Analysis of the Relationship of Waterspout Day Frequency in the Florida Keys to Synoptic-Scale Patterns

Waterspouts are likely to occur more frequently along the Florida Keys coastal waters than anywhere else in the world, and pose a potential hazard to the marine community and coastal residents. The distribution of waterspout days in this region exhibits a clear annual cycle, with the peak of waterspout activity seen during the locally defined wet season (June through September) with one or more waterspouts occurring on approximately 20 percent of days. In the wet season’s transition months (May and October) waterspouts occur on approximately 12 percent of days, with a substantial drop in activity outside these months. In addition to this seasonal variability, monthly waterspout activity has a large interannual variability component: using the month of August as an example, there were ten waterspout days in 2015, compared to just one in 2012. Prior analysis has demonstrated a strong statistically significant relationship between waterspout probability in the region on a given day and radiosonde-derived data from the morning sounding at a single station (Key West). This relationship was shown to be strong enough to allow for the construction of a well-performing daily statistical forecast model (Devanas and Stefanova 2018), which is used daily by National Weather Service Key West forecasters.

In order to explore the feasibility of producing a waterspout outlook on a longer timescale, the connection of waterspout activity to synoptic-scale variability during the wet season is assessed. Statistically significant relationships are diagnosed between waterspout activity and synoptic scale patterns obtained from atmospheric analyses (NCEP-DOE Reanalysis 2 and NCEP Daily Global Analysis). For example, a strong relationship is found between waterspout occurrence in the Florida Keys waters and the strength and position of the Bermuda High, the total column precipitable water values over the southeastern United States and southwestern Atlantic, and the 100 hpa heights over the Eastern Seaboard (including the Florida Keys and coastal waters). The exploration of these statistically significant synoptic patterns in place during, and preceding, waterspout occurrence lays the groundwork for development of a synoptic scale based waterspout outlook.