*CSU released its first seasonal forecast for 2024 on Thursday, April 4th, and an updated forecast on August 6th.
+A measure of a named storm’s potential for wind and storm surge destruction defined
as the sum of the square of a named storm’s maximum wind speed (in 104 knots2) for each 6-hour
period of its existence.
We anticipate an extremely active 2024 hurricane season in the Atlantic basin.
We have maintained our forecast for an extremely active Atlantic hurricane season in 2024. We have reduced our forecast number of named storms slightly but have maintained all other numbers from our July update. Sea surface temperatures averaged across the hurricane Main Development Region of the tropical Atlantic and Caribbean remain near record warm levels. Extremely warm sea surface temperatures provide a much more conducive dynamic and thermodynamic environment for hurricane formation and intensification. We continue to anticipate cool neutral ENSO or La Niña during the peak of the Atlantic hurricane season, resulting in reduced levels of tropical Atlantic vertical wind shear. This forecast is of above-normal confidence. We anticipate a well above-average probability for major hurricane landfalls along the continental United States coastline and in the Caribbean. As with all hurricane seasons, coastal residents are reminded that it only takes one hurricane making landfall to make it an active season. Thorough preparations should be made every season, regardless of predicted activity.
CLICK HERE for real-time statistics and other tropical weather resources
DESCRIPTION OF CLIMATE FACTORS INFLUENCING ATLANTIC HURRICANE ACTIVITY
Forecast Factors
In 1984, Dr Bill Gray at Colorado State University documented that Atlantic hurricane activity responded to a variety of large-scale atmospheric and oceanic parameters spanning various portions of the globe. These large-scale factors interact with the global climate system in such a way that then alter the environment of the tropical Atlantic, where most major hurricanes develop and intensify. For example, hurricanes are more likely to develop when they traverse through an environment of low vertical wind shear (the change in wind direction and speed with height in the atmosphere), high sea surface temperatures and high mid-level moisture. Several parameters that have been documented to impact Atlantic hurricanes are discussed here in more detail.
Atlantic Ocean Thermodynamics
A warmer-than-normal tropical North Atlantic Ocean is known to typically create conditions more favorable for hurricane formation and intensification. In addition to enhancing latent and sensible heat fluxes that fuel tropical cyclones, a warmer tropical North Atlantic also drives lower pressures and reduced low-level trade winds, which also feed back to a more conducive environment for hurricanes.
It is important to note that the warming of the tropical North Atlantic Ocean also be measured relative to warming in other areas. One frequently utilized metric is relative sea surface temperature, which measures the difference between tropical North Atlantic Ocean sea surface temperatures and sea surface temperatures in the rest of the tropics (Vecchi et al. 2008). This index is important because it dictates where anomalous rising and sinking motion is likely to occur. When the tropical North Atlantic Ocean is anomalously warmer than the remainder of the tropics, anomalous rising motion is likely in this area, favoring enhanced tropical cyclone activity.
Averaged sea surface height anomalies measured by Jason 2 in the Pacific Ocean, observed at the beginning of October 2015. Credit: NASA
An additional index that is often utilized to assess the favorability of the tropical North Atlantic Ocean is the Atlantic Meridional Mode (AMM) (Kossin and Vimont 2007). The AMM assesses both the thermodynamic and dynamic state of the tropical North Atlantic Ocean through analysis of low-level winds as well as sea surface temperatures. A positive phase of the AMM is associated with a warmer tropical North Atlantic relative to the tropical South Atlantic, as well as with reduced trade winds in the tropical North Atlantic. Positive phases of the AMM are generally characterized by above-normal Atlantic hurricane activity.
El Niño Southern Oscillation
At the interannual timescale, the prime driver of Atlantic tropical cyclone variability is generally considered to be El Niño Southern Oscillation (ENSO). ENSO is driven by changes in ocean temperature in the tropical Pacific, where above average conditions (El Niño) in the Central and Eastern Pacific shift the convective activity in the tropical Pacific eastward, and modify the Walker cell throughout the tropics. The influence of ENSO on Atlantic tropical cyclone activity is well documented and is understood to occur mainly through local changes in vertical wind shear: during El Niño (La Niña) conditions, the eastward (westward) shift in convection in the tropical Pacific leads to anomalous upper-level westerlies (easterlies) over the Atlantic, which then increases (decreases) the vertical wind shear, thus decreasing (increasing) tropical cyclone activity.
Dust streaming from Mauritania, Senegal, western Sahara and maybe other countries farther East, and heading West toward South America and the Gulf of Mexico. Composite image made with data from VIIRS acquired by Aqua/MODIS on June 24, 2014. Credit: NASA
Sahel Precipitation
Changes in convective precipitation over the Sahel region has been shown to impact zonal winds in the upper-troposphere, which in turn modulate vertical wind shear over the Main Development Region and influence the conditions of cyclogenesis over the tropical Atlantic. It is possible that changes in the nature of the African Easterly Waves coming off the African continent might also be playing a role. Years with higher Sahel rainfall tend to be associated with more active hurricane seasons. The influence of the Sahel precipitation is mostly felt when the thermodynamical conditions over the tropical Atlantic are not conducive to hurricane formation.
Saharan Dust
Dust outbreaks from West Africa impact tropical cyclone activity by reducing Atlantic tropical sea surface temperatures ahead of the hurricane season. Episodes of dust outbreak are also associated with extremely dry air coming from the Sahara, another factor detrimental to cyclone formation.
