Professor Quibb's Picks – 2021

My personal prediction for the 2021 North Atlantic Hurricane season (written May 16, 2021) is as follows:

17 cyclones attaining tropical depression status,
16 cyclones attaining tropical storm status,
8 cyclones attaining hurricane status,
5 cyclones attaining major hurricane status.

I predict that the 2021 season will feature above-average activity, though not at the pace of the record-breaking 2020 season. Every decade, the NOAA revises the 30-year averages for number of cyclones to reflect new data and better track climate change; the 1991-2020 averages were 14.4 tropical storms, 7.2 hurricanes, and 3.2 major hurricanes, up from 12.1 tropical storms, 6.4 hurricanes, and 2.7 major hurricanes in the 1981-2010 period. My forecast therefore exceeds this new average, but not by a great deal.

In making this prediction, I first consider the El Niño Southern Oscillation (ENSO) index, which measures sea surface temperature anomalies in the equitorial Pacific ocean. Warmer than normal temperatures (El Niño) correlate to decreased Atlantic activity (and increased Pacific activity) and cooler than normal temperatures (La Niña) the opposite. A La Niña event, the strongest in nearly a decade, is currently ongoing and began in the latter half of 2020, contributing to the frenzy of activity of that season.

Nevertheless, recent data indicate that the La Niña is waning and models generally show this should continue into the summer (see the above graph - the vertical axis indicates the relevant temperature anomaly). However, the ensemble average still indicates ENSO neutral to negative conditions. Overall, I predict lingering La Niña effects will still boost activity this year.

In the same vein, most of the 21st century has been in the positive phase of the theorized Atlantic Multidecadal Oscillation, which has led to elevated sea surface temperatures, and, generally, more hurricanes. It's hard to disentangle such long-term climate cycles from modern anthropogenic global warming, and the headline regarding ocean temperatures is largely the same as the last few years: the Atlantic, Caribbean, and Gulf of Mexico will be warm, even by recent standards. The largest anomalies are likely once again to be in the subtropical Atlantic.

A few other factors that influence tropical cyclone formation are relative humidity of the atmosphere and wind shear. A moist atmosphere allows nascent tropical disturbances to develop thunderstorm activity and grow. Wind shear refers to a change in wind direction and speed between the lower levels and upper levels of the atmosphere; higher values of wind shear hamper tropical cyclones because they prevent them from becoming or remaining vertically stacked. Long-term models can give at least some sense of what average conditions to expect during peak hurricane season (see above: the top figure shows expected precipitation anomalies for August-October 2021 and the bottom zonal wind shear anomalies for the same period). Using these and a few other factors, I'll give a finer analysis of the risks by region. My estimates are on a scale from 1 (least risk) to 5 (most risk).

U.S. East Coast: 3
A mixed bag of conditions leaves the U.S. east coast with middling risk. Indications are that the summer will be wet in this region, with ample heat and moisture for tropical cyclones to form and strengthen. On the other hand, the Bermuda high looks a bit weaker than usual given the neutral to negative ENSO index, suggesting that hurricane tracks might veer east out to sea. With neutral ENSO more likely late in the year, expect fall fronts to reduce east coast risk by the end of September; if there are landfalls here, it will be in the front end of the season.

Yucatan Peninsula and Central America: 2
After a devastating 2020, these areas will (hopefully) experience much less hurricane activity this year. Preciptation forecasts (see above) anticipate a drier west Caribbean, and while wind shear will be lower than average in most of the basin, the same is not true for the eastern Caribbean, where threats to the Yucatan and Central America could form. If hurricanes do affect this region, I'd expect it to be in October and November; rapid intensification episodes close to land are the primary risk.

Caribbean Islands: 5
Even in the most active season in history last year, tropical cyclone activity in the main development region in the tropics between the Windwards and Africa was lackluster. Things will probably be different this year. The dusty Saharan Air Layer (SAL), and its suffocating effect on east Atlantic tropical waves, looks to be less prominent than usual. Furthermore, shear is low, temperatures are warm, and precipitation anomalies are at least around neutral over the tropical Atlantic. This could open the door for some long-track hurricanes à la 2017. The biggest question mark is whether these will avoid land or not, but everywhere from the Lesser Antilles to the Bahamas should be on high alert.

Gulf of Mexico: 4
Though maybe a little dry, the Gulf may have the highest sea surface temperature anomalies outside the subtropical Atlantic this summer. Storms forming near the Bahamas and homegrown hurricanes in the Gulf are both likely to occur at sometime in the year, putting this region at above-average risk.

Overall, I expect the 2021 Atlantic hurricane season to be above-average, though not exceptionally so. Nevertheless, this is just an amateur forecast. Individuals in hurricane-prone areas should always have emergency measures in place. For more on hurricane safety sources, see here. Remember, devastating storms can occur even in otherwise quiet seasons.



Sources: https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf, https://www.tropicaltidbits.com, https://www.trackthetropics.com/saharan-air-layer-sal-tracking/, https://www.wunderground.com/hurricane/articles/wind-shear-explainerhttp://www.webberweather.com