An unusually late start to the season for USA’s Tornado Alley




Tornado

In a twist that would ruin the storyline to the Wizard of Oz, the USA’s ‘Tornado Alley’ has been strangely quiet this year.

In fact, if there are none reported in Oklahoma or Kansas on Thursday, 2018 will officially be the quietest start to the tornado season in both states …on record!

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.

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?

This year has been possibly the quietest tornado season in recorded history. Kansas and Oklahoma would expect to see close to 19 tornadoes between the start of the year and now, with around 13 or 14 in April alone.

But this April, there hasn’t been one! Even in the tornado-barren Aprils of 1987 and 1988, Oklahoma had already seen them earlier in the year.

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, 18 of the last 24 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). There has also been only one day of significant rainfall, and just two where thunder was reported.

The reasons behind this shift of weather pattern are not straightforward, but it is possible that the Sudden Stratospheric Warming (SSW) event that occurred earlier in the year could have played a big part.

It led to a greater amplification of waves in the jet stream producing more ‘blocking’ to the weather patterns. That same SSW was also responsible for the cold, snowy start to spring in the UK and for the heavy spring snowfalls in the northeast of the USA and eastern Canada.

Storms on the way?

The latest “first tornado” occurrence in Oklahoma was 26th April in 1962, so if no twisters are spotted on Thursday, it will officially be the quietest start to the tornado season since daily records began in 1950.

Current forecasts from the USA’s National Weather Service are predicting only a ‘marginal’ risk of any thunderstorm development through the rest of this week. There is however, a small chance that conditions for storm development may increase later next week.

For the time being though, it looks like Dorothy may have to find Oz by her own accord!




Author: admin

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