Last night, I shared the video of the tornado in North Dakota. As you may have noticed, there was no precipitation surrounding the tornado. We often see supercell tornadoes that are either wrapped in rain, or on the brink of becoming wrapped in rain. The supercells in the southern US tend to form in much more humid environments. These humid environments are made possible due to the close proximity of the Gulf of Mexico. As one gets farther away from the moisture source, there’s less moisture to work with. There are “hiccups” to this idea, in that areas with lots of agriculture (ie. Corn fields of Indiana) have noticeable jumps in humidity due to evapotranspiration of the crops (moisture evaporating from the crops).
The main culprit for yesterday’s tornadoes was what we refer to as a cold core low. This is basically a cold pocket of air spinning in the atmosphere. It is not based on the surface of the earth, like a hurricane’s low pressure center, but it is based much higher up within the atmosphere. This spinning pocket of cold air is rather common across the Plains, especially during the spring months. Now, if you can get a lot of sunshine underneath these lows, that sunshine will warm the earth’s surface. Since we’re getting a higher sun angle every day that we closer to the first day of summer, it can really warm the ground up this time of year! That warm air underneath that pocket of cold air above can create the perfect environment for clouds to develop, as that warm air rises into the cold air and cools and condenses. The winds with these cold core lows increases pretty dramatically with height, so that adds a component of shear. In other words, the faster winds aloft allow the updraft to tilt, which allows it to gain/maintain strength without being choked out by the nearby downdraft.
With winds increasing with height (speed shear) and instability (sun heating the ground), the ingredients were in place for thunderstorms. An area of surface low pressure then formed just southwest of where the tornadoes were reported. This development produced a frontal boundary across eastern North Dakota and provided directional shear for the winds. Speed shear is when winds increase with height, directional shear is when winds change direction with height. We now have southeast winds at the surface, increasing and becoming southwesterly aloft, which is the perfect wind combination for rotating supercells. The only thing left to talk about is surface moisture and that’s what leads us to the next point: why is there no precipitation around the tornado?
Interestingly, a cold front had made its way across the Dakota’s earlier in the afternoon. That front robbed the atmosphere of some of its moisture. However, dewpoints were still able to climb to around 60, which was just enough moisture to create thunderstorms. That’s enough to create the rotating thunderstorm, but not enough to produce a lot of precipitation. We call these ‘low precipitation supercells’ and they’re actually rather common across the northern Plains. Remember the guy mowing his yard during that very visible tornado in Canada a week or so ago? Same situation! There’s enough moisture to create a storm, but too little moisture for the storm to produce heavy precipitation. In the southern Plains we may see dewpoints in the 70s with supercells, which is more than enough moisture to not only create storms, but create storms that produce copious amounts of moisture that can wrap around the rotating supercell and hide the tornado.
If the sun hadn’t come out and heated everything up so much, there wouldn’t have been enough instability to create thunderstorms. Plus, without that warming the surface low that formed wouldn’t have been as strong, and the wind fields would have been weaker. It really all boils down to that sunshine and that cold pocket of air that we call a cold core low.
The tornado has been rated an EF-0 by the NWS. Cold core lows often produce weak tornadoes, and that certainly seems to be the case here.
I hope this helps you understand how the tornado originated and why we were able to see it! If you have any questions at all, please don’t hesitate to let me know!
For more radar, satellite, storm environment, etc information, go to http://www.weather.gov/fgf/2017_06_…