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Where have all the hurricanes gone?

Hurricane Sandy

Hurricane Sandy ravaged the east coast last year. Photograph: Nasa/Getty Images


Powered by article titled “Where have all the hurricanes gone?” was written by Harry J Enten, for on Friday 13th September 2013 17.24 UTC

Forecasting anything is hard, and forecasting the weather is extremely difficult. I’ve seen storms predicted to drop two to three feet of snow on New York City leave only three inches. Long-range winter forecasting? Forget about it. Hurricanes usually fall under a different category, though they too prove the limits of modelling in this day and age.

You might have heard that NOAA (National Oceanic and Atmospheric Administration) was predicting 13 to 20 named storms (which have sustained winds of at least 39mph) just before hurricane season started in June. In addition, they called for seven to 11 hurricanes (maximum sustained winds of at least 74mph) and three to six major hurricanes (winds of at least 111mph).

The estimates from other weather agencies and private entities were basically the same: somewhere between 12 and 20 named storms, five and 11 hurricanes, and three to six major hurricanes. The record so far suggests these early forecasts were too bold.

However, we are below the number of storms we should have had at this point. On average, there are five named storms after 10 September. There have been eight named storms, with only one hurricane just formed. That hurricane, Humberto, was almost the latest forming hurricane of the past 40 years. The chances we get the 13 predicted named storms are decent.

The chances we come anywhere close to the number of hurricanes or major hurricanes forecasted by the NHC (National Hurricane Center) are far lower, though. On average, only three more hurricanes, and only one more major hurricane, form this late into the season.

Basically, fewer hurricanes form under the conditions we’ve had this year: too much wind and dry air in the middle and upper atmosphere. (NOAA has pointed this out.)

What caused the inaccurate hurricane forecast (inaccurate so far, anyway)? Long-range tropical storm and hurricane forecasts are based off of a number of environmental factors, such as the development of an El Niño or La Niña in the Pacific, and wind shear and sea surface temperatures in the eastern Atlantic. These variables are then put into historical context to create a type of analog forecast.

Not surprisingly, forecasters update these prognostications throughout the year and even during the hurricane season, which lasts from June to November. If the El Niño weakens, then one would think there would be a more hurricanes. The opposite would occur with a La Niña. An increase in the wind shear decreases the chances of a hurricane, while an increase in sea surface temperatures has the opposite effect.

This year the lower atmospheric conditions were predicted correctly. The sea surface temperatures have been warm. That has meant that tropical storms have been able to develop. The upper atmospheric conditions were not forecasted accurately, though. Hence, storms haven’t been able to intensify into hurricanes.

If the westerly winds that have disrupted hurricane growth die down, then we could end the season with a bang. If we don’t see a particularly impressive end, then it tells what past data hints at; we’re not nearly as good at long-range hurricane forecasts as we might think.

Last year, the final pre-season forecasts missed the mark by a pretty wide margin. Instead of nine to 15 named storms in 2012, we had 19. And rather than four to eight hurricanes, we had 10. In other words, an average to slightly-above-average year ended up with numbers well above average, with Sandy topping it off.

Tropical Storms Forecast Accuracy

These inaccurate forecasts more represent the norm than the exception. The actual number of storms should fall within the forecast 70% of the time per the NHC. From 2002 to 2012, five of the 11 tropical storm forecasts have fallen outside this interval.


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