It has been a quiet year for Atlantic hurricanes, and it is increasingly likely to stay that way. In the first of a series of columns, Dr Tim Hall from NASA forecasts reduced odds for US hurricane landfalls during the remainder of the season.
There is, roughly, 50:50 chance for a post September 1 hurricane to make landfall somewhere in the US based solely on long-term hurricane behaviour. However, climate conditions that vary month-to-month are known to affect hurricanes. By using the current atmospheric conditions and forecasts for the next few months, it is clear that there is a lower chance of a devastating hurricane in the US than we would usually expect.
This figure shows the difference between a forecast of landfalling probabilities that uses current climate conditions and a forecast based on long term averages.
Using a climate-dependent statistical hurricane model we can estimate close to 20% reduction for the 2014 post September 1 probability of large hurricane landfall compared to the long-term average (that is, a probability reduction from 0.21 to 0.17). There are considerable regional differences - Texas is more than 30% less likely to be hit (0.03 to 0.02), while probabilities on the US mid-Atlantic coast and northeast differ by only a few percent from their long term averages.
These shifts in odds are due mostly to the current and projected states of El Nino Southern Oscillation (ENSO) and Atlantic sea-surface temperatures (SST). ENSO is an irregular Pacific atmosphere-ocean variation that has global reach, and a positive (El Nino) phase tends to suppress Atlantic hurricanes by increasing the wind shear, which inhibits development. ENSO is forecast to be increasingly positive as the year progresses. In addition, SST in the Atlantic are currently (and projected to remain) cooler than the rest of the tropics, a spatial pattern that reduces Atlantic activity.
The variation along the coast is due to the interplay among changes in overall hurricane formation counts, changes in Atlantic regions of this formation, and shifts in mean storm propagation paths. See Hall and Yonekura, (Journal of Climate, 2013) for a discussion of these topics.
This map shows the regions used in this analysis
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/. The ENSO forecast was obtained from Columbia University IRI (http://iri.columbia.edu/our-expertise/climate/forecasts/enso/current), and the SST forecast from NOAA Earth System Research Laboratory (http://www.esrl.noaa.gov/psd/forecasts/sstlim/for4gl.html).
A recent article discussed some of the challenges of hurricane forecasting - Reading tea leaves.
This chart shows other seasonal forecasts
Posted: Monday, September 8th, 2014