A vigorous trough and favorable thermodynamic environment led to the development of numerous severe storms and flash flooding during the day on 20 May through the evening and into the next morning. Storms produced over 25 tornadoes and many large hail and strong wind reports, in addition to flash flooding.

Water vapor imagery from the event showed the trough become negatively tilted as it swept across the Rockies into the plains from the 20th into the 21st (Fig 1). The negative tilt implies differential temperature/moisture advection in the vertical (increasing instability) and increasing wind shear. Combining the water vapor imagery with NWP analyses (in this case, hourly RAP) helps to improve analysis of the overall synoptic picture as features from the model can be connected to features in the imagery. Over time, this can improve ones ability to diagnose features in the imagery alone. Additionally, variances between the imagery and model can be noted and extrapolated into the model forecast. The NWP overlay in this case quantifies the strength of the trough and shows the strong jet rounding its base and advancing over the southern plains.

One-minute satellite imagery from GOES-16 was available to forecasters during this event. Visible imagery from west Texas/Texas Panhandle into western Oklahoma showed rapid thunderstorm evolution, including storm initiation and development of overshooting tops and above anvil cirrus plumes (Fig 2). The quick development of these storm top features was no surprise given the favorable setup, and indicated very strong updrafts and significant severe potential.

One-min IR imagery confirmed the trends and features, with rapid cooling rates implying swift updraft growth, the small cold regions indicating overshooting tops, and downstream warm regions surrounded by colder tops (enhanced V) suggesting the above anvil cirrus plumes (Fig 3).

A 19 hr long IR loop (17Z – 12Z) revealed persistent strong thunderstorm activity from west Texas across much of Oklahoma (Fig 4). Thunderstorms training over the same areas led to numerous reports of flash flooding.

Bill Line, NWS