Peak hurricane season is approaching. What can we expect?

Two months of 2017 Atlantic Hurricane season down, four more to go. With the peak of this year’s hurricane season yet to pass, so far we have seen seven named storms. None of these storms have become hurricanes, but echoes of hurricane Matthew last year still ring in those areas worst affected.

Can we expect any severe storms this year?

Forecasts at the start of the season called for a slightly above average number of hurricanes this season. Although we haven’t seen any hurricanes yet, early indications suggest the season is primed to meet these expectations.

Dr. James Done, Willis Research Network Fellow at the National Center for Atmospheric Research (NCAR), has produced the second in a series of three hurricane season briefings (available here), in which he reviews a range of forecasts from the many major meteorological organisations and experts from around the world. James summarizes that as we approach the statistical peak of hurricane season (normally late August/early September) we need to be ready for some potentially active periods.

The El Niño-Southern Oscillation drives hurricane activity in the Atlantic, although this year, neutral conditions are most likely to remain

All of the major forecasting centres have increased their outlooks to above average. We’ve already seen some early season Atlantic storms in Bret and Don, and we’re also seeing warmer than average sea surface temperatures. Combine these sea surface temperatures with the fact the most intense storms tend to form in the tropical North Atlantic and Caribbean Sea associated with African Easterly Waves (atmospheric ripples that move westward from Africa and can ‘seed’ storms), and we have potential for extremely favourable conditions to align, which can produce very severe storms.

The El Niño-Southern Oscillation is also a driver of hurricane activity in the Atlantic, although this year, neutral conditions are most likely to remain, so while this doesn’t add potential for storms, it doesn’t detract either.

Seasonal damage potential

An experimental approach to relating forecast model outputs to damage has been developed at NCAR in collaboration with Willis Re and the Willis Research Network. Wind speed doesn’t tell the whole story when estimating how much damage a single storm does: size and forward motion of the storm are also key drivers of hurricane damage. Beyond these there are other factors that can influence how much loss is incurred when considering the storms characteristics. In short, it’s a complex relationship. On a seasonal scale, a similar challenge is how to relate damage to season hurricane activity (which is often represented as a predicted number of storms with a range of uncertainty).

The experimental approach from NCAR creates an index based on predicted sea surface temperatures and environmental winds to represent the drivers of hurricane damage on aggregate across the whole Atlantic. This is the seasonal Cyclone Damage Potential (CDP) index.

They used the forecasts from the Climate Forecast System from the National Centers for Environmental Prediction to derive their seasonal CDP index based on forecasts from March and those more recently in June this year. The image below shows the spread of those forecasts, and how they have changed between April and July this year (red and yellow lines respectively), compared to long-term historical outputs (black lines).

We see an increase in the seasonal CDP values indicating higher potential for damaging storms in the Atlantic this year. Note that this is not a forecast of impact from storms that affect the coastline and make landfall, but it does take a step towards assessing potential impacts beyond simply supplying a predicted number of storms. If this information is calibrated against historical losses on a portfolio of assets, then there may be potential to predict future loss potential for a season.

So as the season approaches its peak months, we need to keep a weather eye on the Atlantic. On a year-to-year time scale, any active or inactive season overall can create extreme catastrophic losses. It only takes one storm to form in the right conditions and head for a populated area to have a huge impact on society and insurance losses. This year appears to hold significant potential for some severe storms, so as ever, we’ll need to make sure that we’re prepared.

About Geoffrey Saville

Geoffrey Saville is a member of Willis Towers Watson's Analytics Technology Team, having joined the company in 2013…
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