorize North Atlantic hurricane seasons as being above normal, near normal, or below normal. This index refers to the collective intensity and duration of Atlantic named storms and hurricanes occurring during a given season. Therefore, it is a measure of total seasonal activity and it is computed as the sum of the squares of the maximum sustained surface wind speed (knots) measured every six hours for all named systems while they are at least tropical storm strength.

The fundamental variable that NOAA's forecast has been predicated on since its inception is the continuation of the positive phase of the multi-decadal signal. In the current warm (positive) phase, which began in 1995, this pattern has favored above normal Atlantic basin seasons. In fact, all of the Atlantic basin hurricane seasons since 1995 have been above normal, with the exception of three moderate-to-strong El Ni?o influenced years (1997, 2002, and 2006). This contrasts sharply with the 1971-1994 period of generally below-normal activity, when the multi-decadal signal was in a cool (negative) phase.

Over the North and Tropical Atlantic, the key aspects of the positive phase of the multi-decadal signal that are likely to persist include warmer sea surface temperatures, lower surface air pressure, and increased moisture across the tropical Atlantic; an amplified ridge at upper levels across the central and eastern subtropical North Atlantic; reduced vertical wind shear in the deep tropics over the central North Atlantic; and weaker easterly winds in the middle and lower atmosphere, resulting in a configuration of the African easterly jet that favors hurricane development from tropical waves moving westward from the African coast. This signal is expected to again be the main factor guiding the upcoming seasonal outlook, as it does not switch phases on the time scales of years, but rather decades.

The second factor, which can change on the time scale of months, is the state of ENSO (El Ni?o southern oscillation). As noted above, the only seasons since 1995 that have experienced below-normal activity coincided with moderate-to-strong El Ni?o events (warm phase of ENSO). When the ENSO state was one of neutral or La Nina conditions, near and during the peak of the hurricane season, an above-normal season has resulted. Therefore, while the multi-decadal signal remains in a positive phase, it is unlikely that the Atlantic basin season will be below normal, unless influenced by a moderate-to-strong El Ni?o during the hurricane season.

Recent observations from the equatorial Pacific indicate that a transition from La Nina to ENSO neutral conditions is underway. Therefore, as we enter into the 2009 hurricane season, the state of ENSO is expected to be neutral. Most climate models indicate that at least some warming will continue across the equatorial Pacific throughout the rest of 2009. However, the magnitude of this warming is still uncertain and will be an important factor for the seasonal forecast. If the ENSO state remains neutral into the Fall of 2008, another above-normal year is likely. However, if a weak or moderate El Ni?o develops before the peak of the hurricane season, the Atlantic basin would likely become less active toward the end of the season.

Perhaps the single most important outcome or use of seasonal activity forecasts is that they generate public interest and aid in public awareness of the tropical cyclone threat, especially just prior to the beginning of the hurricane season. However, regardless of the seasonal activity forecast, it is imperative that all potentially impacted residents prepare as if they could be impacted by a hurricane every year. Although history has shown that multiple landfalls are more likely along the U.S. coast during an above-normal year, the probability of a hurricane making landfall somewhere along the U.S. Gulf coast is still around 60 percent even during a below-normal year. The 1983 and 1992 hurricane seasons are examples of below-normal seasons; however, during both of these seasons, land-falling hurricanes occurred along the Gulf Coast (Hurricane Alicia in 1983 and Hurricane Andrew in 1992).