The Epic Chase - 22nd April 2015
The 22nd April 2015 is a day that I will always remember - it could even be one of the most epic days of my life. This was the day that I saw proper hail, the full structure of a supercell and a tornado for the first time. The storms that I witnessed were probably the most beautiful I have ever seen. The storm chase occurred in classic Great Plains terratory, with flat ground and few trees or buildings to obscure the horizon.
As with other storm chases that I had already experienced, the Storm Prediction Center did not pick up on the risk magnitude until very late. The previous day, they had only issued a marginal risk for severe convective weather in north central Oklahoma. Then by the early hours of the following morning, the risk had been ramped up hugely - again, this was quite possibly due to the lower coverage of overnight convection and thus convection that was likely to persist into the following morning. In any case, I was surprised to see the sudden increase in forecast storm activity. A storm chase was made possible by the possibility of very large hail being a threat - this is something that I, and my friends really wanted to see. It had turned out (I only found out about this in the USA), that I had never actually seen "proper" hail, since what I had seen in the UK was technically graupel, as hail must have a diameter of at least 1cm. 
SPC convective outlook issued at 10:29 am local time. This time, the enhanced risk area was issued mainly for large to very large hail. The tornado probability was significantly reduced.
Credit: Storm Prediction Center.  
SPC Probabilistic Large Hail Graphic issued at 10:29 am local time. The hatched area was of greatest interest, since hail greater than 2 inches in diameter was possible here (almost tennis ball sized).
Credit: Storm Prediction Center.
We set of on the storm chase in early afternoon, after morning classes: this was a weekday, so we were all conscious that we needed to be back not-too-late for classes the next day. The initial plan was to head down to the Oklahoma/Texas boarder, which was due south of Norman. There, we would assess the situation and wait, if necessary, for storms to fire in the warm sector. We were hoping to catch any storms that developed ahead of the dry line, as these discrete storms were more likely to produce the most severe weather, and produce the largest hail if they wandered into the hail risk area as well. 
Surface weather map, issued at 4:30 pm local time. The long cold front which originates in the north-east USA, sits over the centre of Oklahoma. This then becomes a stationary front in the Texas Panhandle, and a dry line is positioned to the south of this. 
Credit: NOAA/Weather Prediction Center
Having driven into Texas, directly south from where we started, no isolated storms had developed in the warm sector. The forecast was for some storms to develop here, before dry line-initiated storms formed a quasi-linear mesoscale convective system that would move across the hail risk area during the night. Unfortunately for us, the gamble that we took on chasing warm-sector storms did not pay off. Storms did fire along the dry line through the afternoon, with a particularly potent HP supercell that developed to the south of Amarillo. I remember watching this storm on radar all day as it grew and developed - at one point, it had a radar reflectivity value of 76 dBz, which represents hail the size of grapefruits! This is still the highest reflectivity value I have ever seen in a storm. Watching this storm intensify all day, we had two choices: we could either drive further west into the Texas Panhandle or head home. Having already driven for several hours, we decided to commit to the chase, and carried on driving away from home. 
Having driven at incredible speeds towards this storm, which was about 90 minutes drive from where we based ourselves, we arrived at around 7pm. The storm was huge and looked evil - it was spitting out lightning left, right and centre and had a massive hail core. This storm was a great example of how long-lived a supercell thunderstorm can be - it formed at around 2pm, and ultimately dissolved into a messy MCS at around 10pm.
The HP "beast" supercell mentioned above. By the time we arrived at the storm, it was starting to become downdraft-dominated and was gusting out. 
This was the first storm we chased that evening, and it seemed the rest of the storm chasing community had come along too. On a straight, two-lane highway in the centre of the Texas Panhandle, we encountered chase traffic. This is a term for the hundreds of storm chasers who congregate on the best storm and compete for the best viewing spots and places to park. However, because storm chasing can be fast paced, especially when a storm is moving rapidly, things can become dangerous. In this case, there were many storm chasing vehicles travelling south on this road, away from the storm. People were driving well above the speed limit, away from the storm (it still had massive hail in it, which no-one wanted to encounter), which itsself was moving at around 50 mph. A line of storm chasing vehicles formed, and cars would suddenly brake harshly from high speed, often with another vehicle following closely behind, when they saw a good viewing spot on the side of the road. Chase traffic often adds a new element of danger to storm chasing.  
