An isolated severe thunderstorm developed at the intersection of a dry line and warm front in northeast Texas during the late afternoon and evening of 08 April 2021.
GOES-East split window difference imagery captured the eastward progression of the dry line during the day (west-to-east rapid transition from low values/dark gray to higher values/light gray), filling in the temporal/spatial gaps between sfc obs. The severe thunderstorm developed just ahead of the dry line where it interacted with the warm front. An overlay of cold IRW BT’s adds information about cloud top features not apparent in the SWD.
Day Cloud Phase distinction RGB imagery at 1-min intervals could be used to gain confidence when and where convective initiation was become most probable. The following six figures include 30 minute periods of 1-min DCPD RGB imagery ending at key points (Fig 2a-e).
The first period shows a transition of the cu fiend in the southwest part of the scene near KBWD, setting it apart from the rest of the scene. Individual cu begin to transition from cyan to green (slight vertical growth and glaciation), and exhibit vertical growth and clumping. From across the full scene, this area appears most favorable for future CI.
The “green-up” and clumping trend continues during the next 30 minute period ending 30 minutes later, but with enhanced growth and a brighter green (more glaciation) for one such cu ESE of KBWD.
Ending 18 minutes later, the aforementioned cu initiates but develops an orphan anvil. Although failed initiation, it represents increasing potential for imminent CI.
Ten minutes later, a cu cloud further to the northwest transitions cyan to green and bright green, representing additional glaciation and vertical growth.
Eighteen minutes later, this area of cu continues to grow, becomes red/orange (continued glaciation and cooling of cloud top), and eventually produces lightning (GLM FED).
The full evolution from both the DCPD RGB and VIS is shown in Fig 3 and 4. While one can analyze cu trends in the VIS such as clumping, vertical growth, orphan anvils, additional phenomena such as glaciation and cooling are easily diagnosed in the RGB as well.
One-min feature-following VIS-IR Sandwich imagery during the day characterizes the cloud top trends of the supercell. Of note is the large and persistent overshooting top (OT) and downstream cold region (thermal couplet), and a long-lived above anvil cirrus plume extended downstream from the OT (Fig 5). Additionally, rapid expansion of the anvil is observed.
A longer, 5-min, VIS-IR animation extending into the evening includes NWS warning polygons and local storm reports (LSRs; Fig 6). The thunderstorm had numerous reports of large hail associated with it (up to baseball size), along with damaging wind gusts.
Finally, an animation of GLM Flash Extent Density shows the rapid uptick in lightning activity during the middle of the storm life cycle, followed by a downtick in activity prior to weakening and decay of the storm.
Bill Line, NESDIS and CIRA
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