The Right Side of a Storm

By: - July 3rd, 2014

The insurance industry often focuses on media graphics that depict a storm’s path and the “cone of uncertainty,” but many of these graphics fail to explain the physical structure of a hurricane. The extent of hurricane damage doesn’t solely depend on the strength of the storm. It is also greatly influenced by the way the storm makes contact with land, and whether the left or right side of a hurricane strikes a given area.

The “right side of the storm” refers to the storm’s motion. For example, if the hurricane is moving to the west, the right side would be to the north of the storm; if the hurricane is moving to the north, the right side would be to the east of the storm. In the Northern Hemisphere, the strongest winds in a hurricane are generally found on the right side of the storm because the motion of the hurricane contributes to its swirling winds. Therefore, the right side of a hurricane packs more punch, since the wind speed and the hurricane’s speed of motion align. Conversely, on the left side, the hurricane’s speed of motion subtracts from the wind speed. The National Hurricane Center (“NHC”) forecasts take this asymmetry into account and often predict that the highest winds are generated on the right side of the storm.

The image above illustrates why the strongest winds in a hurricane are typically on the right side of the storm.

Hurricane Arthur is now less than 12 hours from impacting the North Carolina coastline, with a forecasted intensity of a strong Category 2 storm. Knowing the exact track of Arthur is critical to predicting the expected damage. If Arthur follows a more easterly track and skirts North Carolina’s Outer Banks, as suggested by the Geophysical Fluid Dynamics Laboratory (“GFDL”) model and current NHC forecast, it would mean the strongest winds (i.e., the right side) would remain away from the Outer Banks and offshore. However, forecast adjustments have been increasingly trending to the west, and with most U.S. models favoring a landfall near Morehead City, NC, the worst possible conditions would hit the Outer Banks as the storm tracks up Pamlico Sound.

Above is a view from BMS iVision, which, using model guidance from Verisk Respond, currently puts the right side of the storm and the strongest winds directly over the Outer Banks. This real-time wind forecasting information within iVision will enable clients to view the effect of Hurricane Arthur’s wind swath on their policy base, therefore providing a better estimate of exposed locations and possible losses. This westward track also increases concern for storm surge. The islands of the Outer Banks flood very easily, and the latest forecast by the NHC suggests up to three feet of water over US-64, which is one of two roads crossing the Outer Banks. However, Arthur’s forecasted approach along the North and South Carolina coastlines should limit the impact of a large storm surge.

While the Outer Banks is no stranger to hurricane-force winds, or even storms named Arthur (which occurred in both 1996 and 2002), this storm is forecasted to be one of the strongest to impact the area since Hurricane Emily in 1993. With an estimated return period of a hurricane passing within 50 miles of the Outer Banks occurring every five years, property has generally been upgraded to withstand such storms. However, the strongest winds staying to the right side of the current NHC track will determine the final outcome of damage and loss.