Squall Line Development and Downburst Potential over eastern Montana

 1. Satellite Morphology and Convective Mode (2000 UTC)

On the afternoon of June 4, 2026, the focus for severe weather shifts to the border region of Montana and South Dakota as depicted in Figure 1 below.

Synoptic Forcing: An eastward-moving upper-level disturbance and an associated surface cold front are acting as the primary triggers for convection.

Convective Evolution: The 2000 UTC composite satellite image captures the development of a broken-line squall line and bow echo complex. This area of concern for downburst winds currently extends across Carter County, MT, and into northwestern South Dakota.

Precipitation Structure: There is a notable correspondence between the bright yellow shading in the satellite imagery and high radar reflectivity. This indicates intensifying thunderstorms with a large ice-phase precipitation content, a signature highly favorable for robust downburst generation as the ice melts and evaporates during its descent.

2. Thermodynamic Environment: Rapid City Regional Analysis

Sounding data from earlier in the day, shown in Figure 2 below, confirms that the environment was primed for the wind event currently unfolding.

Data Integration: A comparison between the 1700 UTC METOP-C NUCAPS sounding—retrieved approximately 135 km northwest of Rapid City—and the 1800 UTC UNR radiosonde (RAOB) shows excellent agreement in vertical profile structure.

Thermodynamic Signatures: Both profiles exhibit a classic hourglass + inverted-V signature. This setup, characterized by a dry and deeply mixed boundary layer beneath a relatively moist mid-level, is a textbook environment for evaporatively cooled convective outflows.

Downburst Ingredients: The analysis highlights favorable wet-bulb zero heights located near the 700-mb level. Combined with a Wind Gust Potential (WGP) of 49 to 50 knots, the thermodynamic evidence strongly supports the potential for severe downbursts as the squall line impinges on this air mass.

Figure 1: Composite SZA cloud phase distinction, MRMS reflectivity, and LightningCast probability product image at 2000 UTC 4 June 2026.

Figure 2. Comparison between the 1700 UTC METOP-C NUCAPS sounding retrieval 135 km NW of Rapid City, SD (left) and the Rapid City (UNR) RAOB sounding at 1800 UTC 4 June 2026 (right).

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