to near 90 °F (32-33 °C) in summer. Average overnight lows range from the upper 50 °F (10 °C)s in winter to the mid and upper 70 °F (21 °C)s in summer. Southern Florida is the warmest region of the U.S. mainland in winter. Southern Plains/Lower Midwest/Middle East Coast/Mid-Atlantic The region from the southern Plains, to the lower Midwest, eastward to the central East Coast (the New York City/coastal Connecticut region southward to Virginia) has a humid temperate climate, transitional between the humid continental and humid subtropical climate zones, becoming semi-arid in the western plains. This region has cool to cold winters and hot, humid summers. Daytime highs range from in summer to in winter. Lows range from the s in summer to in winter. Cities in this region include Louisville, Kentucky, Wichita, Kansas, Cincinnati, St. Louis, Missouri, Springfield, Illinois, Indianapolis, Columbus, Ohio, Pittsburgh, Philadelphia, Washington, D.C., Richmond, Virginia, New York City, New Haven, Connecticut, and Atlantic City, New Jersey. Precipitation is spread fairly evenly throughout the year, though as one travels from Indiana westward there is an increasingly prominent early-summer concentration, with a May maximum in northern Texas and Oklahoma, and a June maximum increasingly evident from (central/northern) Indiana westward to Kansas. As one travels from east to west across Texas, Oklahoma and Kansas, average annual precipitation steadily decreases. Far western Texas (El Paso area) is desert, and average annual precipitation is less than in westernmost Kansas and the Oklahoma Panhandle, where the climate qualifies as semi-arid. In the lower Midwest (and southern Plains states, especially), temperatures can rise or drop rapidly; winds can be extreme; and clashing air masses, including hot, dry air of Mexican and/or Southwestern origin, warm, moist air from the Gulf of Mexico and cold, dry air from Canada can spawn severe thunderstorms and tornadoes, particularly from April to June. The "dryline", separating hot, dry air of Mexican/Southwestern U.S. origin from warm, moist air from the Gulf of Mexico, often causes severe, occasionally violent, thunderstorms to fire in central and eastern Texas, Oklahoma and Kansas; these sometimes contribute toward the hailstorms and tornado outbreaks that the Southern Plains are well known for. Reflecting these air-mass conflicts, central Oklahoma, including the Oklahoma City and Moore–Norman areas, has the highest frequency of tornadoes per unit land area on planet Earth, with May the highest-risk month for tornadoes throughout "Tornado Alley", from northern Texas north-northeastward toward western and