Undergraduate Research and Creativity

URECA

2007-2008

Atmospheric Conditions Associated with Tornadoes on Long Island

Alexander Titus and Brian A. Colle
School of Marine and Atmospheric Sciences

The goal of this research project paper is determine the atmospheric conditions that favor tornadoes on Long Island. Both the Atlantic Ocean and the Long Island Sound play an important role in modifying severe weather on Long Island. Typically, the relatively cool water stabilizes the low-level atmosphere, thus severe storms often weaken over the island. However, there have been 27 tornado reports on Long Island from 1958 to 2007. One objective was to determine the time of year and day that favors tornado events on Long Island as well as the large-scale atmospheric flow patterns around the Northeast U.S. associated with these events.

First, a database of tornadoes on Long Island was compiled using a program called SVR Plot2, which is a national database for severe storm reports across the U.S. The Long Island tornado events were separated into tornado location, the times in which the storms occurred, and the month. The dates of the tornado occurrences were composited on a 2 x 2 deg grid using the National Center for Atmospheric Research composite website (http://www.cdc.noaa.gov/Composites/Hour/), in which horizontal plots of the average weather conditions at each grid point was obtained by summing a particular meteorological variable (surface temperature, pressure, wind, etc…) from all tornadic storms at a grid point and then dividing by the total number of storms in the composite. Plots were compiled of the different layers of the atmosphere, as well as certain thermodynamic properties.

Overall, tornadoes on Long Island occur just as frequently in the morning as the evening, with an equal chance between 8 AM and 8 PM. This is in contrast to other locations in the Northeast U.S., which have a significant peak in tornado activity in the late afternoon. The most active month for tornadoes on Long Island is August as compared to June and July in other parts of the U.S. This may be attributed to surface temperatures reaching their peak on Long Island by August, since the surrounding waters also reach their maximum temperature around this time. The composite surface pressure pattern across the Northeast U.S. suggests that many tornado events have a trough approaching Long Island from the west, which results in large-scale ascent during these events. The low-level thermodynamic composites suggest a wedge of warm, moist, and unstable air extending northward along the mid-Atlantic coast to Long Island. The approaching cyclone advects this moist and unstable air northward into the region, which is likely the reason why there is little diurnal preference to tornado events on Long Island.