Severe thunderstorms developing across central Texas on 26 April 2023 resulted in very large/damaging hail, including up to 4.5″ diameter near Waco. GOES Water Vapor Imagery captured the mid/upper level feature that aided in the development of thunderstorms (Fig 1). During the morning of the 26th, WV analysis from SPC included: “The primary mid/upper-level feature influencing this forecast is a compact cyclone — apparent in moisture-channel imagery over the southern Rockies and centered near SKX…” and from NWS Norman: “Water vapor imagery shows a closed upper low slowly translating southeastward over northeast New Mexico early this morning… expect activity to expand this morning as the upper low pivots eastward into the panhandles and lift increases over the area.” The animation in Fig 1 also included NWP 500 mb height, which aids in the depiction of the mid-level feature, 500 mb wind speed (shades of blue) to show the location of the mid-level flow in relation to the low, and surface Theta-e, to capture the moisture return ahead of the low. As the low drifts southeast during the period, thunderstorms develop just ahead of the vorticity maxima as the strongest mid-level flow overlays the increasing low-level moisture.
Forecast offices studied the GOES imagery during the late morning and early afternoon in there mesoscale analysis. The following NWS interpretations go along with the Day Cloud Phase Distinction RGB animation in Fig 2, which ends at 1831 UTC. As has been shown in numerous case studies on this blog, the DCPD RGB can be thought of as an enhanced version of visible imagery as it combines the high resolution detail of the VIS with phase information from the NIR and cloud-top-temperature information from the LWIR. The imagery shows the increasingly vertical growth of the cu field, especially near the boundaries. Throughout the period, the unstable region, with respect to surface-based convection, is obvious as denoted by the cumulus field south of the w-e boundary near Abilene, east of San Angelo, and well west of Waco. A stable environment is apparent over the eastern part of the scene, as denoted by widespread stratus and stable wave clouds, or billows. Elevated/mature convection is apparent in the northeast part of the scene. By the end of the period, glaciation is being observed in the tallest cu (color changing from cyan to green) within the unstable region near and southeast of Abilene, with numerous orphan anvils, indicating imminent CI. The imagery is annotated in Fig 2a.
From NWS FWD: “Visible imagery is depicting partial clearing is ongoing with temperatures approaching 80s degrees.” This location is in the western part of the FWD CWA.
From NWS SJT: “Visible satellite showing better cu bubbling up around and south of the outflow boundary and appears that convection will be developing before 3 PM. Again, roughly we are talking about an area from Abilene south to Ballinger and east towards Coleman, Brownwood, and Clyde line for initial development, with development spreading south into the afternoon.”
And from SPC: “Visible-satellite imagery shows a swelling cumulus field south of an outflow boundary and east of a dryline… Water-vapor imagery shows the leading edge of cirrus and implied ascent spreading quickly east across the Permian Basin and into the Big Country. As a result, convective initiation is expected in the next hour or so near Abilene.” The SPC mesoscale discussion was accompanied by a quick analysis of the visible imagery with relevant boundaries highlighted (Fig 3).
During the two hour period following the end of Fig 2, convection would initiate (vertical growth and and transition from green to yellow/red) and quickly strengthen into strong/severe thunderstorms (Fig 4).
At this time, switching to a product like the GOES VIS/IR Sandwich Imagery allows one to monitor storm top features and their trends as a proxy for updraft strength and potential storm severity, as a supplement to radar analysis. For example, many of these storms exhibited persistent and large Overshooting Tops and Above Anvil Cirrus Plumes, the latter which has been associated with the strongest thunderstorms. Other notable features include enhanced-V’s (or cold-U), and thermal-couplets. These features are annotated on one of the storms (near Waco) in Fig 6. Storms that are weakening exhibit a collapse of the OT and warming of the cloud top brightness temperatures, in addition to loss of texture in the VIS.
Bill Line, NESDIS/STAR
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