At least four people, including a baby, were killed after a series of tornadoes struck Oklahoma on Saturday, amid a weekend of extreme weather that left dozens injured and a trail of destruction across the midwest. Local authorities confirmed that a four-month-old infant was among the two people dead in Holdenville – one of the hardest hit towns in Oklahoma, located 80 miles south-east of Oklahoma City – where about 20 tornadoes hit late Saturday, leveling buildings and ripping off roofs. The victims have not been named, but at least four others were injured as the tornado left a path of devastation through the town of around 6,000 people. A third death occurred near Marietta on Interstate 35 (I-35) which was closed on Sunday at the border with Texas “due to overturned vehicles and power lines across the highway”, according to the Oklahoma office of emergency management. The body of a woman, the fourth known victim, was recovered from the debris near a bar in downtown Sulphur, said governor Kevin Stitt, who has issued a state of emergency for 12 Oklahoma counties. Search and rescue efforts continued through Sunday as emergency services responded to extensive damage caused by the hail, high winds and flooding. Meteroligists consider tornadoes among the most violent and destructive phenomena of all atmospheric storms, resulting in billions of dollars of damage each year, according to the National Oceanic and Atmospheric Administration (Noaa). A tornado is a narrow, violently rotating column of air that extends from a thunderstorm to the ground. Wind is invisible, making it hard to see a tornado unless water droplets, dust and debris form a condensation funnel. By Sunday afternoon, around 25,000 Oklahoma customers (residential and commercial) – and almost 30,000 in Texas – were without electricity, according to poweroutage.us, which tracks electric utility outages More than 100 tornadoes are reported to have struck since Friday. The weekend flurry started in Lincoln, Nebraska, where hundreds of homes and structures were destroyed and damaged by tornados close to Omaha, which was followed by heavy rain and winds on Saturday. Several dozen people required hospital treatment for mild to moderate injuries but no deaths were reported, according to Omaha health officials. The National Weather Service (NWS) issued a tornado warning for several counties in Texas on Sunday, and said there was an “enhanced risk of severe thunderstorms” through Sunday evening across parts of east Texas, north-west Louisiana and south-west Arkansas. Flood and storm alerts also remain in effect for millions of people across parts of Oklahoma, Kansas, Missouri, Arkansas and Texas. Peak tornado season depends on the region beginning on the Gulf coast in the cooler spring months, before shifting toward the southern and central plains in May and June, and the northern plains and midwest during early summer, according to Noaa. However, tornadoes have been reported in all fifty states and can happen at any time of the year. Most tornadoes occur between 4 and 9pm. About 1,200 tornadoes hit the US each year, but changes in records and reporting make long-term changes in trends hard to track. The climate increase may be impacting the location of tornadoes. Studies have shown an increase in tornadoes in parts of Mississippi, Alabama, Arkansas, Missouri, Illinois, Indiana, Tennessee and Kentucky – the Dixie valley – and a slight decline in the Great Plains, known by some as tornado valley.