An impressive supercell thunderstorm produced damaging softball sized hail overnight on March 1st-2nd in south Texas around Dilley. The storm developed on the eastern edge of GOES-18 (-West) Meso sector 1, capturing the rapid development of the storm minute-by-minute (Fig 1). IR imagery indicated brightness temperatures dropping rapidly as the storm emerged from beneath a cirrus canopy, cooling from -49C at 0314 UTC to -67C at 0324 UTC. The storm would eventually cool to -85C in the large and long-lived overshooting top per the GOES-West Imagery, similar to what was later sampled from GOES-East. Other significant thunderstorm characteristics exhibited by this storm included a rapid outward expansion of the anvil, an Enhanced V (or cold U), and thermal couplet. An Above Anvil Cirrus Plume is difficult to confirm in this nighttime case.

Five-Minute CONUS sector IR imagery from GOES-East captures the full evolution of the storm (Fig 2), including the above mentioned features. Warming of the cloud top is observed as the storm drifts east and weakens. The animation captures later storm development around San Antonio, which resulted in hail up to 1.75″ in diameter.

One may prefer to overlay cold C13 IR BTs on the Nighttime Microphysics RGB in order to analyze the low cloud situation (boundaries, moisture movement, etc) in conjunction with mature storm motion (Fig 3).

Overlaying GLM Flash Extent Density (FED) on the IR imagery, we diagnose a rapid increase in lightning activity during the storm’s early development, including a jump from 5 to 64 Flashes/5min from 0316 UTC to 0321 UTC, and to 93 flashes in the next 5 minutes (Fig 4). The storm would max out at 272 flashes/5min at 0626 UTC (per GOES-East GLM). Rapid decrease in total lightning activity is observed near the end of the animation, with increasing lightning highlighting the San Antonio storms.

Corresponding GLM Minimum Flash Area (MFA) imagery traces the movement of the updraft core well, as the region of lowest MFA (Fig 5).

We can combine the FED and MFA in an RGB to give us a single image, or experimental “GLM RGB”, that tells us where many flashes are occurring coincident with small flashes (or many small flashes may be occurring), as a signal of exceptionally strong updraft growth/sustainment (Fig 6). In this RGB, yellow indicates high FED and small MFA (many small flashes), red indicates few small flashes (with brighter orange representing increased number of small flashes), and blue represents few large flashes (with brighter cyan representing increased number of large flashes). The main southern storm consistently exhibits yellow within the main updraft region throughout it’s evolution prior to decaying. The northern San Antonio storms show comparatively lower total lightning activity, resulting in RGB appearance of updraft regions with bright oranges (many small flashes, but less than the other storm). This is a testament to the fact that such an RGB may have to be calibrated from event to event as a high lightning rate in one situation may be relatively low in another. Such changes are easily made in AWIPS using the “Composite Options”.

NOAA-20, NOAA-21, and S-NPP VIIRS Imagery were available over the region as the thunderstorm was weakening. However, the VIIRS imagery did capture the development of thunderstorms impacting San Antonio. During this evening, two NOAA-21 swaths sandwiched single swaths of S-NPP and NOAA-20, resulting in a sequence of imagery as follows: N21 (25-min) NPP (50-min) N20 (25-min) N21 (Fig 7). Prior to NOAA-21 becoming available, this scene would have been covered by only two VIIRS (N20 and S-NPP), separated by ~50-minutes. The sequence captures a lack of detail in the initial storm top as it drifts east out of the scene, while also revealing, in detail (375-m) the cooling and cool cloud tops of individual thunderstorms as they advance through northern San Antonio.

Figure 7: 02 Mar 2023 early morning NOAA-20, NOAA-21, and S-NPP VIIRS Band I5 IR Imagery. NOAA-21 VIIRS Imagery is preliminary, non-operational.

A three-satellite (NOAA-21 (25-min) S-NPP (50-min) NOAA-21) sequence is created for the VIIRS Day Night Band/Near Constant Contrast product during the same period over a broader region (Fig 8). The product captures visible-like imagery at night, including cloud details, city lights, and lightning flashes. The decay of the initial thunderstorm is obvious in this imagery, along with the abundant texture associated with the developing San Antonio storm.

Figure 8: 02 Mar 2023 early morning NOAA-20, NOAA-21, and S-NPP VIIRS DNB/NCC Imagery. NOAA-21 VIIRS Imagery is preliminary, non-operational.

Bill Line, NESDIS/STAR