Risk of Florida hurricanes is 29% above average

It is now late enough in the year to get good readings on the two meteorological conditions that are known to correlate with hurricane activity. NASA's Tim Hall has run his model to predict a 29% higher frequency of hurricanes in Florida when compared to the long term average.

Forecast percent changes in hurricane (category 1 and higher) landfall rates for 2017 (June 28-Dec 31) compared to the long-term (1944-2015) mean for the same period. Colors indicate percent change, as labeled, with red positive and blue negative. Individual values with uncertainty estimates are listed for Texas, Florida, and North Carolina. Over the full coast the change is 22%.

The 2017 season is now underway, with three named storms to date. None reached hurricane intensity, but Cindy caused significant flooding on Louisiana.  Looking into the bulk of the season I predict US landfall odds to be 22% above average based on forecasts of two major hurricane factors, Atlantic seas-surface temperature (SST) and the state of El Nino/Southern Oscillation (ENSO).

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 ENSO Nino34 indicator is currently near neutral, and its forecast for the next several months is neutral to moderately positive (El Nino), weakly inhibiting activity. Meanwhile, subtropical North Atlantic sea-surface temperatures (SST) are forecast to be warmer than average, enhancing hurricane activity.

To estimate the net effect on US landfalls 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 simulated hurricanes crossing coastal US states is then computed and compared to long-term (1944-2015) rates.

The figure shows the results, a state-by-state map of the percent change in hurricane landfall probability from Jun 26 to Dec 31 compared to the 1944-2015 mean of the same annual period. The North Gulf states and Florida have the largest increases in hazard, while Texas and the east coast are less affected.

The uncertainty is large---seasonal forecasting is a difficult business. When I rerun the calculations using the most favorable and least favorable ENSO and SST combinations of their forecast uncertainties I get a range of 1% to 50% about the 22% increased full-coast landfall rate. Best-estimate values and uncertainty ranges are listed individually in the figure for Texas, Louisiana, Florida, and North Carolina.

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/


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Posted: Monday, June 26th, 2017