Chasing the world's wildest storms

Mulvane, Kansas

Eric: I'd missed out on a major storm outbreak a few days earlier because I had to prepare for a conference, and I felt like I needed a decent chase to make up for it. Most chase days you find something pretty mediocre, so missing an event forecast well in advance, as I had, is twice as annoying. I made plans for a chase on this day with a chaser friend, Scott Currens, and gambled on the best target we could reach. As it turned out, it was a good gamble because I had probably the greatest chase of my life!

We get to McPherson, Kansas, but the storm isn't so great and gradually dissipates, leaving us little time to regroup. As we're preparing to head home with nothing to show for the day, we hear about storms firing 70 miles south of us: an hour's drive. Perhaps normally we'd cut our losses, but today we're desperate and head towards Wichita.

On the way down we hear about a tornado striking west of the interstate. We can see storms developing along a new outflow boundary from a complex of thunderstorms, and a magnificent rotating updraft. Everything seems to be slotting into place, and judging by the violently rotating base of the cloud it seems we've caught it just in time.

A large funnel sprouts from the base and rapidly turns into a tornado that crosses the road about 100m in front of us! There's something about a tornado crossing a road that is totally exhilarating – it's like crossing your path with a perfect view.

The tornado kicks debris as it uproots trees in the distance. Hail starts to hit and the tornado strengthens and knocks down power lines near the road. We're able to drive over them and continue south to observe it from the west. Typically, this is a poor position as rain can wrap around the funnel, obscuring the view, but this one stays relatively dry and we can see the whole thing. A few chasers have windows shattered by softball-sized hail (about 11 cm in diameter), but we manage to avoid it.

Almost as quickly as the tornado had come, it weakens and disappears in a strange way, with no visible sign of a complete occlusion. It just seems to vanish, leaving small pieces of debris to fall from the sky for another 10 minutes.

The circulation of the storm moves southward on our radar, without producing another funnel until about 30 minutes later, when the leading edge of the outflow catches up with the storm and creates a new tornado against the backdrop of the setting sun. It's a pretty weak tornado and we gradually lose the detail of the storm in the darkness, but it's a great scene to close the day.

O'Neill, Nebraska

Mike: After an hour of ploughing through the no-man's land of the Sandhills on the gravel roads of the Nebraska-South Dakota border, I reach a new updraft. Apparently the storm tops are reaching 65,000-70,000 feet. It's barely moving. This is easily the best backlit tornado I've ever seen. This thing is huge!

The east-west inflow area is going crazy. A snaky tornado forms rapidly. It starts to widen and I finally get some precipitation at my location. It hardly moves at all and stays like this for more than five minutes. The rain kicks in and it turns into a very wide barrel, all wrapped up in rain. Somewhere in there are two houses that got taken out. I position myself in front of the tornado and plan to drop south quickly when I notice a new tornado form to the west.

I mess around trying to alter my course as it changes direction, turning from heading south-southwesterly to go south and then east. I am doing 70mph on the highway and it's trying to pass me overhead. I am southeast of North Loup and what a monster it has become!

I finally get a chance to pull over and take in the view, looking west... and north. I told you it was huge: it was covering the entire sky.

Sioux City, Iowa

Mike: I almost didn't chase this day because it didn't look so great. I had been checking out the data online and there was nice directional shear, but it wasn't very strong, so I figured it wasn't worth the drive. Thankfully, Steve Peterson convinced me to check it out. We headed out to Spencer, Nebraska. After some work it starts to crank... and boy does it crank. It now has raging inflow and my eyes are quickly full of dirt. It keeps picking up in strength and I'm keeping close to it. I move east and get a whole lot more excited about the day's prospects. It's got to be one of the coolest scenes I've seen while chasing and it is about to get a whole lot better. The storm is sucking inflow in extremely hard and creating some crazy structures. The cloud base is away from the centre – the speed of the inflow being sucked into that base must be incredible. I don't get how anyone can drive under this and not feel the need to pull over and watch.

It looks like a structure-only storm (no real chance of tornadoes), so we keep our distance to make sure we get the best of it. An arm of the storm swings around and feeds back west into it, creating a big coil. We see a little sculpted lowering at the base and it looks like it might produce a tornado, but it doesn't. Still, I'm not disappointed with a storm this great!

We get closer to Sioux City and the supercell just keeps on changing into something more crazy as it forms into a doughnut shape. As the sun lowers to below the cell, I get the shot on page 30 – one of my all time favourites.

Even closer to Sioux City we hear the warning sirens blaring. I wonder what the folks there are thinking as they see this thing approaching. Some guy pulls up to me: "You'd better seek shelter!" I tell him it will be OK and figure out what to do next. I decide to jump ahead of the storm to check out the hail and wind, but as I rush down the I-29 interstate it gets weaker and more linear, and it really doesn't look like it's got anything left in it, so I figure it's not a bad time to call it a day.

This extract was taken from 'Adventures in Tornado Alley' by Mike Hollingshead and Eric Nguyen, published by Thames & Hudson, £14.95. To order your copy for a special price, inc p&p, call 08700 798897

HOW TORNADOES DO THE TWIST

Tornadoes need certain conditions to form – particularly intense heat, which characterises 'Tornado Alley' in the central United States. As the ground temperature increases, moist air rises. When the warm, moist air meets cold dry air, it explodes upwards and a thunderstorm develops. The updraft can become very rapid and when there is strong vertical wind shear (difference in wind speed or direction with height) the storm begins to rotate and a supercell storm forms. About 20 per cent of these form tornados: a funnel drops out of the cloud towards the ground. The vortex varies in size and shape, and can be hundreds of metres wide. A typical tornado has winds of 110mph.