Increased risk of Gulf hurricanes in 2016

NASA's Dr Tim Hall has run his multi-factor model and read the tea leaves. In 2016, the  chance of landfalling hurricanes in the southern states is significantly higher than the long term average. Mississippi has experienced the greatest increase in risk: 48% above the average.

The 2016 season is now underway, with three named storms to date, including Hurricane Alex, a rare January storm. Here I make probabilistic forecasts of hurricane activity state by state from June 15 through the end of season.

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Forecast percent changes in hurricane landfall rates for 2016 (June 15-December 31) compared to the long-term (1950-2013) mean for the same period. The smaller numbers indicate the uncertainty.

The state of El Nino/Southern Oscillation (ENSO) is well known to affect Atlantic hurricane activity, with El Nino suppressing activity and La Nina enhancing it. The moderate and intense El Nino states in 2014 and 2015 respectively were factors in the quiet Atlantic hurricane seasons of the past two years. The intense El Nino state of this past winter has now relaxed, and ENSO is forecast to enter a moderate to strong La Nina state in the coming months, increasing the probability for an above average 2016 season.  At the same time the subtropical North Atlantic sea-surface temperature (SST) is forecast to be warmer than average in the coming months, another factor enhancing tropical cyclone activity.

To make forecasts of the impact this climate state I drive a stochastic North Atlantic tropical cyclone model (Hall and Yonekura, 2013) 1,000 times with best estimate forecasts of Jul-Aug-Sep ENSO and North Atlantic SST. The rate of synthetic hurricanes (category 1 and higher) crossing coastal US states is then computed and compared to long-term (1950-2013) rates.

The above figure shows the resulting forecasts, a state-by-state map of the percent increase in hurricane landfall probability from June 15 through the end of 2016 over the long-term mean of the same period. The Gulf coast states have the largest increases in risk, while the northeast is essentially unaffected. Over the full coast I forecast an increased probability of 30%. To get a rough estimate of the impact of uncertainty in ENSO and SST forecasts, I rerun the calculations with eight different combinations of ENSO and SST above and below their best-estimate values (eight evenly spaced points about the two-sigma contour of the ENSO-SST error ellipse) and select the minimum and maximum rates as the uncertainty range.  The uncertainty is large: the range about the full-coast 30% increase is 9%-48%, a reminder of the difficulty of the task.

Dr Hall is a climate scientist at the NASA Goddard Institute for Space Studies in New York City. He specialises in hurricanes and their relationship to climate.  He develops statistical hurricane and weather hazard models, and has worked as a consultant to RMS and reinsurance companies. Dr Hall can be reached at www.linkedin.com/pub/timothy-hall/a2/709/b27/

Posted: Monday, June 20th, 2016