Going out with a Bang: 16th May 2015
The 16th May was to be the last storm chase of my 10 month stay in the USA: but it was a cracker! Myself and two friends chased what was arguably the best set-up we'd had all season. We were feeling optimistic of witnessing something epic: we were all novice chasers, but had the most experience of everyone, and having just three people chasing together seemed like a good number: usually we had a lot more. Ultimately the SPC issued a moderate risk for severe weather: 
SPC convective outlook, issued at 2:00 pm Central Time, on the 16th May. 
Credit: Storm Prediction Center
This moderate risk included a greater risk for tornadoes, including the potential for some significant events (EF-2 or greater). The enhanced tornado threat was in response to the development of a cyclone centred to the north. An area of high pressure over the eastern half of the USA helped to reinforce the southerly moisture return over the Great Plains. As well as moisture, this cyclone introduced cold air aloft, along with a strengthening westerly upper-level jet and strengthening low-level jet as the trailing cold front approached from the west. This low level jet was forecast to increase the low-level speed shear that could potentially sustatain a strong tornado. 
SPC convective outlook, issued at 2:00 pm Central Time, on the 16th May.
Credit: Storm Prediction Center
Surface pressure analysis, valid for 3:00 pm Central Time, on the 16th May. Much of the southern Great Plains is sitting in the warm sector of a mature cyclone, with ample moisture return from the Gulf of Mexico. This is a classic set-up for a significant severe weather event across the Great Plains.
Credit: NOAA/ Weather Prediction Center
After assessing the model output, analysing closely the SPC reports and the opinions of other storm chasers, the decision was made to head down to southwestern Oklahoma, on the border with Texas. Here, it looked like the dewpoints would be highest, favouring the development of low cloud bases, along with the highest low-level wind shear, which would favour the development of supercell storms and tornadoes. As we travelled down to the chase destination, it seemed that every other chaser had exactly the same idea: we ended up following the radar trucks from OU! Ultimately, this turned out to be the right decision. 
We ended up chasing after the southernmost cell in a line that developed through Oklahoma - known as the "Trailing Charlie" cell. This storm is more often than not the best storm to chase, since it has an undisrupted flow of warm, moist air from the south to feed off: the flow of air into this storm is not intercepted by other convective storms beforehand. We drove to Eldorado in southwestern Oklahoma in an attempt to get south of the developing storm. However, it looked like the storm was not maturing as we expected it to: radar signatures were not showing promising signs of a tornadic supercell. 
We decided to take a country road across a flood-swollen river to arrive at Route 283. After racing south across the Red River, we were just about able to cross ahead of the storm, onto its southern flank. All the while, the storm had rapidly organised to become a tornado-warned supercell (in about 20 minutes). All the classic signatures of a supercell thunderstorm were there: a rapidly rotating mesocyclone, a low wall cloud and inflow band. The radar signatures were becoming very impressive, with a hook echo feature beginning to develop. The storm felt completely overpowering: it was rotating incredibly quickly (I've never seen clouds move so fast) and there was a roar from the inflowing wind as well as the constant barrage of thunder. From a safe distance, we were able to observe the development of a wedge tornado - although, unfortunately, it was mostly enveloped in rain, which made it difficult to see. The tornado maxed out the wind velocities measured by radar and produced beautiful hook echo signatures. 
Maxed-out wind velocites measure by the radar network. Indicated speeds were close to 150 mph in both directions (around the funnel). 
Classic hook echo radar signature for the storm. Slight increase in reflectivity at the centre of the hook is likely due to flying debris. Note the hail core of the storm, indicated by the deep purple colours - several storm chasers had their cars destroyed by baseball sized hail. The circle indicates our location, a few miles south of the tornado. 
This strong tornado crossed the road about 15 minutes after we had driven south on it, tracking in a north-eastward direction. As we tried to head north again, in an attempt to catch up with the storm, our progress was halted by downed power lines and debris (including the roofs of outbuildings) strewn across the road. We had to travel south into Texas in order to find the next north-south road that might allow us to catch up with the storm. Unfortunately, this took us about 20 miles out of the way, and we fell well behind the storm. That was the last we'd see of the storm and tornado after some slow driving due to torrential rain and a few wrong turns. During the remainder of the day, we attempted to chase a few other cells, but saw little else before the storms merged to form a squall line into the night.  
This was a great way to end my first season chasing in the USA and it was certainly a chase that I'll never forget. Being so close to the storm made me feel incredibly small and insignificant, but it was also a beautifully orchestrated example of the atmosphere in motion.