Hawaii threatened by two hurricanes, Iselle and Julio

Hawaii is being threatened by two hurricane landfalls in the coming days, as first hurricane Iselle makes its way towards a direct hit on Big Island, before hurricane Julio follows along just a couple of days later.

Hurricane Iselle is the main threat, based on the current forecasts. Once a category 4 hurricane, Iselle is now a strong category 1 hurricane with maximum sustained winds of around 90mph. The forecast path takes Iselle directly onto Big Island, with a landfall predicted to be at category 1 strength.

Hurricane Iselle forecast path or track

Risk modelling firm AIR Worldwide said:

The main hazards facing Hawaii are tropical storm force winds on the Big Island, especially in Hilo, the largest city on the island, which sits right at the center of Iselle’s CPHC cone of uncertainty. Large waves are also a hazard along the coast and may lead to erosion. In addition, heavy rains are expected, which could lead to mudslides and flash floods.

Residents and the many visitors to the Hawaiian Islands have been warned to prepare for tropical cyclone Iselle. Other impacts could include sea surges and flooding, as well as power outages, airport departure delays, cancellations of cruise ship dockings, and curtailment of charter boat and private boat activities.

According to AIR, with wind speeds down to tropical storm strength at landfall, the primary exposures at risk from Iselle will be residential structures. Most single-family and duplex homes in Hawaii are of wood frame construction, with about 40% of these being single-wall wood frame construction. Generally load-bearing walls in these buildings are made of thin plywood boards and thus are quite susceptible to wind damage. (Buildings of this construction did not fare well when Category 4 Hurricane Iniki struck.) With tropical–storm-force winds, significant damage is not expected to residential structures built to code. Older structures, and those not built to newer building standards, may see some impact to roofing cover or poorly attached siding from tropical–storm-force winds. As with any storm, debris from failed roof covers or other sources could be mobilized to generate further damage.

Hurricane Julio will pass slightly further to the north than Iselle and is expected to weaken to tropical storm strength before it makes its closest approach to Hawaii. The main threat therefore will be from the additional rainfall levels Julio will bring, which on top of Iselle will exacerbate flooding conditions, particularly on Big Island.

Hurricane Julio forecast path or track

With these two hurricanes threatening Hawaii at once it’s a useful reminder that the threat from named storms and hurricanes to the reinsurance industry is not only in the Atlantic, but U.S. exposures also exist in the Pacific. A few of the outstanding catastrophe bonds which cover named storm risks are also on-risk for a Hawaiian hurricane, but it would need to be considerably larger than either of these in order to worry the market unduly.

Latest update on hurricane Iselle from Aon Benfield’s Impact Forecasting unit:

COORDINATES: 18.5° north, 150.6° west (previous location: 16.9° north, 144.1° west)
LOCATION: 305 miles (490 kilometers) east-southeast of Hilo, Hawaii
MOVEMENT: west-northwest at 17 mph (28 kph) (previous movement: west-northwest at 13 mph (20 kph))
WINDS: 80 mph (130 kph) with gusts to 100 mph (160 kph) (previous sustained winds: 90 mph (145 kph))
RADIUS OF TROPICAL STORM-FORCE WINDS: 140 miles (220 kilometers) from the center of circulation
RADIUS OF HURRICANE-FORCE WINDS: 35 miles (55 kilometers) from the center of circulation
CENTRAL PRESSURE: 986 mb (29.12 inches of mercury) (previous pressure: 979 mb)
SAFFIR-SIMPSON SCALE RANKING*: Category 1

LANDFALL AND LOSS PROBABILITIES
FORECASTED LANDFALL LOCATION: Hawaii’s Big Island
FORECASTED LANDFALL TIMEFRAME: Thursday evening local time HST
24-HOUR SIGNIFICANT LOCAL INSURED LOSS POTENTIAL: MEDIUM

DISCUSSION
Hurricane Iselle, located approximately 305 miles (490 kilometers) east-southeast of Hilo, Hawaii, is currently tracking west-northwest at 17 mph (28 kph). Although the satellite presentation of Iselle has degraded slightly during the past couple of hours, a persistent eye feature continues to be observed and Iselle remains a hurricane. Data from an Air Force reconnaissance aircraft corroborate the slight weakening trend that has been observed by satellite as both flight-level and surface winds have diminished. Because of this, the CPHC has lowered the initial intensity to 80 mph (130 kph).

The initial motion estimate remains towards the west-northwest as Iselle continues to be steered by a mid-level ridge of high pressure located to the north. The updated CPHC forecast track remains consistent with the last several forecasts in the short term, and remains in good agreement with the consensus that has been consistent in depicting landfall along the Big Island this evening. In 36 to 48 hours Iselle is expected to reach the western periphery of the ridge as a weak cut off low digs southward along the forecast track, resulting in a slowing of the forward motion through 72 hours. As the low moves south and Iselle weakens, a new ridge aloft is forecast to build north of a weakening Iselle, and the forward motion will again increase. The new forecast track only offers minor changes to the previous one.

The intensity forecast calls for only slight weakening before landfall tonight, despite the forecast of moderate northwesterly wind shear. Iselle remains south of a ridge aloft centered west of the main Hawaiian Islands and a ridge centered over the Baja Peninsula. A weakness between these two ridges provides a lighter shear environment in which Iselle would be able to maintain the current intensity before landfall. However, if Iselle were to move west of the weakness and closer to the ridge centered to the west, the wind shear would be more debilitating. The CPHC notes that the intensity forecast problem surrounds the global models offering different solutions as to the interaction between Iselle and the flow aloft in the short term. Some models track Iselle and the weakness westward in tandem; while others move Iselle west of the weakness and into an area of stronger wind shear. Interaction with the terrain of the Big Island is expected to weaken Iselle tonight, while a southward-moving cut off low developing northwest of the Hawaiian Islands on days 2 and 3 will bring stronger wind shear. Interaction with this low is expected to lead to the demise of Iselle, with remnant low status forecast by Day 5.

The onset of tropical storm conditions is expected on the Big Island of Hawaii this afternoon, with hurricane conditions expected tonight. Tropical storm conditions are expected over Maui County tonight, over Oahu on Friday, and over Kauai County on Friday afternoon.

Very large and damaging surf is expected to rapidly build along east and south facing shores today and tonight, especially on the Big Island.

Rainfall totals of 5 to 8 inches – with isolated maximum amounts to 12 inches – are expected along the track of Iselle. These rains could cause life-threatening flash floods as well as rock and mud slides.

The combination of a dangerous storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters. The water could reach the following heights above ground if the peak surge occurs at the time of high tide:

Big Island windward and Kau: 1 to 3 feet