Severe thunderstorms were in the forecast across southwestern Oklahoma on Wednesday, May 10th. The potential for supercells (a rotating thunderstorm) existed as well, particularly along the intersection of the dryline and outflow boundary. The forecast was quite complicated, with early morning convection and persistent cloud cover across much of Oklahoma that made for a complex afternoon for the evolution of severe weather.
In addition to analyzing operational and statistical model guidance, it is also important to look at realtime observations the morning/day of any event as it can provide you a great deal of information as to what the current state of the atmosphere is. Having a solid grasp as to what the current observations are sampling is critical in the decision making for forecasting severe convective storms.
On the morning of Wednesday, May 10th, observed sounding data from weather balloons that were launched across the region at 1200z / 7:00am CDT revealed a deeply moist environment, with a stout capping inversion. The morning sounding out of Norman, Oklahoma, sampled a dew point depression of 3°F at the surface — indicative of high moisture content in the low-levels of the atmosphere. The dew point depression is calculated by subtracting the observed temperature and dew point (T-T?) at a given height in the troposphere.
The smaller the difference between the temperature and dew point in the low-levels of the atmosphere, the richer the moisture content will be. This can result in low cloud base height (lifted condensation level; LCL) after the genesis of convection, and can increase the potential for heavy rain, damaging downbursts, severe hail, and even tornadoes.
In the image above, the capping inversion is circled. A capping inversion can be analyzed by looking for an increase in the temperature with respect to height. The warming of air aloft suppress air parcels from rising freely, thus preventing the formation of thunderstorms. When the capping inversion erodes in a severe weather environment, the genesis of thunderstorms is likely.
Surface observations across the south-central Plains indicated that the dryline was located in the Oklahoma and east Texas Panhandles. The dryline mixed slowly east with time throughout the morning and into the afternoon hours. Given that there was residual thunderstorm activity left over from the overnight convection, a few outflow boundaries were analyzed on the visible satellite across the region. As the dryline mixed eastward in the afternoon into southwest Oklahoma, it began to interact with the remnant outflow. The predicted afternoon thunderstorms began to form thereafter.
Storm spotters reported golf ball size hail as it moved through the vicinity of Eldorado, Oklahoma. A few reports of tennis ball size hailstones were reported as well. Dual-polarimetric radar imagery indicated that there was a relatively high concentration of water coated hailstones within the updraft, as the precipitation core on the base reflectivity imagery was >70dBZ (decibels).
Positioned south of the supercell, the precipitation core passed to our north as the storm moved east. Trailing behind the supercell, we observed a spectacular optical phenomenon. A ground rainbow was observed east of Eldorado, Oklahoma, as sunlight refracted from the falling raindrops. Storm spotters reported tennis ball size hail within the supercell as it progressed eastward towards Altus, Oklahoma, as well.
Storms took on a splitting profile Wednesday due to strong, unidirectional winds in the middle to upper levels of the troposphere. However, due to weak low level winds sampled around 15 knots as per the evening sounding from Norman, Oklahoma, the organization of discrete updrafts was hindered.
The storms that split into left and right-movers ended up congealing together into a quasi-linear convective system (QLCS). As a result, the overall tornado threat diminished tremendously along the line. There was, however, reports of tornadoes along the southern flank of the QLCS as a supercell produced brief, weak tornadoes near dusk.
Behind the storms, we observed a few isolated supercells form off the outflow boundary. The left-moving supercell as seen above was progressing to the north, and albeit beautiful structure, it was entering into a zone where the air was much more stable. The storm began to decay about twenty minutes after this photo was taken. It was quite photogenic, however.
Eventually, storms began to decay across southwestern Oklahoma around sunset. The backside of the supercells were beautifully illuminated as the sun began to set, unveiling some photogenic iridescence within the anvils.
The image below (via Twitter) is a view from the front side of the supercells as they moved across southwestern Oklahoma. A strong updraft was beautifully illuminated by the sun as it set, revealing some fine details within the structure of the supercell.
— Jacob DeFlitch (@WxDeFlitch) May 11, 2017
As the sun began to lower to the horizon, a distant supercell in the Texas Panhandle scattered the light particles into a beautiful array of crepuscular rays. The dark contrast of the supercell unveiled a clear depiction of the overshooting top and back-sheared anvil. For the days ahead, the pattern looks quiet. Next week; however, may present some convective activity across the central Plains.