Data Quality and Methodology Questions
Frequently Asked Questions
1. How did the CSU seasonal hurricane forecast get started?
Dr. Bill Gray first started seasonal Atlantic hurricane predictions at Colorado State University in 1984. Prior to
his development of these forecasts, not only were there were no publicly issued predictions for overall Atlantic basinwide activity,
but Atlantic TC teleconnection patterns had only been rudimentarily investigated (Ballenzweig 1959 J. Meteor.). His final
Ph.D. student, Phil Klotzbach and the CSU tropical weather and climate research team continues the legacy and regularly issues the forecast to the present day.
2. Why do you study hurricanes in Colorado?
"Storm Surge can't get you at 5,000 feet!" ~ Dr. Bill Gray
3. What features of weather and climate do you look at when preparing a forecast?
In 1984, Dr. Bill Gray at Colorado State University documented that Atlantic hurricane activity responded to a variety of large-scale atmospheric and oceanic parameters spanning various portions of the globe. These large-scale factors interact with the global climate system in such a way that then alter the environment of the tropical Atlantic, where most major hurricanes develop and intensify. For example, hurricanes are more likely to develop when they traverse through an environment of low vertical wind shear (the change in wind direction and speed with height in the atmosphere), high sea surface temperatures and high mid-level moisture. Several parameters that have been documented to impact Atlantic hurricanes are discussed here in more detail HERE
4. What data sources are you using for your calculations?
Real-time estimates of global tropical cyclone activity are calculated from the operational b-decks as archived on the University Corporation of Atmospheric Research
These are operational estimates of intensity and have not been subject to any thorough review. When tropical cyclones are active, these files are updated every six hours with estimates of location, maximum wind, minimum sea level pressure and a variety of wind radii information.
More information on the b-decks is available on the National Corporation for Atmospheric Research’s Tropical Cyclone Guidance Project.
5. How good is the data?
As you go back in time, the quality of the best track data generally degrades. The quality of this data is also very heavily basin dependent. For example, the North Atlantic is the only basin that currently has operational aircraft reconnaissance. For all other basins, intensity estimates are currently based virtually entirely on satellite data. The Dvorak technique is the primary technique currently used today to estimate tropical cyclone intensity from satellite imagery. This technique was first initiated in 1972; however, the quality of satellite data in the 1970s through the mid-1980s likely led to significant underestimates in intensity, especially for the most intense systems.
Here are the time periods that I typically consider to be the most reliable for each TC basin.
Basin
Time Period of Highest Quality Data
Reason
North Atlantic
1966-present
Geostationary satellite data available
Eastern North Pacific
1988-present
National Hurricane Center took over observational responsibilities
Western North Pacific
1985-present
Joint Typhoon Warning Center deems these best tracks to be of highest quality
North Indian Ocean
1985-present
Joint Typhoon Warning Center deems these best tracks to be of highest quality
South Indian Ocean
1985-present
Joint Typhoon Warning Center deems these best tracks to be of highest quality
South Pacific
1985-present
Joint Typhoon Warning Center deems these best tracks to be of highest quality
Here is a list of a few publications discussing various issues with data quality:
Extremely Intense Hurricanes – Revisiting Webster et al. after 10 Years – Discusses changes in observational platforms that may have led to a spurious increase in Category 4-5 hurricanes prior to 1990.
6. How do you make your climatological calculations?
Climatological calculations are made from the best tracks provided by the National Hurricane Center, the Central Pacific Hurricane Center and the Joint Typhoon Warning Center as archived on the International Best Track Archive for Climate Stewardship.
Best tracks are the best estimate by the operational center for a tropical cyclone’s intensity for every six-hour period when a tropical cyclone is active. Other times may be included in the best track when additional information is provided (e.g., landfall, aircraft reconnaissance).
7. What is Accumulated Cyclone Energy (ACE)?
ACE is calculated by summing the square of the maximum sustained winds of each tropical cyclone (in knots) every six hours when the system is classified as either tropical or sub-tropical. The resulting value is then divided by 10,000. Details of the calculation are available on Wikipedia.
8. Why do the hemispheric totals not equal the global total in a particular year?
The Northern Hemisphere season is defined to run from January 1 through December 31, while the Southern Hemisphere season is defined to run from
July 1 through June 30. Global totals are based on the calendar year (January 1 – December 31), but the Northern Hemisphere and Southern Hemisphere
totals have a six-month offset.
9. How do you account for tropical cyclones that cross basin boundaries?
Occasionally tropical cyclones will cross boundaries between basins. This most frequently occurs between the Northeast Pacific and Northwest Pacific basins, as well as the South Indian and South Pacific basins. In this case, days as a tropical cyclone are counted in the basin where the storm was located. So, if a storm was in the Northeast Pacific as a tropical storm for 4 days, then passed into the Northwest Pacific as a tropical storm for 2 more days, it would count as 4 named storm days for the Northeast Pacific and 2 named storm days for the Northwest Pacific. In terms of storm counting, this particular tropical cyclone would count as 1 named storm in the Northeast Pacific and 0 named storms in the Northwest Pacific (in order to avoid double-counting).
As another example, if a system becomes named in the Northeast Pacific but intensifies into a typhoon in the Northwest Pacific, it would count as 1 named storm and 0 hurricanes in the Northeast Pacific and as 0 named storms and 1 hurricane/typhoon in the Northwest Pacific.
All statistics for an individual storm are displayed under the basin in which it formed. This is to avoid confusion of having TCs listed in multiple basins. Consequently, if there are basin crossers in a particular year, the named storm days for each tropical cyclone in a basin may not add up to the total days observed in a basin.
