The February 2009 North American storm complex occurred on February 10 and February 11, 2009, affecting portions of both the Central and Eastern United States. During the two-day period, 15 tornadoes touched down in seven states. Oklahoma was struck by six tornadoes, the most of any state. The first day of the outbreak produced the most tornadoes; the second brought mainly high wind damage and rain or snow in most of the Northeast. The storm system responsible for the tornado outbreak resulted from the unusual congruence of a cold, dry system, originating in the Four Corners and a warm, moist system, moving north out of Texas. Complicating factors included daytime heating and a strong wind field favorable to the creation of circulating thunderstorms. On the second day, the stronger cold front limited discrete supercell activity and the risk of tornadoes decreased significantly. A squall line, however, produced high winds and rain along the river valleys, primarily those of the Mississippi and Ohio Rivers. This squall line continued to renew its energy as it passed through the Midwest, Pennsylvania, New Jersey, New York, and New England, causing wind and water damage, and dumping of snow in central and eastern Massachusetts. The resulting power outages affected homes throughout the northeastern seaboard. The most destructive of the weather events, a long-track EF4 tornado, traveled for nearly an hour through four counties in Texas and Oklahoma during the evening of February 10. It destroyed 114 residences in Lone Grove alone. Almost all of the casualties (8 deaths and 46 injuries) from the outbreak were due to the tornado in the Lone Grove area in Carter County. It was the deadliest tornado to hit Oklahoma since May 3, 1999 and the strongest tornado during the month of February in Oklahoma since 1950. Meteorological synopsis February 10 During the afternoon hours of February 10, 2009, a strong, middle and upper storm system moved across the Four Corners region into the central and southern Great Plains. A moist, low-level air mass streamed northward across Texas and Louisiana into eastern Oklahoma and the Ozarks ahead of the system. Daytime heating of the moistening boundary layer increased throughout the afternoon across Oklahoma and Texas as cloud cover shifted eastward. Cooling aloft, in conjunction with the approaching storm system, led to atmospheric instability, resulting in thunderstorms. A very strong wind field increased with height through the lower troposphere, resulting in