2021 is likely to be “near normal” in terms of tropical activity, though there is more risk of an active season. Impacts from landfalling hurricanes could shift eastward this season toward the U.S. East Coast and the Leeward Islands.
The forecast for the 2021 Atlantic Hurricane Season from June-November shows the likelihood for a near normal season, with
- tropical cyclone activity at ~107% of normal anticipated
- range of activity from 97-119% of normal
- 17 named storms (average 14)
- 8 hurricanes (average 7)
- 3 major hurricanes (average 3)
Unlike last season, the 2021 outlook does not include a hyperactive season within the expected range of outcomes, though there is very little chance for below normal activity this season. It should be noted that a “normal hurricane season” now represents higher levels of tropical activity in all aspects because of the climatology update uses 1991-2020 as the baseline instead of 1981-2010 period used previously. If 2021 outlook was issued based on the previous climatology, the forecast would call for an active season instead of a near normal one.
The Greek alphabet will no longer be used to extend the named storms’ list. If all names in the first list have been used, a supplemental list will begin.
Background:
The major ocean basins’ data support a near to above normal season of tropical activity once again. Beginning with ENSO (El Niño Southern Oscillation), there is an 80% chance of neutral or La Niña conditions being in place by the August-October peak of hurricane season, with only a 20% chance of El Niño.
La Niña is the most favorable state for active Atlantic seasons as it supports low vertical wind shear needed for tropical cyclone formation and intensification, so the strong likelihood of neutral or La Niña conditions in 2021 suggests an active year while the slight El Niño chance reduces that potential.
The Atlantic Multi-decadal Oscillation (AMO) shows an 80% chance to be in its favorable warm Sea Surface Temperature (SST) phase for Atlantic tropical activity.
The largest question is the Indian Ocean Dipole (IOD), which has a connection to Atlantic occurrence of dry air that suppresses tropical cyclone formation. In 2021, there are questions about the state of the IOD by August-October, which supports a nearer to normal hurricane season.
The most reliable forecast variable is Accumulated Cyclone Energy (ACE), which is widely viewed as the best measure of cyclone activity instead of the total named storm number and hurricane number. Since tropical cyclones vary wildly in duration from 1-10+ days, similar numbers of storms in different years can still represent very different levels of activity.
The spread among the storms is relatively narrow, with 20% of the years showing below normal activity while the other 80% showed above normal activity. Due to the narrow range among prior years relative to the new normal level of activity, 100% of the prior years used in the forecast are in the “near normal” range. This results in a high confidence outlook for near to above normal activity in 2021, with the direction of ENSO and the IOD being the key issues to monitor.
The data used in the forecast and current SST anomalies both indicate the U.S. East Coast being at the greater risk for higher impacts than usual, based on warm ocean waters off the coastline. If the model holds, any developing tropical cyclone that moves across the Western Atlantic approaching the U.S. will have ample energy to become a high-impact hurricane if other environmental conditions allow. There is also a consensus for slightly warmer than normal SST around the Leeward Islands of the Caribbean Sea, making that another area to watch for high-end impacts this season. Gulf of Mexico SST is not low but not near the record warmth of last year.
