Oklahoma tornado: 'Tornado Alley' explained In contrast, yesterday's tornado registered speeds of almost 200mph (only one per cent of twisters ever reach speeds this high) but the death toll is so far close to double. In 2003, Moore was yet again put in the record books after another powerful tornado hit - one of over 400 to strike 19 US states during a seven day period in May. While property damage was again high, they did not suffer any fatalities. The residents of Moore join a vulnerable line of towns and cities that are home to 90 per cent of the world's tornadoes. Covering the prairies that stretch south from the Canadian border, through the Great Plains to the hills and river deltas of the Deep South, "Tornado Alley" runs north to south between the Rockies and Appalachian Mountains. Cold, dry air from the Canadian border is pushed south where it meets warm, moist air from the Gulf of Mexico and hot, dry air from the Sonoran Desert. It is the meeting point of these three weather systems that produces a hot-spot of atmospheric instability, breathtaking thunderstorms, and tornadoes like the one that left 91 dead yesterday. It's no wonder the strip draws storm chasers from across the country. Just recently our own reporter joined the team behind the "Tornado Alley" US television series, tracking down some of the 1,000 tornadoes to hit the US each year. But it is just to the east of Oklahoma where some of the deadliest and most powerful twisters hit the region. Known colloquially as "Dixie Alley," it covers much of the lower Mississippi Valley and causes more tornado-related deaths per year than any other region in the US. In contrast, Oklahoma is usually victim to just under 50 deaths a year, with neighbouring Texas reaching the highest average of 137.