An isolated severe storm brought large hail to northern portions of Oklahoma City on 23 March 2019. GOES-16 1-min imagery was available over the region to support severe weather forecast and warning operations.
Water vapor imagery shows convective development across Oklahoma south of a broad upper low (Fig 1).
Prior to convective initiation, stable wave clouds (aka billow clouds) were apparent over the OKC area and east, signaling the presence of low level stability. As the day progressed, destabilization was apparent west of OKC as a cumulus cloud field developed. A southwest to northeast oriented axis of enhanced cumulus cloud growth (towering cu and clumping cu) eventually become apparent, signalling the most likely zone of imminent convective initiation (Fig 2).
The Day Cloud Phase Distinction RGB further highlights imminent initiation through the transition of cu from “light blue” to “green” within the cu field (Fig 3). This color change comes from the blue contribution (snow/ice band) decreasing (ice increasing in clouds), while the green contribution (red VIS) is relatively high (highly reflective clouds). The red contribution (IR window) is low, but may also be increasing as the cloud tops cool. In other words, glaciation is starting to occur within the cloud.
Moving forward another hour, convection initiates within the zone of increasingly agitated cumulus clouds (Fig 4).
Plotting GLM Flash Extent Density during the first hour of initiation, a rapid increase in lightning activity is apparent leading up to the first NWS warning and hail report (Fig 5). The increased lightning activity signals a strengthening updraft.
Finally, looking at storm evolution with respect to GOES-16 derived CAPE, convection developed within a local max of CAPE of around 800 j/kg (Fig 6).
Bill Line, NWS