Waterspout outbreaks over areas of Europe and North America: Environment and predictability

Waterspout outbreak occurrences are examined for the areas of the Aegean and Ionian Seas, Eastern Mediterranean, the Baltic Sea in Northern Europe and the Great Lakes of North America. Notable outbreaks included a series of 13 waterspouts on September 5, 2002, and 25 plus waterspouts on September 21, 2006 off the Cretan north coast in the Aegean Sea. Two outbreak days with 10 waterspouts each, occurred on August 11 and 13, 2006 over the Baltic Sea. During the fall of 2003 an outbreak produced about 66 waterspouts during a week over the Great Lakes of North America. A preliminary climatology of waterspout outbreak occurrences showed August as the most active month for the Baltic Sea and the Great Lakes and September for the Aegean and Ionian Seas; a feature primary related to the warmest sea surface temperatures during these months. Synoptic data indicated that a closed low (CLOSED) type pattern prevailed during waterspout outbreaks over the Baltic and the Aegean and Ionian Seas and a longwave (LW) type over the Great Lakes. Thermodynamic, wind and moisture data indicated a marginally unstable environment for most waterspout outbreaks. A steep thermal gradient between the water surface and lowest layers proved to be a primary factor for Great Lakes waterspout outbreak occurrences. For the Baltic Sea, the primary factors were balanced between the thermal contrast in the lowest layers and the total positive buoyancy. For the Aegean and Ionian Seas, positive buoyancy was the dominant factor. Selected waterspout outbreak case studies are investigated in order to present specific synoptic, thermodynamic features and predictability for the three areas. The Szilagyi waterspout nomogram, which is used as a forecasting tool, was applied together with the Szilagyi waterspout index (SWI) and found to be strongly correlated with outbreak occurrences for the three geographical areas.