A compact shortwave trough brought widespread severe weather to the southern US 10-11 January 2020. A Moderate Risk for severe storms was issued by the Storm Prediction Center for 10 January for a significant severe wind gust threat, with secondary threats of large hail and tornadoes. The threat on the 10th was centered over Texas, Oklahoma, Arkansas, and Louisiana, shifting east to Mississippi/Alabama on the 11th.
GOES-West upper-level water vapor imagery during the 48 hours leading up to convective development on the 10th show the evolution of the shortwave trough from off the northwest US coast, south along the west coast, and then east across the southwest US and into Texas (Fig 1).
The potent system drove enhanced low-level moisture into the region along with strong mid-upper level winds. This VISIT blog post highlights the utility of layer PW products for tracking the evolution of moisture and the EML leading up to the convective event.
Convection was ongoing/developing across Oklahoma during the morning/early afternoon on the 10th. Further south into Texas, convection developed a little later during the afternoon as the upper trough and associated strong forcing shifted closer, and inversion associated with the EML eroded.
By the late morning, GOES-16 derived motion winds indicated a swath of 60-70 knot SSW winds around 6 km AGL out ahead of the trough across north Texas (prior to CI here). Using the satellite-derived winds in combination with surface obs below to compute a vector difference, we achieve 0-6 km bulk shear values of around 55 knots, supporting the development of supercell thunderstorms (Fig 2). Further, the strong unidirectional deep layer flow is parallel to the surface/low-level boundary as diagnosed in visible imagery and surface obs (SSW to NNE). This orientation along with the strong shortwave forcing imply a likelihood of linear thunderstorm mode and wind threat quickly upon development.
GOES-16 1-min Day Cloud Phase Distinction RGB imagery showed widespread low stratus clouds (cyan) and showers (darker green indicating ice in cloud) across north Texas by early afternoon (Fig 3). During this period, convection rapidly develops southward across the region, clear in the 1-min RGB imagery per the vertical growth and transition to reds and yellows (result of cooling in the IR and increasing reflectance in the VIS as convection deepens).
Now looking at 1-min VIS/IR sandwich imagery for the following hour, convection continues to grow quickly southward across north Texas (Fig 4). The sandwich imagery maintains the high resolution/detail available in the 500 m VIS, while also representing the degree of cooling via the IR. Important storm to features are also easily detectable, including overshooting tops and above anvil cirrus plumes.
During this same period, VIS/GLM FED sandwich imagery showed lightning jumps with the strongest storms embedded in the broader line during early development (Fig 5). The north Texas storm for which a severe thunderstorm and tornado warning was issued saw lightning activity increasing from around 20 fl/5-min to around 90 fl/5-min within a ~15-minute period leading up to warning issuance. This rapid increase in flash density signifies a rapidly intensifying updraft. This particular storm had 1″ hail reported with it.
Analyzing 1-min visible imagery even further south, storms continued to develop south along a narrow line. The low sun angle allows for easy diagnosis of vertical growth as well as relevant storm top features such as overshooting tops and eventual above anvil cirrus plumes. Ahead of the developing storms, billow cloud formations indicate the continued presence of a stable layer inhibiting surface base convection.
Bill Line, NESDIS/CIRA