Thunderstorm Wind — Dawson, Montana

2015-07-27 · near Glendive, Dawson, Montana

Event narrative

The National Weather Service in Glasgow conducted a damage survey in Glendive on July 28, 2015 following a significant severe thunderstorm that hit the evening before coming in from the southwest. NWS staff met with county and city officials before and after the survey to be briefed and brief them on what was discovered. The event was a macroburst (large scale straight line wind event) with some microburst components in between.

There were not enough time and resources to enter the various neighborhoods that were impacted, and due to the number of trees and power lines down, it was not safe to try and go around these areas where crews were working. In order to best evaluate the wind speeds, the structures with the most damage were focused on as recommended by the Emergency Operations Center staff. Several dozen homes were viewed though, and the damage was generally due to trees and branches lost, or shingles torn off of roofs. That includes trees in homes or on cars, some partially blown off roofs, and trailer homes or semi-trailers blown over. The damage survey started with the power lines west-northwest of town on Highway 200, then to the Glendive airport, Seven Mile Drive, back through town on Highway 200S, up Merrill Ave and then up Highway 16 north to Sidney for 10 miles.

Access to the large metal truss electrical transmission towers west-northwest of town on Highway 200S was not possible due to the extreme mud conditions. There were at least 5 noted as having crumbled toward the ground. Further investigation will have to be done with the owner of the structures as to the age of these towers, but at minimum the winds would've been 120 mph. There are also numerous wooden poles snapped at the bases and part way up along Highway 200. The NWS will get total numbers from utility providers once they have a chance to assess all the damage.

At the airport, winds were gusting to 90 mph last night on an anemometer on top of the terminal roof. The winds tore the anemometer off the roof though, so a peak wind was not measured. The FAA AWOS station that is the official observing equipment for the airport lost power, and a maximum gust was not received from it either. There was a large cottonwood tree that was uprooted near the terminal building. There were three metal roof/siding airplane hangars damaged. One was completely destroyed, and the other two had the metal roofing peeled back/off. The one that was destroyed wasn't cemented into the ground and the posts easily ripped out of the ground when the wind blew against it. All the debris was blown towards the north-northeast. The winds are estimated at 90 to 95 mph at this location.

The next stop was on Highway 200S just southeast of the Interstate turnoff. Significant damage occurred to a large metal building. The failure point was two large garage doors that face southwest. There is nothing to stop the wind as it rolls through the valley towards it. The garage doors blew in, and the roof blew off. One of the employees mentioned that sheet metal from another building southwest of them was blown towards their building. The impact of that metal hitting the doors may have helped cause the doors to fail and be blown in. This metal building had solid steel I-Beams in the ceiling that were bent back where the roof peeled back. Power poles with transformers on them were snapped off near this facility and just east-southeast of it. The transformers are pretty heavy and the resistance and weight of them against the wind made them easy targets to be snapped. All the debris was blown in a north-northeasterly direction. Between the building and the power poles, winds at this location are estimated to have been around 100 to 110 mph.

7 Mile Drive was next. There were many homes with shingles peeled off, and large trees split and large branches down, metal buildings damaged and sheet metal blown around. A few power poles were also snapped. We stopped at an office building and a newer, well-built warehouse after noticing the garage doors on the warehouse were bent out. There was also some insulation missing on a portion of a wall. They had already cleaned up the south side doors, which had blown in as the winds came off the hill and crumbled some of the north side doors. There was very little noticeable roof damage, which is something you will often see with garage doors blowing in, a testament to how well the building was built and the steel I-beams used in the trusses. Winds at this location are estimated at 100-105 mph.

Another metal building structure was viewed south of the interstate off of 7 Mile Drive. A large overhead door that faces south was blown in and the roof right above it peeled off and blown to the north. On the north side of the building, the majority of the roof (with wooden trusses) was either collapsed into the building or blown north-northeastward. Winds at this location are estimated at 95 to 100 mph.

As we went down Highway 200S into downtown, the most notable damage was tree branches down. We went down Marsh Road, and there were a lot of trees damaged in that area from 1 to 3 miles south of town. House damage was generally due to branches falling on them or against them. Heading north on Merrill Ave, tree damage was the most noticed damage, specifically near the high school football field. A large blue spruce was noted down near I-94 and roof damage with shingles and fascia torn off were noted on a new hotel at that location. Winds in this portion of town are estimated to have been 80-90 mph.

We continued north on Highway 16 towards Sidney. The number of damaged trees and sheds quickly reduced as we headed north and by 10 miles we were really looking hard to see any damage. At four miles north of town, there was a home with a new barn that had all but the west wall collapse. With no one home, we did not go out into the field to assess the barn closer, but it was newly built. Winds at this point are estimated at 100 mph.

Wider weather episode

Significant instability, a low pressure circulation over southeastern Montana, favorable winds, and warm, moist air all combined with an approaching strong upper-level storm system to quickly develop and maintain well-organized severe thunderstorms over many locations and a macroburst in the Glendive area.

View location on OpenStreetMap → (47.0806, -104.7987)

Source: NOAA Storm Events Database, event_id 597453. Narrative written by the NWS forecast office that issued the report.