Phenomenal waves with a transitioning tropical cyclone (Luis, the Queen, and the Buoys)

A significant portion of tropical cyclones in the North Atlantic reach mid latitudes, affecting New England, eastern Canada, and their adjacent waters. While many of these systems are in the decaying stage, they still possess an extraordinary amount of energy. In Canada, these storms in transition–-referred to at the Canadian Hurricane Centre (CHC) as post tropical–-pose a unique prediction problem and threat.

In 1995, Hurricane Luis was downgraded to a Category 1 hurricane before entering Canadian offshore waters. During its passage over the edge of the continental shelf, Luis was already undergoing transition from tropical to extratropical. The Tropical Prediction Center’s re-analysis–-using Canadian buoy data and reports from the QEII luxury liner–-revealed that Luis actually strengthened before making landfall on the Avalon Peninsula of Newfoundland; it is now quantified in the data set as being a Category 3 hurricane at landfall. Despite this re-classification to a higher Saffir Simpson number, the qualities of a transitioning storm were still evident as it passed through southern Canadian waters, and the accompanying threat of "huge" waves was realized to the right-of-track.

Only hours before Luis made landfall, the QEII felt the brunt of the hurricane and reported maximum winds of 130 knots. A post-diagnosis revealed that a "trapped fetch" with the storm gave rise to significant wave heights of 17+ metres, and phenomenally high maximum waves in excess of 30 metres (reported by both a NOMAD weather buoy and the QEII). However, the speed of Luis greatly exceeded the wave phase speed, such that waves were not in resonance with the storm; conditions could actually have been much worse if the acceleration of the storm through Canadian waters was not as great. Wave reports during Hurricane Luis will be presented, including a discussion of the reports from the Canadian buoy network and the QEII.

Since the wave event with Luis, CHC meteorologists have investigated the problem of "group velocity quasi-resonance" waves—waves that are "trapped" within a weather system and move in harmony with it—and their particular connection to tropical cyclones that move into mid-latitudes. Companion presentations outline the underlying theory of trapped fetch waves with transitioning tropical cyclones and the development of an analytical and predictive model for use by CHC forecasters.