Topic: Why does the US (Tornado Alley) get so many tornadoes?

[Jimmy Deguara]

Tornado Alley has its name for a reason: it experiences the most violent tornadoes on this planet. This region basically covers the states of Texas, Oklahoma, Kansas, Nebraska, North and South Dakota, Missouri, Iowa, Minnisota, Louisianna. Other states particularly futher east also have tornadoes but not as frequently as this region. Outbreaks of tornadoes in the Tornado Alley states each season are not uncommon and they can be deadly. One of the most recent deadly outbreaks was one that occurred in May 3, 1999 in Oklahoma and southern Kansas.

This region has a set of features and conditions that explains why it receives so many tornadoes (on average just under 1000 per year reports):

- a very warm sea known as the Gulf of Mexico. Winds blowing from the Gulf provide the moisture storms need to develop

- the Rocky Mountains help divert upper level winds across the Great Plains which helps set up outbreaks conditions when cold air aloft overrides the very warm moist air below

- dry air form New Mexico often overrides the warm moist air creating what is know as a cap. This cap acts as a lid and helps delay the development of storms. As the energy builds up, eventually this cap cannot hold down the energy and releases this energyusually later during the afternoon or evening and explosive development occurs.

- what most people don't know is that the land in Tornado Alley rises gradually from sea level to well over 1000 metres up to the foothills of the Rockies and yet it still looks flat! This gradual rise helps force the moisture upwards but not too quickly. The moist air meets the drier air originating in the arid region of New Mexico. Where they meet is known as the dryline. - The dryline often sets up in eastern New Mexico or western Texas during the spring and early summer months. Storms often develop along this dryline.

- when strong low pressure systems approach from the Rocky Mountains, the winds in the atmoshere are often strong in the form of a jet. Because winds increase further up in the atmosphere, it has a forward rolling effect known as shear. As the moist air from the Gulf of Mexico heats up near the dryline, it rises into the atmosphere forming clouds, the rolling action is disturbed and basically the clouds follow this same rolling effect but more vertically. Rotating thunderstorms known as supercells often develop in these type of conditions and are responsible for most of the violent tornadoes we often here about.

Regards,

Jimmy Deguara

[Dave Nelson]

 ok  following on from that ....

   do we get a dryline boundry forming in Oz ?   or is the topography not conducive to that ?

  if we do ....  what are the atmospheric conditions that cause it  and where would it form in relation to
a troughline ?

cheers

Dave N

[Jimmy Deguara]

Dave,

A dryline by definition requires that moisture changes a particular amount over a particular distance. Yes we do get drylines in Australia and some spectacular storms have occurred here with those drylines - refer to this outbreak for an example

http://www.australiasevereweather.com/storm_news/2002/docs/200210-01.htm


The difference in the US is that due to perhaps the topgraphic effects, the dryline in late spring propagates near the border of west Texas and New Mexico and does not advance a great deal during outbreaks compared to some of the outbreaks here. Our dryline often shifts due to dynamics rather than just heating processes. Perhaps Bangladesh and east India has a similar setup comparable to the United States Tornado Alley dryline.

Now without going through complications, the source of the dry air in west Texas comes from the elevated deserts of New Mexico. Now this only gradually 'eats' away the moist air mixing eastward advancing the dryline. This dry air also overrides the moist layer which increases in depth eastwards simply because the elevation of the land slowly decreases. This means that at some point there is sufficient thermals that can escape with this moisture to form clouds and perhaps storms. The systems that often are associated with outbreaks also coincidently provide much need wind shear, destabilsation and other factors that lead to supercells and tornadoes more frequently than any other place on earth.

Regards,

Jimmy Deguara