eVENT Willis Research Network Hurricane Commentary

Hurricane Irma Executive Summary IRMA remains a major hurricane, despite weakening slightly overnight. It is a strong Category 4 storm and continues on a west-northwest track towards Florida. Uncertainty remains in the exact time of the northward turn. The timing will determine the severity of the impacts on the U.S. mainland, but currently southern Florida is likely to see the most severe conditions, although northern Florida, Alabama and southeast coastal states should also prepare for significant impacts. is expected to drive significant coastal damage, dependent on the storm’s track.

Date/Time 11am AST Sep 8, 2017 Location 22N, 75.3W Windspeed/Pressure 150mph, 927mb (Cat4) Speed/Direction 14mph WNW

Forecast Summary In yesterday’s update, IRMA was producing sustained Category 5 winds as it scoured the northern Caribbean. The Lesser have taken the brunt of this powerful storms at the most severe category, with islands form to the Turks and Caicos experiencing significant damage from 15-20 foot storms surge. The extreme intensity of IRMA, and rarity for these Islands is likely to have caused destruction beyond living experience.

Forecast tracks are in very good agreement for at least the next 24 -36 hours (Figure 1). Overnight IRMA went through an -wall replacement cycle, which has led to a drop in maximum wind speeds. This means that the storm is now Category 4, albeit on the upper end of the category’s wind thresholds. However, the renewal of the eye-wall has also led to an expansion of the wind field, so a larger area is now experience hurricane force winds. Over the next 24 hours the surrounding environment will remain favorable, with light wind shear and warmer sea surface temperatures. Therefore the forecast for this period confidently predicts that it will not weaken and may still re-intensify to become category 5 again before landfall. However, if IRMA makes a closer pass to ’s coastline, this may interfere with the circulation and weaken the storm, but, this is unlikely (Figure 1) and so IRMA should be expected to remain Category 4 or 5. In the last, Hurricane Advisory the Cyclone Damage Potential calculated for the storm was a formidable 4.8, compared with a value of 2.9 for Hurricane Andrew (1992).

The influence of the high pressure system that has guided it over the past few days will diminish as IRMA approaches the U.S. Instead, IRMA will be influenced more by a low pressure weather system ahead of its path. The transition between two influencing weather systems has created some uncertainty on the exact track of the storm, and therefore the timing of the northward turn. During the last few days the intensity of the low pressure system ahead of the storm has become more clear and so the forecast tracks are converging, but the interaction between the low pressure and the high pressure remains a source of uncertainty.

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The forecast tracks have taken IRMA to the east of Florida in recent model runs, but now the consensus is for a direct hit on the southern tip of Florida. However, regardless of whether the track stays to the east, or takes a more direct approach to the southern counties, including the exposed Florida Keys, most of Florida should expect significant impacts. A Category 4 or 5 hurricane landfall in Florida should be prepared for, and despite the current forecast track being further west than previously, severe impacts in Miami-Dade county cannot be ruled out.

In the long range, northeast Florida, Georgia and South Carolina are now less likely to see the direct landfall. However, due to the size of the hurricane, the storm surge risk for these coastal states remains high. In the worst-case scenario, surge heights exceeding 15 feet are possible in some locations across and southern Florida, although this is unlikely to be widespread for the eastern Florida coastline due to bathymetry. The region is also at risk of extreme rainfall, as we saw with Hurricane Matthew last year, with 12- 18 inches possible in many areas.

There are other storms currently active. The occurrence of three simultaneous active hurricanes in the Atlantic is relatively rare, especially considering the severity of IRMA, and it hasn’t happened for 7 years. Hurricane JOSE is also an extremely dangerous Category 4 hurricane threatening the Northern , less than a week after IRMA, despite the cold-wake in sea surface temperatures left behind by Irma. JOSE is expected encounter increased wind shear, and turn northward as a Category 1 or 2 hurricane. It’s early to forecast confidently, but JOSE may yet threaten . The third in the trio is , which is now a Category 2 hurricane, and is slowly moving towards the east coasts of Mexico, to the north of Veracruz.

Figure 1: Model forecast tracks (left) and intensity (right) for at 1200UTC 09/08/17 (NCAR/TCGP). For updated forecasts, please visit: http://www.ral.ucar.edu/hurricanes/realtime/current/

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Geoffrey Saville Roy Cloutier Senior Research Manager Catastrophe Analytics Willis Research Network Atmospheric Hub Willis Re Inc. Willis Towers Watson 8400 Normandale Lake Blvd Willis Group Limited Bloomington, MN 55437 20th Level The Willis Building 51 Lime Street EC3M 7DQ. Tel: +1 (952) 841-6652 Tel: +44 203 124 8858 Email: [email protected] Email: [email protected] Website: http://www.willisresearchnetwork.com/ Website: http://www.willisresearchnetwork.com/

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