Tropical Update: Arthur

By: - July 1st, 2014

With a month of the Atlantic hurricane season in the books, one might think that the quiet Atlantic hurricane season is unusual. Historically, however, the year-to-date Atlantic hurricane season typically only experiences an Accumulated Cyclone Energy (ACE) index value of 1, based on the 1981 – 2010 climatology. Also, on average the first named storm typically does not form until the first week in July, with the first hurricane not showing up until mid-August. According to Roger Pielke Jr.’s normalized economic hurricane loss dataset, when looking at damage from tropical cyclones, historically only 2% of hurricane damage occurs in July, with 95% occurring in August and September. In fact, with the development of the first named tropical storm of the 2014 Atlantic hurricane season (Arthur) off the southeast coast of the U.S., the 2014 season is matching nicely with climatology, and by July 4 it should be ahead of climatology.

Earlier this spring, in our first look at the 2014 hurricane season, it was mentioned that not all El Niño seasons are the same. Even if an El Niño develops, it does not mean that the Atlantic hurricane season will have limited impact. In that post we highlighted past seasons, such as 2004, where El Niño had a high impact and we further detailed the importance the warmer- than-normal Sea Surface Temperatures (SST) off the East Coast could have on the upcoming season. Arthur is currently centered over these warmer-than-normal SSTs and is expected to strengthen into the first hurricane of the 2014 season.

Above is the National Hurricane Center (NHC) official track and intensity forecast, as of 11 AM EDT, showing Arthur tracking along the southeast coast of the U.S. over waters of at least 26 degrees Celsius. This water temperature is warm enough to support hurricane development. According to the NHC, Arthur is expected to just by pass the Outer Banks of North Carolina as a category 1 hurricane on Friday July 4th.

Another factor that will aid in hurricane development is the natural curve of the southeast coastline. Historically, the curve of the coastline has helped similar storms develop in this area, by providing a natural pressure/wind gradient that allows for counter-clockwise rotation. In 2004, Hurricane Alex battered the outer banks and strengthened in a 42-hour period from a minimal 35 kts tropical storm to a 85 kts hurricane, as it tapped into the warm waters of the Gulf Stream. Hurricane Alex produced light damage in the Outer Banks, primarily from flooding and high winds. Over 100 houses were damaged and damage totaled approximately $7.5 million (2004 USD) in economic loss.

As Arthur develops, an approaching trough of low pressure that is moving into the central U.S. will provide an atmospheric pattern conducive to low pressure development on the southeast side of the trough; this low pressure will allow for further intensification later this week. However, this approaching trough will not only keep the upper Midwest and parts of the East Coast cool for the July 4th holiday weekend, it will likely provide the steering flow to push Arthur off shore and provide minimal impact to insured property along the East Coast. This would be similar to the impact of Alex in 2004.

The greatest threat will be to the North Carolina Outer Banks on the 4th of July, as the storm tracks 50 – 100 miles east as a possible strong category 1 hurricane. It has been 1 year, 10 months and 1 day since the last hurricane hit the U.S. (Hurricane Isaac). With the understanding that Superstorm Sandy was officially downgraded miles off the NJ coastline, keep in mind that hurricane Sandy rapidly strengthened, due to a warm gulf stream and Arthur has access to similar warm waters to spur it on. It is these warmer-than-normal SSTs that need to be watched all season.