Forecast group Tropical Storm Risk (TSR) has raised its forecast for the 2023 Atlantic hurricane season for a second time, now citing an expectation of activity levels 10% above the 1991-2020 climate norm.
Back in July, the TSR forecast team said that warm sea surface temperatures in the tropical Atlantic will counteract the increased vertical wind shear and stronger trade winds that occur under El Niño conditions.
This led it to increase its Atlantic hurricane season forecast to 17 named tropical storms, 8 hurricanes and 3 major hurricane of Category 3 or more strength, with ACE forecast at 125, so slightly above normal.
Now, one month later, TSR has said that one of those factors it cited may not have the influence it anticipated.
It explained, “TSR’s reason for raising its forecast is due to the July-September trade wind speed now forecast to be neutral or slightly favourable for Atlantic hurricane activity. In prior years when the July Niño 3.4 SST has been as warm as in July 2023, July trade winds have been stronger than normal which is correlated with increased vertical wind shear through the season and below-average Atlantic hurricane activity. In July 2023, despite the El Niño conditions, the trade winds have been slightly weaker than normal suggesting more favourable atmospheric conditions may persist through August-September.
“Combined with the very warm Atlantic sea surface temperatures an above average hurricane season is now forecast.”
The updated forecast now calls for 18 named tropical storms, 8 hurricanes and 3 major hurricane of Category 3 or more strength, with ACE forecast at 140, so further above the norm.
This now forecasts a 33% probability of an above-normal North Atlantic ACE in 2023, a 54% likelihood of a near-normal ACE, and only a 13% chance of a below-normal ACE season, TSR said.
Citing elevated uncertainty this year, TSR said, “It should be noted that sizable uncertainties still remain. There is good confidence that sea surface temperatures in the tropical Atlantic will be much warmer than average. There is considerable uncertainty in how favourable the trade wind predictor will be for enhanced hurricane activity. Variance also exists in the level of hurricane activity possible from the same climate factors and due to potential influences from variables which cannot be predicted, such as Saharan air outbreaks over the tropical Atlantic, and even if the two August-September predictor fields are anticipated correctly there is still a spread of possible hurricane activity levels.”
If trade winds also prove lower than anticipated, there are now increasing factors that suggest we could be in for an active period at some stage this season, if the forecast numbers are to stand a chance of being reached.
Adding in this latest forecast update to our Artemis average, across the forecasters we track, keeps it at 16 named storms, 8 hurricanes and 3 major hurricanes, with the average ACE seen as 154, still a slightly above the recent historical average season.
Track the 2023 Atlantic tropical storm and hurricane season on our dedicated page and we’ll update you as new information emerges.