Brian Davidson Tornado

It has been confirmed that the first tornadoes of the season to hit Kansas and Oklahoma weren't until 1 May 2018. This means that 2018 has been record-breaking for Oklahoma with their latest start to the tornado season and Kansas's latest start since 1980 when the first tornadic outbreak to hit the state was reported on 28 May.

What is Tornado Alley?

The "Alley" covers an area surrounding north Texas, Oklahoma, Kansas, Nebraska, Louisiana, Arkansas, Missouri and Iowa (as well as the fringes of bordering states).

It is in this zone that the USA records some of its most frequent and devastating tornadoes. Late spring sees the peak of the tornado season in these areas due to the clashing of air masses.

BBC Usual jet stream track across USA

The normal weather set-up at this time of year sees the jet stream dipping southwards just to the west of Tornado Alley. This encourages warm, moisture-laden air to be drawn up from the Gulf of Mexico. Overlaying this is usually a layer of drier air originating from southwest USA and northern Mexico.

This behaves like a lid, trapping all that warmth and moisture in the lower atmosphere. However, as the jet stream continues to bring in colder air above that lid, it eventually breaks down, allowing all that pent up energy and moisture to burst skywards into massive tornado-spawning thunderstorms.

Where are they this year?

Kansas and Oklahoma would usually expect to see close to 19 tornadoes between the start of the year and the end of April, with around 13 or 14 in April alone.

BBC This year's jet stream pattern

The clue as to why it has been so quiet lies in the overriding weather patterns across the USA so far this spring. The jet stream, instead of digging down to the west of Tornado Alley, has pushed further south and east than you would normally expect.

This means the usual flow of warm and moist air from the Gulf of Mexico has deflected further east, and Tornado Alley has been stuck in abnormally colder, drier air.

In Oklahoma City, many April days have seen "below-average" temperatures. On the 7th April for instance, a daytime 'high' of just 8C (46F) was recorded against a daily average of 21C (70F).