On Wednesday, May 24, a supercell thunderstorm developed near the Ruby Mountains in Nevada and produced a rare tornado for this part of the world. The tornado was located in rural terrain and did not produce any structural damage, but it did draw quite a bit of attention in a state that only averages two tornadoes per year.

Image – Tornado with snowcapped Ruby Mountains in the background

Video – Tornado near Halleck

The storm initially developed between 12:30 and 1:00 pm local time, before strengthening and passing just east of the town of Elko. The storm exhibited a "hook" shape on radar, which is indicative of rotation.

Another radar view that meteorologists use to monitor thunderstorms is the velocity scan. Velocity images on radar detect winds blowing toward and away from the radar site, along with the magnitude of wind speeds. Red colors indicate winds moving away from the radar, and green colors indicate winds moving toward the radar. Brighter shades of each color indicate stronger wind speeds. 

When bright red and green colors are located next to each other in a tight "couplet", this indicates strong rotation and often means a tornado is ongoing or imminent.

Radar velocity scans from this storm indicate two instances of strong couplets, which indicate the location of the tornado – during both the first and the second touchdowns.

What caused this tornado to form in such an unusual location?

There were a few factors in play...

First, a trough of low pressure in the upper atmosphere was located over the West Coast, placing Nevada under a persistent southwest flow pattern on the downstream side of the trough. The downstream side of a trough is favored for inclement weather, as energy, moisture, and instability are greater in this region.

Wind shear (changes in wind speed and direction as altitude increases) was high for this region, favoring stronger thunderstorms, and moisture levels were also higher than usual for this region thanks to consistent southwest winds which had transported an abundance of moisture from the South Pacific. 

While conditions in the upper atmosphere were favorable for strong thunderstorms, the pattern near the surface likely played the biggest role.

A stationary front was situated across Northeast Nevada on this day with strongly opposing wind directions on either side of the front. To the west and north of the front, winds at the surface were blowing out of the north, and to the east and south of the front, winds were blowing out of the south.

The frontal boundary was set up right where the tornado developed, and the opposing winds along the boundary likely helped the thunderstorm updraft rotate, eventually producing a tornado.

Tornadoes are relatively rare west of the Continental Divide, due to a lack of warm and moist air originating from the Gulf of Mexico compared to areas east of the Continental Divide.

However, mountain tornadoes west of the Continental Divide can and do happen when conditions are just right. The predictability of mountain tornadoes west of the Divide is also lower compared to the Great Plains and areas east of the Divide where tornadoes are more common. 

Just last year, there were two EF-2-rated tornadoes (on a 0 to 5 scale) confirmed in the Rockies at elevations above 8,500 feet – one in the Uinta Mountains of Northeast Utah, and one in the Gros Ventre Mountains just east of Jackson, Wyoming.

While mountain tornadoes are less common overall, there has been evidence in recent scientific studies that in some instances when conditions are otherwise favorable, changes in terrain over short distances can actually enhance the process of tornado development. 

ALAN SMITH