This is the conclusion of the case study.

Conclusion

This storm had its beginnings in the state of Colorado as a surface low pressure that developed on the lee side of the Rocky Mountains. It formed on the tail end of a cold front that impacted the Central Great Plains on the 16th. The main player for the storm was the jet stream and the associated jet streaks that went along with it. With the jet stream, there is upward motion when there are areas of the faster winds associated with jet streaks. This was one of the key roles in getting the convection not only going but also the deepening of the low pressure.

As the low pressure underwent cyclogenesis, the jet stream was oriented in a dip from the pacific northwest, where there was a strong jet max over the pacific northwest. This would be the jet streak that would later move into the Midwest. A large piece of the subtropical jet stream was in position over the southern Great plains, with a jet max of 140 knots over Texas. At that time, the low pressure that would impact the mid-Mississippi River valley was in the left entrance region of the first jet streak. This would temporarily cause the surface low to stay at the same strength.

However, the large lobe of the subtropical jet stream would go out over the Atlantic, allowing the jet streak that was over the northwest at 0Z on the 17th to move to the southeast, and set up shop over the Midwest at 12Z. At that time, the surface low had moved below the left exit region of the jet streak. This began to rapidly strengthen the storm as it moved to the northeast. This is due to the divergence of winds in the left exit region of a jet streak. At the same time, a stronger jet streak of 140kts was seen over the Rocky Mountains of Wyoming.

Out in front of jet streak in the central part the US was an area of ageostrophic motion due to the exit regions of a jet streak. This is the area where we’d expect increased upward motion that the surface low pressure could tap into to further strength itself. Therefore, the low pressure rapidly strengthened when it moved into the northern and northwestern portion of the state of Illinois. The strong winds that were assocated with the jet streak were often pulled down from the upper portions of the atmosphere. This caused the damaging wind gusts that were common with the severe convection throughout the day.

As the low pressure moved to the northeast, a second low pressure formed on the front. The jet streak’s left exit region moved to the northeast, with the lows in the general area of the divergence that occurs with the left exit region. By this time, the overall jet structure had begun to shift eastward as the upper level low at 250 hPa shifted eastward. The dip in the jet stream had shifted over the state of Illinois, with a jet max sitting in the bottom of the dip. This helped to propel the system eastward.

As this was happening the low pressures were starting to merge into one, with the frontal system becoming occluded at the same time. The low pressure lost the influence of the jet streak when the occluded front becoming more and more wrapped up into the low pressure. With this happening, the low pressure was beginning to dissipate. At 12Z on the 18th, the jet stream’s orientation featured a dip in the center of the US, with a jet streak sitting over the central part of the Illinois and Indiana.