According to one study, May is the most dangerous month for tornadoes in the United States, with an average of 329, while February’s average is the safest with only three. In another study the months December and January were usually the safest, and the months having the greatest number of tornadoes were April, May, and June. In February, tornado frequency begins to increase. February tornadoes tend to occur in the central Gulf states; in March the center of activity moves eastward to the southeastern Atlantic states, where tornado activity peaks in April. In May the center of activity is in the southern Plains states; in June this moves to the northern Plains and Great Lakes area (into western New York). The most costly outbreak of tornadoes occurred in May 1999, when at least 74 tornadoes touched down in less than 48 hours in Oklahoma and Kansas, including an F5 on the outskirts of Oklahoma City causing $1.1 billion in damage.