Tuesday, July 27, 2010

Rare deadly tornado in Montana - 7/26/10

Killer tornadoes are rare in Montana due to the sparse population. Prior to 2010, only 2 deadly tornadoes had been recorded in the state, with the last tornado deaths occurring way back in July of 1983. But yesterday (Monday 7/26/10), sadly, 2 people were killed and another seriously injured when a large tornado struck a ranch in the northeast corner of Montana west of the town of Reserve (see photo and radar images above). This tornadic supercell moved east-southeast from Canada, deviating rightward under westerly flow aloft.

The surface map at early evening (above) showed a surface low over northeast Montana with easterly upslope winds along and just north of a stationary front where surface temperatures (80s F) did not cool too rapidly across the WNW-ESE boundary, and dew points feeding the storm were in the mid to upper 60s F. RUC model soundings had a little trouble depicting the moisture depth in northeast Montana near this boundary (moisture too dry/too shallow just above the ground, not shown) and wind profiles (hodographs a little "flat" in shape, not shown). However, the SPC mesoanalysis effective-layer sig tor parameter (STP, see graphic above) still suggested a "favorable" environment not too far to the east over extreme northwest North Dakota, likely feeding into northeast Montana on the upslope winds near and north of the stationary front.

A RUC analysis sounding at early evening near Reserve MT (last image above) had to be modified for moisture depth and the easterly wind pattern in order to be reasoanbly represnetative of the environment. This modified profile showed excellent deep layer shear (60 kts), good MLCAPE (2800-3000 J/kg), and workable low-level storm-relative helicity (SRH, > 150 m2/s2). Even though the low-level hodograph/wind profile wasn't large or notably strong, there was a "kink" in the profile and enough low-level shear to support strong tornadoes along and near the boundary. It appears this tornadic storm was able to parallel the boundary, taking advantage of the enhanced low-level wind environment there, along with better moisture just north of the stationary front without too much temperature contrast across it. Tornado warnings from the Glasgow NWS blanketed the area traversed by the supercell well in advance of it.

- Jon Davies 7/27/10