A Close Call - 6th May 2015
Like the 22nd April, this was another equally epic and unforrgettable day. The storm season in Oklahoma was in full swing, and it was proving to be an especially active season. Convective rainfall seemed to occur on a diurnal cycle, with thunderstorms forming on the dry line in the afternoon and reaching central Oklahoma around midnight or the early hours of the morning. This was the beginning of what would prove to be the wettest month on record for Oklahoma. (On a side note, the statewide rainfall average was 14.4 inches, 9.58 inches above normal, while the Norman Mesonet site recorded 23.4 inches. That's 594 mm or almost 2ft of rain - in fact, the rainfall total for May was the same as for the previous 12 months leading up to my arrival in Oklahoma in August 2014.) The rainfall obliterated the drought that had been plauging the state for the previous 5 years. 
The excessive precipitation was likely associated with the developing El Nino in the Pacific which generally promotes rainfall in the central and southern Great Plains. Thus, on the day before my final exam, it was perhaps not surprising that thunderstorms were forecast around the OKC metro, with severe thunderstorms possible. 
Surface weather map, issued at 1:41 pm local time. The central and southern Great Plain states are under southerly return flow from the Gulf of Mexico, in response to the low pressures situated under Colorado and Kansas. With a lack of surface forcing, the storms that formed were a result of the unstable airmass and surface heating. 
Credit: NOAA/ Weather Prediction Center. 
Interestingly, the outlook issued by the Storm Prediction Center for this day was initially relatively mundane even up to the night before. However, during the morning of the 6th May, the SPC ramped up their forecast severe probabilities fairly rapidly. I think this was a result of the complex upper-level situation unfolding, which left the forecasters uncertain about what would unfold at the surface. Confidence obviously grew about the potential for severe weather when the next model runs came out, probably at 12 pm local time. I expect the models were struggling to cope with a chaotic atmospheric situation. Whatever the case, by lunchtime, the SPC had issued an enhanced probability for severe weather around Norman and to the south. The main threats were forecasted to be large hail and to a lesser extent, tornadoes, from supercells that would form in the highly unstable warm sector, with high surface dewpoints and around 40 knots of deep-layer shear.  
SPC convective outlook issued at 2:00 pm local time. The slight risk was moved futher eastwards compared to previous outlooks, and the overall risk was significantly upgraded. 
Credit: Storm Prediction Center
SPC probabilistic tornado graphic issued at 2:00 pm local time. There was talk of people heading up to central Kansas to chase the significant severe probability issued there, but most stayed close to home in order to revise. For storm chasers, this proved to be a great decision. 
Credit: Storm Prediction Center
For much of the day, I was stuck inside revising for a mesoscale meteorology exam I had the next day. I was constantly monitoring the weather situation, and was watching the continual upgrading of severe weather potential across Oklahoma. At around 3pm, storms began to initiate over the Witicha Mountains in south western Oklahoma. It is likely that these acted as an elevated heat source, and were a prime location for the development of thunderstorms in the unstable warm sector. These storms immediately became supercellular, as forecast by the SPC, in the highly-sheared environment. In fact, a special sounding from Norman at 1 pm showed the vertical veering wind profile, over 1500 J/Kg of surface-based CAPE, and deep-layer shear of 40-50 knots. The first supercell to develop produced an EF-3 wedge tornado close to Chickasha. This storm went through several stages of producing tornadoes along the I-44 corridor, as it moved northeast towards Norman, dropping other weak, but large tornadoes close to Bridge Creek. 
All this time, I was attempting to revise supercells and theories behind the development of tornadoes. This became more a case of trying to drag my eyes away from the TV as the situation became more and more intense! By around 5pm, it became more and more apparent that the storm and tornado were heading straight for Norman. It was very strange being able to listen to the Weather Channel Meteorologists emphasising how dangerous the storm was, and then heading outside and being able to hear the thunder from the same supercell. At around 5:20 pm, the decision was made to head to the nearest storm shelter (a 10 min walk). As we were heading there, I suddenly felt in awe of the whole situation: lighting was striking all around in a constant barrage, the bowl-shaped mesocyclone could be clearly seen behind, rotating rapidly, and the entire storm was laced with that wicked green tinge that can only mean large hail is close by. The tornado sirens went off as we were walking, which hurried us up a bit! By this time, I was relatively sure that the tornado would track through north Norman, around 3 miles north of where I was.
I did manage to capture some epic radar reflectivity images of the storm showing classic tornadic supercell characteristics: 
Screenshot from RadarScope of the supercell as it was producing a weak wedge tornado to the south-west of Bridge Creek. The classic "V" shape of precipitation, inflow notch and hook echo can all be seen. 
Screenshot from RadarScope of the supercell about an hour later. It has cycled through the previous stage of tornado development by this point, and has just produced another wedge tornado.
My view of the storm as the tornado passed through north Norman. At this point, the storm probably does not have a tornado on the ground, but the low wall cloud is below the baseball stadium to the right of the black car. In my location, we were lucky in that we recieved no precipitation, so cars and property were spared the large hail that the storm could have thrown at us. 
Ultimately, the entire town was spared the worst. Although there was a large wedge tornado (close to a mile wide), it was weak (rated as EF-1) - this indicates wind speeds of 86-110 mph. 
However, more supercells had developed off the Witicha Mountains to the south-west and were taking very similar tracks towards Norman. At this point, I decided to go chasing - I was too jangled from the experience of the previous storm to revise! The chasing was right on our doorstep - we only had to drive for 20 mins to be able to see the mesocyclone of a second tornado-warned supercell over Newcastle. 
Mesocyclone of the second supercell, looking north-west towards Newcastle. 
"Mothership" structure of the mesocyclone. The wall cloud is located to the right of the mesocyclone, but was very hard to distinguish in the fading light. The white stripes across the panorama are from lightning strikes that occurred while I was taking the image - the storm was very electrically active! 
Meteogram from the 6th May . Just before 6:00 pm, the Mesonet site (located at Max Westheimer Airport in north Norman) recorded simultaneous drops in air temperature, dewpoint temperature, and pressure, while there was a substantial increase in the wind speed, up to almost 60 mph. This may have been associated with the mesocyclone of the first supercell passing over the station. 
We watched the second supercell for as long as we could see, by which point we decided to head home as it is generally too dangerous to chase at night. We were briefly caught in the strong rear-flank downdraft winds of the storm, but this aided our progress home as we were heading in the same direction. Fortunately, this storm never produced a tornado, although it always looked like it wanted to as it had a massive hook echo. It headed straight for Norman where it produced a lot of rain and some hail. As can be seen from the meteogram above, this was an incredibly wet day, with over 5 inches, or 130 mm recorded by the end of the day. The rainfall was so severe, that it prompted the Norman National Weather Service Forecast Office to issue a flash flood emergency for Oklahoma City.