The inflow into this cell was also insane. I estimate that inflow winds were between 40 and 50 mph - the best part was that dust from farm tracks acted as a tracer for the wind motion, and you could see the air being drawn right up into the storm. This experience was also one of the most intimidating that I have experienced from the atmosphere. However, we soon realised that this storm was not the one to chase, as another, smaller cell had begun to develop to the south of this one. It was in a favourable position for warm, moist inflow air and was rapidly starting to rotate.  
The HP supercell after we had begun to drive south after the second sueprcell. The low-hanging cloud is the gust front, which formed over the downdraft air behind. This indicated that the storm was weakening. 
It turned out that chasing the second cell would be an unforgettable experience! Once we had driven out of the low-lying cloud from the first storm, the entire structre of the classic supercell could be seen, from the ground all the way up to it's top in the stratosphere. For the first time, we arrived at a storm and were able to witness the formation of funnel clouds and two tornadoes, all from the base of a perfect wall cloud below the perfect storm. What made it unforgettable was the truly unobstructed view, with the flat and tree-free horizon and the setting sun behind, giving an orange tint to the storm. Although we were around 4 miles from the storm, we could see the spinning motion in the wall cloud and the tornadoes. Thankfully, the tornadoes were all very weak - they were on the ground for less than a minute each, and occurred over open farmland, well away from buildings. All of us stood on the side of the road, gaping with our mouths wide open, staring at what we could hardly believe was happening in front of us. As meteorologists, what we witnessed was essentially weather pornography:
The storm, with a classic "beaver's tail" inflow from the left, and the coorkscrew-like swirling cloud above, indicating the rotating motion of the updraft. A small wall cloud is developing at the base of the storm - this was where the small funnel clouds and tornadoes spawned from.
Panorama of the storm, with the rotating updraft in the centre, and rainy downdraft towards the right. (Here, precipitation was being mixed with that from the earlier HP supercell.)
Close-up view of the updraft and beaver's tail. A well-defined wall cloud can be seen below this, on the left of the image.
Earlier photo of the storm, with a small funnel in the center. The supercell produced multiple funnel clouds for around 40 minutes. 
This storm was a classic supercell because the updraft region was not obscured by precipitation, as is often the case with HP supercells. Instead, any precipitation fell away from this region (to the right in this case). The storm provided an excellent opportunity to get time-lapse footage of a rotating supercell.         video is the best that I have seen on the internet. If you are interested in watching some (extended) highlights of the chase from my perspective, then click           
After the epic storm structure we witnessed, along with two tornadoes, we were very happy and content with an excellent day's chasing. However, the chase was not totally over: as day turned to night, these two storms, along with several others that formed along the dry line late in the evening, conglomerated to form a messy MCS that had several tornado warnings and nasty hail cores in its early stages. We chased this into the night, and we strategically waited for a tornado warned storm with a large hail core to pass over the road ahead of us. When we dived in behind the storm, we found the large hail we had been looking for...
The stones were generally 2 - 3 inches at their longest points, with the largest being close to 3 inches in length. Although hail of this size is not uncommon in intense supercells in the USA, coming from the UK, we were not hard to please! 
The largest individual hailstone we found. Note, that these hailstones had been melting for around 10 minutes before we picked them up. Also, I suspect that we drove through the largest hail in the swathe. 
This was a truly epic chase, with all that a group of amateur storm chasers could want, and I think it changed my perspective on what power and beauty the atmosphere can produce. The 22nd April 2015 is a day that I shall never forget! In the end, we didn't arrive back until around 3am, after cumulatively driving close to 600 miles and being on the road for 12 hours. Even so, I still managed to make it to the 10 am lecture the next morning!