Persistent Severe MCS and Downbursts in Southwestern South Dakota

 1. Convective Initiation & MCS Evolution (1900 UTC)

The early afternoon of 2 June 2026 was marked by the development of a persistent mesoscale convective system (MCS) over Fall River County, SD as shown in Figure 1 below.

Satellite Observations: The 1900 UTC composite satellite image shows the system advecting east-northeastward, exhibiting rapidly increasing ice-phase cloud tops and a significant rise in lightning flash rates.

Structural Signatures: At this stage, the MCS already exhibits a well-defined bow echo complex and a prominent rear-flank dry-air notch. These features indicate
that the system is efficiently processing dry mid-level air to generate strong surface outflows.

Figure 1: Composite SZA cloud phase distinction, MRMS reflectivity, and LightningCast probability product image at 1900 UTC 2 June 2026.

2. System Intensification and Dual Bow Echoes (2030 UTC)

By 2030 UTC, Figure 2 shows the convective complex underwent further intensification and expansion as it tracked across southwestern South Dakota.

Complex Organization: The system evolved into a more formidable MCS consisting of two distinct bow echo complexes.

Severe Potential: Each bow echo was accompanied by its own rear-flank dry-air notch. This dual-notch presentation signaled a marked increase in downburst potential, as the system became increasingly organized and capable of producing widespread damaging winds.

Figure 2: Composite SZA cloud phase distinction, MRMS reflectivity, and LightningCast probability product image at 2030 UTC 2 June 2026.

Figure 3. Comparison between the GOES-19 (G19) sounding retrieval 90 km SSE of Rapid City, SD (left) and the Rapid City (UNR) RAOB sounding at 1800 UTC 2 June 2026 (right).

3. Thermodynamic Environment: Rapid City (1800 UTC)

An analysis of the 1800 UTC environment in Figure 3 explains the high-end wind potential observed.

Sounding Profile: A comparison between the GOES-19 sounding retrieval and the Radiosonde observation (KUNR) at Rapid City, SD, reveals a textbook "hourglass-inverted-V" profile.

Instability & Wind Potential: The environment was characterized by large CAPE (1014 to 1584 J/kg). Correspondingly, the Wind Gust Potential (WGP) was calculated between 41 and 47 knots, providing strong evidence for the severe gusts that followed.

4. Regional Validation & Surface Verification

Regional sounding data further confirmed the widespread nature of this volatile environment.

NUCAPS Comparison: A NOAA-21 NUCAPS sounding retrieved at 1919 UTC approximately 150 km southeast of Rapid City showed excellent structural agreement with the UNR radiosonde (RAOB).

Key Ingredients: Both profiles highlighted favorable wet-bulb zero heights for evaporative cooling and robust downburst generation.

Ground Truth: This thermodynamic setup culminated in a significant surface event: a severe 55-knot downburst wind gust was recorded at Cactus Flats, SD, at 2043 UTC— aligning well with the high WGP and structural signatures identified earlier in the afternoon.

Figure 4. Comparison between the 1919 UTC NOAA-21 NUCAPS sounding retrieval 150 km SE of Rapid City, SD (left) and the Rapid City (UNR) RAOB sounding at 1800 UTC 2 June 2026 (right).

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