High Wind — Southern Lewis and Clark, Montana

2021-03-29 · Southern Lewis and Clark, Montana

Event narrative

About 20X30 feet of metal roof torn of a building. Tree branches 6 inches in diameter snapped at bottom of a tree.

Wider weather episode

An anomalously deep Pacific trough initially brought H700 (mountain top) westerly winds of 50 to 80 kts on Sunday March 28th, 2021, which eclipsed the 99th percentile of climatology over many areas during the height of the storm. A tight surface pressure gradient, mountain wave activity, along with a well mixed atmospheric profile all contributed to transferring the elevated winds to ground level. Minimum relative humidity values fell into the 15 to 25 percent range as high temperatures warmed into the 60s to near 70 degrees. These conditions increased the risk for grassland fires and contributed to the explosive growth of the Greenhouse wildfire. On the backside of the storm, a cold front provided cold air advection and pressure rises for another round of strong winds Sunday night into Monday. Robust H700 cold air advection 60+ kt winds were also instrumental for the strong surface winds and snow shower development for that time period.

Source: NOAA Storm Events Database, event_id 951347. Narrative written by the NWS forecast office that issued the report.