As the likelihood of Atlantic hurricanes builds to a peak on September 10, forecasts for the remainder of the year become increasingly skilful. InsuranceLinked's resident meteorologist has used the latest observations to calculate that 2015 should experience 43% fewer hurricanes than the long term average. The biggest falls in hurricane probabilities are in the north-eastern states.
Back in April, we forecast this year's hurricane season would be relatively quiet. Where do we stand, now that we’re near mid season? And what are the prospects for the remainder of the season?
April’s forecast was driven by two factors: a moderate El Nino and near-neutral North Atlantic sea-surface temperatures (SST) anomalies. Since April, El Nino has strengthened, and may even reach the historic levels of 1998 by winter. Meanwhile North Atlantic SST anomalies have increased somewhat. El Nino tends to suppress hurricane activity, while increased SST tends to enhance it. Taken together (strong El Nino and modestly elevated SST anomalies), they moderately suppress activity.
And yes, the season has been relatively quiet to date. The Atlantic has had three named tropical cyclones through August 13 (Ana, Bill, and Claudette), compared to an average of 4.7 for this date (1950-2013). None has reached hurricane status, compared to an average of 1.3 for this date. So far, the accumulated cyclone energy (ACE) is only about 4% the average value over a full season. This is in stark contrast to the North Pacific which has been extremely active.
To make a quantitative forecast for the remainder of the 2015 Atlantic season, we use the stochastic hurricane model of Hall and Yonekura (2013) to simulate multiple realisations of the 2015 season, starting August 14, driven by the El Nino forecasts and the Atlantic SST anomaly forecasts. We then compare the ensemble-mean forecast landfall rates to long-term (1950-2013) landfall rates.
The figure shows the probability of there being at least one hurricane landfall over the rest of the season state-by-state compared to the 1950-2013 mean probability from this date. There is fractional reduction everywhere, ranging from 5% to 25%. The geographic structure is a result of the complex interplay between the seasonal phasing of cyclogenesis and tracking and their variations with El Nino and SST. There is considerable uncertainty due to the intrinsic stochastic nature of the model and to the uncertainty in El Nino and SST forecasts driving the model. Overall there is an approximately 10% reduction in the hurricane landfall rate.
Still, there is less uncertainty now than back in April, in large part because the hurricane model is ingesting El Nino and SST forecasts at shorter lead times: the next three months versus the next seven from April. Currently all El Nino models forecast a significantly positive state over the remainder of the season, while back in April the spread of results included the neutral state.
What about the full-season landfall counts? We haven’t had a hurricane landfall yet this season, compared to a 1950-2013 mean rate of about 0.5 US landfalls by August 13. When we combine the season already observed through August 13 with the simulations of its remainder we estimate a 43% reduction in the 2015 full-season US landfall count compared to 1950-2013. This is in good agreement with other independent estimates.
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, August 17th, 2015