Field Methods – Day 12

Posted by: Kate Wilson

Today seemed like the last promising chase day of the trip. The setup involved a surface low in the northern central plains, with a warm front situated west to east across the Midwest and a cold front stretching from South Dakota through Colorado. Southerly flow off the Gulf of Mexico brought up moist air with dewpoints in the upper 60s and lower 70s. The surface showed clearing in southeastern South Dakota and eastern Nebraska. CAPE values were expected to exceed 5000 J/kg, but a strong cap was in place as well. However, it looked like a break in the cap would drape from central Nebraska up to eastern South Dakota. While the strong CAPE seemed able to break any remaining cold cap by late afternoon, the main issue with the setup was wind sheer. Winds aloft were very weekly veering, and there were low helicity values, meaning smaller chances of powerfully rotating supercells and thus tornados. Looking for a break in the cap, some students set their sights as far north as Sioux Falls, but we decided to remain in Nebraska and head towards the town of Taylor in the central part of the state.

Throughout the day, a mesoscale discussion was issued by the Storm Prediction Center which described the situation we were anticipating: large amounts of instability with a good chance of strong thunderstorms, which would likely start as discrete (individual) cells, becoming a joined line of storms with time. This was due to the mainly unidirectional vertical wind field. It was key to be on a storm as soon as it formed for its best chances of rotation before congealing into a line of other storms.

Storms began firing around 1900 UTC to the southwest and north of Taylor. We decided to pursue a storm to the north which produced wall clouds and several gustnados, one of which was mistakenly reported as a tornado by another storm chaser. This is somewhat easy to do as a gustnado is a circulation visible as a dust whirl on the ground due to turbulence from the main storm. However, there is no upper level support in the form of a rotating updraft or wall cloud, so this cannot be classified as a tornado, although they may mimic one in appearance. Eventually this cell became more organized and had a well-defined circulation, but we had to abandon it when it started right-turning and the limited road access prevented our chase.

We then watched cells fire on the tail end of the quasi-linear convective system. This term simply describes a “line” of storms that is not definitely linear joining together multiple cells displaying linear characteristics. After watching the new storms form, we made the decision to move northward towards O’Neil to get a better view of more mature, possibly right-turning storms. A cell that begins to turn to the right of the mean wind flow is usually a good indicator of imminent rotation and tornadogenesis. This is due to the increase in helicity and wind sheer from the motion of the storm in the shifting wind field. We were satisfied with our choice to go north as the storms to our south were bowing out and overall becoming more linear. This linearity shows that a storm probably only has one strong burst of wind in a unified direction, and as it bows may also produce damaging straight-line winds. A tornado was reported on our way to O’Neil, and by the time we arrived, the entire complex of storms had become substantially more linear. There were brief episodes of rotation indicated on reflectivity, but as the sun set, we decided to end our chase. The team headed to Norfolk, Nebraska, awaiting the conclusion of our trip in the coming days.

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