Eastern Montana and Western South Dakota Facing Hail Producing Supercells with Hen Egg Size Hail Possible as Jet Stream Energy Drives Threat Thursday June 4

RAPID CITY, SOUTH DAKOTA — A hail-focused severe weather threat is developing across eastern Montana and western South Dakota on Thursday June 4, 2026, as HRRR model data initialized June 4 at 12Z via AguaceroWX graphics shows a pulse of jet stream energy moving overhead combining with sufficient instability to fire hail-producing supercells, while weak low-level flow keeps tornado risk near zero across the region.

Hail Producing Supercells Possible Across Eastern Montana and Western South Dakota

A few hail-producing supercells are possible across eastern Montana and western South Dakota today as a pulse of jet stream winds moves over the region during the afternoon hours. The jet stream energy is sufficient to support supercell development and organized storm structures capable of producing significant hail, but the lack of adequate low-level flow is a critical limiting factor that will suppress tornado potential to little or none across the threat zone. Residents should focus their preparedness entirely on the hail threat rather than tornadoes for today’s storm activity.

Isolated Hail Up to Hen Egg Size Cannot Be Ruled Out Across Core Threat Zone

AguaceroWX hail threat graphics for Thursday June 4 show a concentrated isolated hail zone across the eastern Montana and western South Dakota corridor where hen egg size hail cannot be ruled out from the most intense supercell activity. A broader scattered hail zone extends across a larger swath of western and central South Dakota, covering communities from Rapid City through Pierre and into the central part of the state. The isolated hail zone sits within the scattered hail area, indicating the potential for very large hail in localized spots within an already active hail-producing environment.

Weak Low Level Flow Limits Tornado Risk Despite Jet Stream Energy Overhead

While jet stream winds are providing sufficient upper level energy to support supercell development today, forecasters are specifically noting that weak low-level flow across the region is preventing the development of the surface to mid-level wind shear needed for tornado formation. This means storms will likely remain in supercell mode with organized updrafts capable of producing large hail but without the rotating characteristics needed to generate significant tornado risk across eastern Montana and western South Dakota today.

Surface Low and Dry Versus Humid Air Boundary Setting Up Across the Region

HRRR surface analysis shows a surface low pressure center positioned near the Montana and Wyoming border with a dry air mass across the western portion of the region and humid air pushing in from the east across the Dakotas. This moisture boundary combined with the overhead jet stream energy is the primary forcing mechanism for storm initiation across the eastern Montana and western South Dakota corridor during Thursday afternoon.

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