My first storm chase - 25th March 2015
After living in Oklahoma for over 6 months and after months of building excitement and anticipation, the first severe weather day of the year had arrived. That day was to be an experience unlike no other day I had ever seen, although I would become more accustomed to it in the weeks to follow. 
Having known that severe weather was on the cards for several days leading up to the event, I was buzzing with excitement and could hardly sleep the night before. The day began muggy and cloudy, with a murky stratus deck moving up from the south, which rapidly dissipated leaving clear skies by mid-morning. A stalled front draped across Oklahoma was forecast to slowly move southwards as a cold front through the day. To the south of the front, winds were southerly, advecting warm and moist air from the Gulf of Mexico. The dry line was also forecast to move eastwards across Oklahoma from the Texas panhandle as the day progressed.
Surface weather map, valid for 4 p.m. CDT on March 25th. Credit: NOAA/Weather Prediction Center.
As the cold front moved south, it would act as the trigger mechanism for storms to develop in the increasingly unstable airmass to the south. Ample CAPE existed to the south of the front across central Oklahoma, with typical values ranging from 2000 - 3000 J/kg. Any storms that developed in the unstable airmass away from the cold front were more likely to become surface based, although the cold front was likely to prevent such storms from forming along it. Therefore, the highest probability of tornadic supercells was away from the cold front, along the advancing dry line. This, ultimately, was where we decided to chase, although there would be several unexpected surprises that day. 
NAM forecast counding for 3 p.m. CDT for Norman, Oklahoma. Over 3500 J/kg of surface-based CAPE was available, if the cap could be broken. 
Deep layer wind shear was also present, enhanced by the jet stream, as well as a low-level jet which forms at around 850 hPa in general. A relatively weak low level jet enhanced the advection of warm, mosit, tropical air from the Gulf of Mexico, as well as increasing the low-level wind shear, which is essential for initiation of tornadoes. On that day, wind shear, coupled with substantial CAPE, was significant enough to produce initially discrete supercells before they would rapidly merge into a quasi-linear mesoscale convective system. 
Deep-layer (0-6km) vertical wind-shear (knots) at 4 p.m. CDT. Values are typically 40-50 knots where convective initiation was expected. 
Credit: TwisterData. 
Although low-level wind shear and moisture were generally not high enough to sustain long-lived tornadoes, especially since any surface-based supercells which did form were likely to have high bases, we decided to chase the advancing dryline in south central Oklahoma. Although wind shear was generally lower here, any storms which did form were more likely to remain descrete and produce tornadoes and severe weather. However, the dryline never properly fired that day, probably because of the cap being too strong, and storms here were weak and short-lived. This being the first storm chase that myself and my friends had been on, we were very unprepared. We lacked any means of mobile internet access - chasing in rural Oklahoma meant that mobile signal is very sketchy outside of major urban areas. As a result, we were driving around looking for the nearest McDonald's with free wifi, so we could check the radar and satellite imagery. This is not what you want to do, as we often had to drive well out of our way to get to one - becasue of this, we ended up mising the storm initiation along the cold front. This occurred in a north east to south west oriented band across Oklahoma. Storms fired along this boundary from about 3.30 pm onwards, moving in a roughly eastward direction. In north east Oklahoma and north western Arkansas, surface based supercells produced several tornadoes, including an EF-2 that destroyed a mobile home park in Sandy Springs, Oklahoma. 
     After failing to intercept anything along the dryline, we raced towards a supercell that had formed around 50 miles to the north west of Oklahoma City at about 5.30 pm. After becoming supercellular, it began to right-move towards Oklahoma City, and produced an EF-2 tornado that hit Moore, a southern suburb of OKC that was hit my a deadly moster EF-5 tornado in 2013. This was not at all expected, especially as the storm formed well behind the surface cold front boundary. However, storm inflow casued the cold front to be "sucked back" towards the storm, allowing the supercell to become surface based. It is likely that the cold front helped to enhance low-level vorticity and increase the chances of a tornado forming. 
A developing supercell over Norman, Oklahoma. This storm would eventually go on to produce an EF-2 tornado in Sandy Springs. 
It wasn't until around 7.00 pm that we caught up with this storm, only 15 minutes after the tornado had dissipated. We were treated to some nice mammatus displays, decent storm structure; being able to see the rotating updraft of the storm and some very strong outflow winds. Unfortunately it soon got dark, and the storms merged to form a messy MCS that could not be chased. After stupidly driving through incredibly heavy rain and strong wind, we decided to head home for the day. 
The Moore supercell, from a position around 25 miles away from the storm. The shear size of the storm struck me, both it's height and it's vertical extent, probably extending 14 - 15 km in the vertical. 
Some lovely mammatus clouds on the underside of the supercell. They are generated by pockets of cool air from the outflow of the storm sinking and creating these bag like clouds on the underside of the main storm cloud. 
Panoramic image of the supercell. The bowl-shaped area close to the surface in the centre is the rotating updraft of the storm. The underside of the anvil is dotted with mammatus clouds. 
Collectively, we had blundered our way through this first storm chase, but we had seen some great mammatus and nice storm structre, as well as being able to stare in awe at the shear dominance and power of a supercell. At the end of the day I realsied I'd caught a fever - chase fever, and I wanted more!