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NWS Great Falls Conducts Storm Damage Survey in the Tobacco Root Mountains

Microburst damage survey
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Posted at 4:02 PM, Aug 19, 2019
and last updated 2019-08-19 18:02:51-04

From Great Falls National Weather Service: On Sunday, August 11 at approximately 2:48 PM MDT, a microburst occurred in the Tobacco Root Mountains one to two miles northeast of Granite Lake in Madison County, Montana. A storm damage survey was conducted by a team of Beaverhead-Deerlodge National Forest Service and National Weather Service personnel on Sunday, August 18. It is estimated that around 250 acres of trees were blown down, which resulted in the blockage of off-highway vehicle roads and trails. Four people became stranded in the backcountry due to these trees blocking US Forest Service Road 9605 that passes through the area. Personnel from the Madison County Search and Rescue, National Forest Service fire team, and two military helicopters were dispatched to the Granite Lake area to rescue those stranded.

Thunderstorms developed near the Continental Divide on the Idaho/Montana border between Noon and 1:00 PM MDT moving southwest to northeast across Beaverhead and Madison counties. NWS meteorologists in Great Falls issued several Severe Thunderstorm Warnings between 1:15 PM MDT and 4:15 PM across Beaverhead, Madison and Gallatin counties. Large hail ranging from half-dollar (1.25 inches) to golf ball (1.75 inches) were reported near Dillon around 1:45 PM MDT and at 2:30 PM MDT in Sheridan, Montana. At the time of the golf ball (1.75 inches) size hail report in Sheridan, the severe thunderstorm was at its peak strength with a well defined mid-level rotational signature apparent in the lowest elevation scan from the Missoula radar. The storm was beginning to weaken/collapse as it moved over the Tobacco Root Mountains and produced a microburst.

The survey concluded that the storm-related damage was caused by a microburst with estimated wind speeds between 90 to 100 mph. These wind speeds are equivalent to those that can occur in an EF1 tornado. The surveyed damage indicated debris spread in a uniform direction over a broad area, unlike damage associated with a tornado. A microburst is a localized column of sinking air (downdraft) within a thunderstorm and is usually less than or equal to 2.5 miles in diameter.

Microbursts can cause extensive damage at the surface, and in some instances, can be life-threatening.