Colorado State University hurricane researchers are maintaining their forecast 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 portions of the tropical Atlantic are near normal, while the subtropical Atlantic is much warmer than average. This type of sea surface temperature configuration is also considered favorable for an active 2021 Atlantic hurricane season.
The tropical eastern and central Pacific currently has cool neutral ENSO conditions, that is, the water temperatures are slightly below average. CSU researchers anticipate that these waters will likely remain near average for 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.
While the tropical Atlantic currently has water temperatures near their long-term averages, the warmer-than-normal subtropical Atlantic typically forces a weaker subtropical high and associated weaker winds blowing across the tropical Atlantic. These conditions then lead to warmer waters in the tropical Atlantic for the peak of the Atlantic hurricane season.
18 named storms
The CSU Tropical Meteorology Project team is predicting 18 named storms in 2021, including Ana, which formed in May. Of those, researchers expect eight 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. This forecast is almost identical to CSU’s initial prediction in early April, with the addition of one named storm (from 17 to 18) to include Ana’s pre-season formation.
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 25-40 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 (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, 2006, 2008, 2011, and 2017. “1996, 2008 and 2017 had above-average activity, 2001 and 2011 had near-average activity, and 2006 had slightly below-average activity,” 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 US hurricanes, including Category 4 Hurricane Laura which battered southwestern Louisiana.
The CSU team will issue forecast updates on July 8 and August 5.
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 in the same department. 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.
Landfalling probability included in report
The report also includes the probability of major hurricanes making landfall:
- 69% for the entire U.S. coastline (average for the last century is 52%)
- 45% for the U.S. East Coast including the Florida peninsula (average for the last century is 31%)
- 44% for the Gulf Coast from the Florida panhandle westward to Brownsville (average for the last century is 30%)
- 58% for the Caribbean (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 US 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.
Extended range Atlantic Basin hurricane forecast for 2021
Released June 3, 2021
Tropical Cyclone Parameters Extended Range
(1991-2020 Climatological Average Forecast for 2021
Named Storms (14.4)* 18**
Named Storm Days (69.4) 80
Hurricanes (7.2) 8
Hurricane Days (27.0) 35
Major Hurricanes (3.2) 4
Major Hurricane Days (7.4) 9
Accumulated Cyclone Energy (123) 150
Net Tropical Cyclone Activity (135%) 160
* Numbers in ( ) represent averages based on 1991-2020 data.
** Forecast numbers include storms that have already formed (Ana)