Large area of convection with prolific lightning activity southwest of Tropical Storm Bertha’s center
1-minute Mesoscale Domain Sector GOES-19 (GOES-East) Infrared Window images (above) showed a large and persistent cluster of deep convection that remained southwest of Tropical Storm Bertha’s center during the nighttime hours leading up to sunrise on 23 July 2026.
Pulses of overshooting tops exhibited very cold infrared brightness temperatures of -90 C or lower (below).
1-minute GLM Flash Points (below) revealed prolific lightning activity associated with the large and persistent cluster of convection. The NHC Forecast Discussion at 0900 UTC mentioned that “there has been a persistent supercell thunderstorm with a mesocyclone moving westward through Bertha’s southwestern quadrant”.
Some of the GLM Flash Points exhibited an unusually long duration and large flash area (below). Over oceans, the average GLM flash duration is 0.345 sec, with an average flash area of 570 km2 (reference).
While the official intensity of Bertha was 40 knots at that time, Ultra High Resolution ASCAT winds from Metop-B (below) showed 2 pockets of surface winds in excess of 50 knots (lighter shades of violet). These higher surface wind speeds may have been related to the aforementioned supercell thunderstorm with a mesocyclone.
A nighttime NOAA-20 VIIRS Day/Night Band image (below) displayed concentric mesospheric airglow waves (reference) that were propagating southward and westward from the large convective cluster. A few bright “lightning streak” signatures were apparent closer to the coast, which was where the core of the convective cluster was located. The Moon had already set over the western Gulf of Mexico prior to this time, so there was only atmospheric airglow to faintly illuminate any cloud features.
