Triple tornadoes in Colorado - 28th May 2018
The 28th May provided the best chance to see tornadoes of the trip so far, with a small region extending from north-east Colorado east toward far north-west Kansas primed for tornadogenesis. Starting the day in Goodland, Kansas, we drove west toward Limon, Colorado, where we had lunch at Pizza Hut. The decision was then made to drive to the aptly named Last Chance, about 30 miles to the north. We waited here for about 2 hours, in a rather cloudy, cool and damp environment, which didn’t initially indicate explosively developing storms. Even though the forecasts said otherwise, I was rather sceptical about the chances of a good storm - the morning and early afternoon weather conditions went again all my prior experience and instincts to suggest there would be expolsive storm development in the afternoon. 
Surface weather map, valid for 13:00 (Central Time) on the 28th May 2018. With an upper low in place over Utah, south to WSW winds aloft above Colorado would provide 50-60 knots of deep-layer shear, prime for supercell development. 
Credit: NOAA/Weather Prediction Center.​

SPC convective outlook, valid for 15:00 CDT. A large Enhanced threat was issued in a zone where instability, lift, moisture and wind shear were maximised. The entire Ehanced area also had a 30% risk of significant severe hail larger than 2 inches diameter. Credit; Storm Prediction Center. 
SPC tornado outlook issued for 15:00 CDT. A 10% tornado threat was issued for the region where low-level wind shear was maximised, especially during the evening hours, when the low-level jet was expected to kick-in. Credit: Storm Prediction Center. 
However, the stratocumulus deck soon evaporated to reveal a rapidly developing cumulonimbus about 30 miles to our east. This developed on an outflow boundary, and the storm remained near-stationary for a time - we bode our time at our position in Last Chance, waiting for a clear indication of a healthy supercell thunderstorm on radar. Once it became clear that this storm was developing alone, with no interference from nearby cells, we chased east after it. A lot of dirt road driving then ensued - with the all-wheel drive, traction control and terrarin response of our chase vehicle coming in handy once again - as the storm steadily tracked due east over open farmland. Several landspouts developed and dissipated as we drove towards the storm. Finally, a big cloud of rotating dirt was kicked up from the surface - although it was hard to distinguish a funnel connecting it to the cloud base, we believe this was a tornado (rather than a landspout). By this point, we we still about 10 miles from the updraft base, but driving closer we witnessed another, better defined funnel develop, which produced another cloud of dust and debris at the surface. Both tornadoes were on the ground for around 10 minutes together, a hugely impressive sight. 

The first tornado then dissipated, while the second, with a broad rotating column of dust, remained on the ground. Finally, a small and thin funnel developed to the south of the second tornado, and again we had two tornadoes on the ground at the same time. We managed to find a small ridge above the surrounding flat farmland, where we had a great view of these two tornadoes underneath the updraft of the storm - an awesome sight! 

With the storm moving away from us, we had a job to keep up with it on the dirt roads, and eventually our view of the mesocyclone was obscured by rain and hail wrapping around it from the north. We did drive through a small patch of tornado damage, about 50m wide, as one of the tornadoes passed over a dilapidated barn, stripping wooden roofing and walls from it, as well as snapping a telegraph pole and rolling several large hay bales about 100m across the road. We attempted to approach the storm from the rear of the mescyclone - where the rear flank downdraft was - but were forced to retreat as inched-sized hail was flung horizontally at us from the back of the storm. 

With the storm rapidly losing it’s supercellular characteristics, and merging to form a line of strong convection, we raced east on I-70 back towards Goodland, and then headed north towards the small town of St Frances (where we had been the previous day), to watch a huge shelf cloud roll over our heads from the east. Although we would have liked to see some large hail as the storm passed over us (the car was parked under a gas station), it was still a fitting way to end a successful chase day. These were the first tornadoes I'd ever seen with well-defined condensation funnels and cones of dirt swirling around at the base - to see two on the ground simultaneously felt like a real blessing! 
The supercell thunderstorm that would eventually go on to produce several tornadoes. This shot was taken just after the stratocumulus deck had started to dissipate, by which point temperatures had started to rocket up.
Dual tornadoes on the ground. Although difficult to discern, it is likely that this storm produced several landspouts (non-mesycyclonic), as well as mesocyclonic tornadoes. 
Vertical wide-angle image, illustrating the sale of the tornado compared with the parent updraft.
The undulating and chaotic storm base, as seen from the gas station in St Frances, once the gust front and shelf cloud had passed through. The main precip core petered out before reaching our location, with only heavy rain eventually falling (no hail). 
Super-res velocity and reflectivity images of the supercell thunderstorm that produced multiple landspouts and tornadoes. This image likely shows a cluster of several cells at different stages of development, as indicated by the multiple velocity couplets on the right. Credit: Radarscope.