The 2016 Atlantic Hurricane Season

This year's Atlantic Hurricane season will be stronger, forecasts suggest, than that of the previous two years, and stronger than the average year.

The Atlantic Hurricane Seasons starts on June 1st. But, there was a hurricane that happened already, either late in last year's season or very early in this year's season, called Alex. That hurricane had to go somewhere, and I suppose the keepers of the records had already put their spreadsheet to bed when Alex came along on January 7th, so that storm gets counted as part of the season that will nominally start at the beginning of next month.

A lot of factors determine the number and strength (and path) of hurricanes in the Atlantic. One is sea surface temperatures. An El Niño in the Pacific tends to cause vertical wind shear, which attenuates hurricane formation. Saharan dust also reduces the chance of formation or strengthening of these storms. There may be an association with La Niña conditions and a stronger hurricane season.

A small number of agencies or groups put out their forecasts. Often, the forecasts are similar to each other, and typically they are reasonably accurate. One of the more famous groups comes out of Colorado State University and until his recent death, was led by Hurricane Expert William Gray. NOAA also puts out a forecast. The Earth System Science Center (ESSC) at Penn State has been issuing forecasts since 2007.

The following graphic shows the relationship between the median number of named storms predicted each year by those three sources and the actual number of named storms in the Atlantic.

The first thing I notice is that the total range of variation in the predictions per year is less than the total range of variation in actual numbers over several years, and that the predictions roughly track the actual number of storms. This indicates that something is working. But I also note that the actual number of storms is often outside the range of predictions. So, while the predictions, in terms of number of named storms, give a reasonable estimate of the overall severity of the hurricane season, it is hard to predict these storms accurately months or weeks before the season starts.

This year, we are coming off an El Niño, which added surface heat to the already warm tropical seas, which of course is a result of human caused global warming. Also, we are probably heading into a La Niña season. If you look at the chart, you'll notice that the last two seasons were relatively weak in the Atlantic. Last season's El Niño and, probably, Saharan dust the season before, probably contributed to this.

This year, however, the two plotted forecasts (NOAA and ESSC) suggest a stronger season. There are two other forecasts, not shown on this graph, that also suggest a stronger season.

The average number of named storms for a base period of 1981-2010 is 12.1, so this year is predicted to be above average. The record high number of named storms was 28, in 2005 a year that also produced the record number of full-on hurricanes (15) and major hurricanes (7). That was the year of Katrina, Rita and Wilma, which were very memorable. (2005 was the year they ran out of names, remember?)

The ESSC report has, naturally more detail. From that report (emphasis added):

ESSC scientist Michael E. Mann, alumnus Michael Kozar, and researcher Sonya K. Miller have released their seasonal prediction for the 2016 North Atlantic hurricane season, which officially starts on June 1st and runs through November 30th.

The prediction is for 18.9 +/- 4.4 total named tropical cyclones, which corresponds to a range between 14 and 24 storms with a best estimate of 19 named storms. This prediction was made using the statistical model of Kozar et al. (2012, see PDF here). This statistical model builds upon the past work of Sabbatelli and Mann (2007, see PDF here) by considering a larger number of climate predictors and including corrections for the historical undercount of events (see footnotes).

The assumptions behind this forecast are (a) the persistence of current North Atlantic Main Development Region (MDR) sea surface temperature (SST) anomalies (0.88 °C in late-April 2016 from NOAA’s Coral Reef Watch) throughout the 2016 hurricane season, (b) development of a La Niña (Niño3 anomaly of -1°C) in the equatorial Pacific during boreal Fall/Winter 2016-17 (Climate Prediction Center April 2016 ENSO Discussion), and (c) climatological mean conditions for the North Atlantic Oscillation in Fall/Winter 2016-17.

If no La Niña develops (Niño3 anomaly between +/- 0.5 °C), then the prediction will be lower: 16.1 +/- 4.0 storms (range of 12-20 storms with a best guess of 16).

Using an alternative model that uses "relative" MDR SST (MDR SST with the average tropical mean SST subtracted) in place of MDR SST yields a slightly lower prediction (11.4 +/- 3.4 total named storms). This alternative model also includes the development of a La Niña.

So, as you can see, if there is no La Niñ this fall, there may be fewer storms in the Atlantic.