LITTLE ROCK, Ark. – Forecasters at Colorado State University’s Department of Atmospheric Sciences have released a forecast regarding tropical activity for the 2021 Atlantic Hurricane Season.
In the April 8, 2021 forecast, the forecasters are called for 17 named storms, 8 hurricanes and four major hurricanes. These numbers are projected to result in a more active season than NOAA’s new average season.
While it’s too soon to tell exactly how many storms and hurricanes will occur, forecasters look at a variety of factors that may influence the formation of tropical activity.
Some factors that Colorado State Forecasters have noted for this year’s season include: a positive Atlantic Multi-Decadal Oscillation (AMO), El Niño Southern Oscillation (El Niño for short), Sahel precipitation and Saharan dust.
The AMO is based on the variability of seas surface temperatures in the northern Atlantic Ocean based on a timescale of multiple decades. A positive AMO is associated with an active hurricane season. A negative AMO usually gives way to a less active hurricane season. Based on last year’s over-active season, forecasters have high confidence for a positive AMO cycle for the 2021 season.
Forecasters are also taking note of the El Niño pattern that may be more likely to occur this year than last. With an El Niño event, there is more tropical activity in the eastern Pacific Ocean and less in the Atlantic Ocean, Gulf of Mexico and Caribbean Sea. The Pacific tends to have lower wind shear and warmer sea surface temperatures, both of which are conducive for tropical cyclone formation. In the Atlantic basin, though, an increase in wind shear and atmospheric stability are less likely to assist with development for tropical cyclones. It’s uncertain at this time of year, though, how strong El Niño will be during prime hurricane season months
Meanwhile, precipitation changes in the Sahel region of Africa could help with vertical wind shear over the Atlantic Ocean. Vertical wind shear often inhibits hurricanes. Active hurricane years have occurred when Sahel precipitation is higher.
As seen last year and many years before, strong winds carrying Saharan dust are detrimental to hurricane formation. The dust reduces the Atlantic’s tropical sea surface temperatures, and dry air is not conducive for tropical development.
It should be noted that the number of storms doesn’t necessarily translate to how “bad” the hurricane season is. A hurricane season can be be bad if there are few storms, but one major hurricane hits land, likewise a busy season isn’t necessarily bad if the stronger storms stay out to sea.