The chances for above normal hurricane activity in the Atlantic Basin this year have increased according to the newly-released seasonal update from NOAA.
The agency also said El Nino — a climate phenomenon that warms the water in the eastern Pacific Ocean and often reduces hurricanes in the Atlantic — has officially ended. This is the primary reason for the higher chance of more tropical activity.
The official update, which was released Thursday, includes a 45 percent chance of above normal hurricane activity, a 35 percent chance of below normal activity, and only a 20 percent chance of near normal activity. NOAA predicts 10 to 17 named tropical storms, which include Andrea and Barry from earlier in the season. Of those tropical storms, 5 to 9 are forecast to become hurricanes, with 2 to 4 of those reaching major hurricane (category 3 or greater) status.
The demise of El Niño is the primary reason why NOAA increased their odds for an active hurricane season. Dr. Gerry Bell, NOAA’s lead hurricane seasonal forecaster says a weakening El Niño changes the atmospheric setup over the Atlantic.
“El Niño usually suppresses hurricanes, but now that it has dissipated, we’re expecting conditions to be more favorable for storm development through the rest of the season.”
El Niño’s dissipation isn’t the only reason an active season is becoming more likely. Water temperatures are now above normal over most of the Atlantic, except in a small area near the African coast. Warm ocean waters are well-known to contribute to stronger and more tropical storms and hurricanes, if other conditions are present.
Winds high in the atmosphere are lighter than average over most of the Atlantic, except in the Caribbean Sea. These weaker high-altitude winds also promote the formation and longevity of hurricanes.
The monsoon over western Africa and strong African easterly jet stream tend to aid in the formation of thunderstorms that eventually become tropical waves near the Cape Verde Islands. As more of these tropical waves move off the African coast, the greater the chance that a few of them will grow stronger.
Despite the increased chances for more storms, there is still uncertainty in the forecast. In their longer form narrative, the seasonal forecasters say there are still lingering effects from the El Niño and it’s unclear how strong or persistent these leftover impacts will be over the next few months.
Dr. Phillip Klotzbach of Colorado State University, who leads a team of scientists that also publish seasonal hurricane forecasts, is cautious for the same reasons NOAA forecasters are.
“There’s uncertainty with every forecast, but with the August forecast there probably is a little more uncertainty, just given that the El Niño forcing is relatively weak, and also the Atlantic forcing looks to be relatively weak.”
There are still strong winds at high altitudes over the Caribbean Sea, likely left behind from the El Niño. If these strong winds (known as wind shear) hang around longer than expected, it may cause some of the storms trying to form near the African coast to weaken as they move into the Caribbean.
NOAA cautions that its forecast does not predict the number of landfalling hurricanes, or whether those hurricanes will impact a particular location. It’s usually not possible to make those assessments until a few days prior to a landfall. For this reason, it’s important for communities vulnerable to hurricanes and tropical storms to prepare regardless of the seasonal forecasts.