Severe thunderstorms developing off of a E NM/W TX dryline produced hail to the size of baseballs (2.75″) and a few tornados during the afternoon/evening of 17 May 2021. GOES-East Split Window Difference (SWD) – IR Imagery Combo captured the evolution of the dryline early on in the day prior to the development of cumulus clouds and eventual deep convection. The 10.3 – 12.3 SWD with gray scale color table, discussed in previous blog posts, highlights the sharp boundary from drier lower atmosphere (darker gray) to greater moisture (lighter gray). By overlaying cold 10.3 um BTs, we add information where SWD provides little (thick clouds) and can analyze thunderstorm development in the context of the moisture boundaries. In this case, the combo shows convective initiation along a dryline spanning from E NM south through W Texas, and into Mexico. Please note, when using the SWD to track LL moisture, a positive lapse rate is required (temperature decreasing with height). Finally, the SWD also captures blowing dust within the stronger southwesterly flow in the southwest portion of the scene (darkest gray; negative values of 10.3 – 12.3 um SWD). NWS convective warning polygons are also shown in many of these examples. Of note, the NWS Midland, TX office was handicapped in that the WSR-88D was down due to a failed power supply for much of the day, so they had an exceptional reliance on other distant radars and satellite imagery for this event.
Corresponding visible satellite imagery from GOES-East captures the evolution of the atmosphere from clear sky to cumulus clouds to deep convective initiation.
Comparing the SWD and VIS, we see that by late morning, pieces of the dryline, including dryline bulges, can be diagnosed in the SWD, even before cu start to develop along the boundary.
Later in time, visible imagery shows cu developing along and ahead of the dryline as diagnosed in in the SWD, including early deep convective development ahead of the dryline bulges.
Focusing in on the region and overlaying surface obs, we get an idea of how the SWD provides greater spatial and temporal detail as a supplement to the coarser surface observations in tracking the evolution of the dryline. Note the higher dew points east of the dryline, and rapid cooling of dew points as the dryline crosses the obs.
One-minute imagery from GOES-East was available to forecasters over the region. At NWS Midland, TX, the forecaster working the mesoscale desk utilized the Day Cloud Phase Distinction RGB leading up to convective initiation to monitor for relevant mesoscale signatures and for early signs of glaciation. The dryline bulge noted above can also be inferred in this imagery as the region of cu-free airmass southwest of Midland pushing northeast. This feature was noted by forecasters as a potential area of CI (specifically, N/NE of the bulge) given favorable low level moisture convergence and backing of surface flow. Forecasters also noted, from the early imagery, boundaries and HCRs over the northeastern part of the CWA, which indicated to them the amount of low level shear and potential helicity storms could ingest. Based on trends in this RGB, the forecaster notified warning staff when towers were starting to glaciate. In the GOES-East 1-min Day Cloud Phase Distinction RGB animation below, towering cu and glaciation becomes obvious just south and south-southwest of Midland toward the end of the animation, ahead of the dryline bulge.
Convective initiation continued over the following hour, as was apparent in the same RGB imagery, ahead of the dryline bulge southwest of Midland. Also included in the next animation is GOES-East GLM Minimum Flash Area (MFA), which was also being viewed by NWS/MAF during this event. Although lightning activity was relatively low with these storms, the early flashes were quite small, indicating to the forecasters of continued storm intensification. Additionally, the presence of lightning prompted the issuance of an Airport Weather Warning for Midland International Air and Space Port (within 10 nm of storm).
As storms matured, forecasters continued to view GLM products (FED and MFA), as well as ProbSevere, to aid in situational awareness regarding updraft trends and identifying storms with greater severe potential. Forecasters also noted the presence of above anvil cirrus plumes (AACPs) with the strongest storms. The 4-panel below includes (cw starting with top left panel): VIS, DCPD RGB, VIS+GLM FED, VIS+GLM MFA, and warning polygons on all panels.
GOES-East 1-min VIS-IR Sandwich Combo imagery captures the first three hours of development of a supercell thunderstorm off of the southern dryline bulge. Of note is the rapid anvil expansion, obvious and large OT that develops, along with an associated Above Anvil Cirrus Plume, indicative of a particularly strong updraft and severe threat. The storm produced very large hail during this time period.
Later, GOES-East 1-min VIS shows the thunderstorm during the period of tornado development. The increased shadowing toward sunset allows texture in the storm top, including features such as the OTs and AACPs, to become even more obvious.
Bill Line, NESDIS and CIRA and Brian Curran, NWS Midland, TX