An image of Hurricane Irma, a powerful hurricane that caused widespread destruction in September 2017. Photo: Cooperative Institute for Research in the Atmosphere
Colorado State University hurricane researchers have reduced their forecast slightly but continue to call for an above-average Atlantic hurricane season in 2021, citing the likely absence of El Niño as a primary factor.
Sea surface temperatures averaged across the tropical Atlantic and Caribbean are now warmer than normal. A warmer than normal tropical Atlantic and Caribbean is considered favorable for an active 2021 Atlantic hurricane season, since a warmer than normal tropical Atlantic provides more fuel for developing storms.
The tropical eastern and central Pacific currently has cool neutral El Niño-Southern Oscillation conditions, that is, the water temperatures are slightly below average. CSU researchers anticipate that these waters are likely to remain cooler than normal for the remainder of the Atlantic hurricane season. Consequently, they believe that El Niño is extremely unlike this year.
El Niño tends to increase upper-level westerly winds across the Caribbean into the tropical Atlantic, tearing apart hurricanes as they try to form.
The primary reason for the reduction in CSU’s forecast from early July was a slight decrease in the statistical and dynamical model guidance that underpins these outlooks.
18 named storms
The CSU Tropical Meteorology Project team is predicting 18 named storms in 2021, including the five named storms that have already formed: Ana, Bill, Claudette, Danny and Elsa.
Of those, researchers expect eight — including Elsa — to become hurricanes and four to reach major hurricane strength (Saffir/Simpson category 3-4-5) with sustained winds of 111 miles per hour or greater.
The team bases its forecasts on a statistical model, as well as two models that use a combination of statistical information and forecasts from dynamical models from the UK Met Office and the European Centre for Medium-Range Weather Forecasts.
These models are built on 25 to 40 years of historical hurricane seasons and evaluate conditions including: Atlantic sea surface temperatures, sea level pressures, vertical wind shear levels — the change in wind direction and speed with height in the atmosphere — El Niño or warming of waters in the central and eastern tropical Pacific and other factors.
So far, the 2021 hurricane season is exhibiting characteristics similar to 1996, 2001, 2008, 2011, 2016 and 2017.
“2001, 2008, 2011, and 2016 had near to slightly above-average activity, while 1996 and 2017 were extremely active seasons,” said Phil Klotzbach, research scientist in the Department of Atmospheric Science and lead author of the report.
The team predicts that 2021 hurricane activity will be about 120% of the average season. By comparison, 2020’s hurricane activity was about 145% of the average season. The 2020 hurricane season had six landfalling continental U.S. hurricanes, including Category 4 Hurricane Laura, which battered southwestern Louisiana.
The CSU team will issue a verification of all 2021 seasonal Atlantic hurricane forecasts in late November.
This is the 38th year that the CSU hurricane research team has issued an Atlantic basin seasonal hurricane forecast. The Tropical Meteorology Project team also includes Michael Bell, professor in the CSU Department of Atmospheric Science, and Jhordanne Jones, graduate research assistant.
Bill Gray, who originated the seasonal forecasts, launched the report in 1984 and continued to author them until his death in 2016.
The CSU forecast is intended to provide a best estimate of activity in the Atlantic during the upcoming season – not an exact measure.
As always, the researchers caution coastal residents to take proper precautions.
“It takes only one storm near you to make this an active season,” Bell said.
Landfall probability included in report
The report also includes the probability of major hurricanes making landfall after 4 August:
- 65% for the entire U.S. coastline (full-season average for the last century is 52%)
- 40% for the U.S. East Coast including the Florida peninsula (full-season average for the last century is 31%)
- 41% for the Gulf Coast from the Florida panhandle westward to Brownsville (full-season average for the last century is 30%)
- 54% for the Caribbean (full-season average for the last century is 42%)
The forecast team also provides probabilities of named storms, hurricanes and major hurricanes tracking within 50 miles of each county or parish along the Gulf and U.S. East Coast, as well as hurricane-prone coastal states, Canadian provinces and countries in Central America and the Caribbean. These probabilities for regions and countries are adjusted based on the current seasonal forecast and its projected effects on the upcoming hurricane season.
Funding for this year’s report has been provided by Ironshore Insurance, the Insurance Information Institute, Weatherboy, Evex and a grant from the G. Unger Vetlesen Foundation.