Severe thunderstorms developed along a cold front across southwest and central Texas during the afternoon/evening of 23 April 2019. GOES-16 1-min imagery was available over the region aid forecasters in monitoring storm development and evolution.
Analysis of GOES-16 water vapor imagery reveals a mid-level low progressing east across northern Mexico, spreading increased southwesterly flow across Texas, increasing vertical wind shear (Fig 1).
A 4-hour long, 1-min animation of the day cloud phase distinction RGB leading up to convective initiation shows its utility in such situations. Recall, the blue to cyan (or similar) colors represent water clouds, while the transition to truer green indicates ice increasing in the cloud top (glaciation, loss of blue component), and the following transition to yellow is caused by the cooling cloud top (increasing red component). The southeast-moving boundary which eventually serves as the focus for convective development is obvious in the imagery as a line of increasingly agitated cu. Since the 1/2 km, 0.64 um visible channel is one component to this RGB, we are still viewing imagery at the highest resolution possible from GOES. Eventually, orphan anvils develop and blow off, and are obvious (more so than in VIS alone) in their rapid transition from ~cyan to green/yellow. These are often indicators that the atmosphere has destabilized, but the CAP is still present but on the verge of breaking. The failed initiation attempts (orphan anvils) become more abundant through the loop leading up to successful initiation by the end of the period.
One of the severe thunderstorms produced hail of at least 2.5″ in diameter southeast of Sweetwater, TX. The intensity of the storm updraft was captured in the 1-min visible imagery, with abundant storm top texture, overshooting tops, and above-anvil cirrus plumes, all apparent (Fig ). In this particular period of time, two plumes extend from the main (southwest) storm cluster, indicating multiple intense updraft cores.
IR imagery confirms a broad overshooting top, with a large cluster of pixels having a brightness temperature of <-67C and as cold as -70C, surrounded by anvil temperatures between -57C and -60C. Nearby soundings indicate the tropopause was indeed around -60C. A downstream warm region with temperatures around -54C completes a thermal couplet. The above anvil cirrus plume is evidenced by relatively warm and nearly uniform temperatures extending well downstream of the overshooting top. An enhanced-V/U (aka cold ring) extends from the overshooting top and around the plume. The overshooting top, thermal couplet, above anvil cirrus plume, and enhanced-V/U storm top signatures have all been noted in the literature as being indicators of particularly strong updrafts/thunderstorms, with the plume often associated with severe storms (Bedka, 2018).
The Sandwich RGB, now available in AWIPS, combines the VIS and IR into one image (Fig 3) for cold brightness temperatures (cold cloud tops), while maintaining VIS for warmer temperatures (surface and low clouds).
Bill Line, NWS