18th April 2015
Our first weekend storm chase meant that we were not restricted by lectures or other commitments, allowing a few new members of the group to embark on their first storm chase. Interestingly, this chase was a bit of a last minute decision, becasue the Storm Prediction Center rapidly upgraded the severe weather threat level over the night. An initial marginal threat for severe weather issued the previous night was rapidly upgraded to an enhanced threat across a large swathe of Oklahoma, Texas and Kansas. The reason for this was probably increased moisture return from the Gulf of Mexico behind a well organized MCS that moved across northern Texas the night before, along with greater solar insolation than was expected. As well as this, there was forecast to be greater low-level wind shear, which would aid with the formation of any tornadoes. 
SPC convective outlook issued at 1:49 pm local time. The enhanced risk area was issued mainly for large hail and severe wind gusts, although a tornado event was not out of the question. 
Credit: Storm Prediction Center
If I had been more busy with work, I may have been inclined to skip this chase, becuase of the low probability of tornadoes, but since it was a weekend, I was definately up for it - it was made more tempting by the fact that the storms were not likely to be too far away from home. In any case, the threat of tornadoes sometimes increases significantly towards the evening hours, when the low-level jet ramps up across the Southern Great Plains, increasing the low-level wind shear. Storms were forecast to fire along the dry line (or stationary front) that was situated across the Texas and Oklahoma panhandles, and this was the direction we headed from central Oklahoma. The target area was south-east Oklahoma, where we planned to catch onto the storms as they migrated westwards through the day.   
Surface weather map, valid for 1:28 pm (Central Time) on the 18th April 2015. Oklahoma, Texas and Kansas sit firmly in the warm sector behind the warm/stationary front. The moisture return from the Gulf of Mexico is rather poor however, owing to the slack flow. 
Credit: NOAA/Weather Prediction Center
The lack of moisture inflow from the south proved to be a nuisance that day - it prevented the low cloud bases from forming on the supercell thunderstorms that are generally required for tornado formation. 
 
Having left Norman in early afternoon, much of central Oklahoma was under a large cumulus field. This was a sign of the solar heating generating some instability, but not enough to break through the cap. Mostly, the storms were expected to form along the dry line, as this provided a meaningful source of lift to overcome the cap. As it turned out, some storms did fire ahead of the dry line, and these produced the most eventful weather of the day. As we headed further west, we noticed that the dewpoints kept decreasing, initially from around 18-19°C to 14°C later in the afternoon. This was likely due to increased mixing out of the moisture in the low levels by small convective clouds, while it could not be replenished fast enough by the slack flow from the south. What ended up happening was that many low precipitation supercells formed along the dryline, with very high cloud bases and little precipitation reaching the surface. A significant threat from LP supercells is often very large hail, but fortunately none of the storms we chased produced this. I was initially dissapointed by the formation of these low precipitation storms, but in retrospect, I realised that they were very beautiful and allowed us to see parts of the storm structure that would otherwise be obscured by rain. 
Pictures of the severe-warned storms that formed ahead of the dry line, in the warm sector - over north Oklahoma. These storms were probably not initially LP supercells, but likely evolved into this form as they encountered a lack of moisture in the low-levels. 
Visibility was especially good that day - we could see the dry line supercells for about an hour before we reached them. The lack of low-level cloud meant that much more of the storm was visible. The storms that formed were also some of the most sheared that I have seen. Close to the Texas border with west Oklahoma, they formed in an atmosphere with over 60 knots of deep-layer shear. The result was a moist updraft with a glaciated anvil that stretched hundreds of kilometres downstream.
LP supercell along the dry line, with mammatus clouds hanging under the precipitation base. 
View from directly underneath the storm above. I am standing in a region of what would be extremely heavy precipitation for a HP or Classic supercell, with mammatus clouds directly overhead. No precipitation was reaching the surface - it was all evaporating in mid-air. 
After getting frustrated with these LP storms, which were weakening continually as they encountered yet drier air, we decided to head back east, after a HP storm that formed to the south. This storm had several tornado warnings and very large hail was evident on the radar. We travelled east for around an hour, and were able to see the rotating updraft of the storm and just make out a very low wall cloud. These features were both a first for me on my storm chasing and so I was a little overexcited! I also witnessed some amazing mammatus hanging from the anvil of this storm. This storm chase led us onto some dirt roads, in our very unsuitable vehicles and, as with my other chases before, things didn't go our way. The storm soon began to weaken as it got dark and that was the end of that. Storm chase #3 was over.  
The view of the rotating updraft of the HP supercell over south-central Oklahoma. A rather high wall cloud is visible behind the wind turbines to the left-centre of the image.