Weekly Weather Event -Week of June 1stJune 5, 2020
June 3rd brought two different thunderstorm systems over Missouri, a normal occurrence for the state given the time of year. The westerly system moved towards the east, something that is typical of storm systems within the United States. The easterly system, however, had a component of the storm that developed more towards the west. This atypical expansion was likely due to the storm developing along a boundary, as boundary regions tend to be more unstable. Ultimately, this resulted in the two storm systems approaching each other, and around 2:30 am on June 4th, merging into one.
This atypical merging of storms ended up producing two confirmed tornadoes. Since the two systems were moving in different directions, wind speeds and direction varied enough with height to produce enough wind shear needed to sustain rotation. The merging systems also produced cloud top gravity waves, as shown from this Suomi NPP VIIRS Day/Night Band image.
While these merging storm events are unusual in the continental United States, they are relatively common in the lee of mountain ranges. In the South American Andes, merging storms are common due to the presence of a type of wave that flows opposite to the prevailing atmospheric flow. This wave known as the undular bore. As seen in Missouri, where there ended up being around six inches of rain, these merging storms can produce heavy rain events.