A Forecasting Curveball

The early morning hours of Tuesday, July 11, 2023 serve as a prime example of how a forecast can change in an instant.

The weather models that we use to help us forecast were firing on all cylinders the previous day. Each of them were trending towards drier air moving into the Southeast U.S. late this week after a system of storms moved through Mississippi and Alabama on Sunday.

However, a large system of showers and thunderstorms rapidly sprung to life around 3:00 AM over central Oklahoma on Tuesday, providing Oklahoma City with roughly four inches of rain within five hours. The massive collection of rain was directed southeast where it would transport moisture ripe for shower and thunderstorm development in Arkansas, Mississippi, and Alabama for the rest of the week. But how did this lump of rain form so quickly?

Air in the upper atmosphere can flow quickly from the West to the East above North America. When these winds disperse or collect at a point, they can cause air to sink or rise at the surface.

The image below shows the flow of the jet stream the morning of July 11. The areas in pink highlight regions in the upper atmosphere where air is diverging as it flows east. Dispersing air aloft in the atmosphere causes air at the surface to rise, often transporting the moisture and the energy needed for showers and storms to develop in the upper atmosphere. A large area of dispersing air was recorded over south-central Oklahoma early Tuesday morning, indicating such rising motion at the surface.

Abundant moisture at the surface gradually lifted into the middle layers of the atmosphere due to the dispersing air above, increasing as the morning continued. A massive amount of energy provided by a warm night in the upper 70s traveled with the moisture as well, supplying the necessary ingredients for a state-wide cloud deck to take shape.

The weak low pressure system maintained its strength and continued to travel southeast throughout the rest of the week, preventing the forecasted drier air from taking its hold on the Southeast U.S. The system provided scattered showers and strong thunderstorms that caused flash flooding and gusty winds for some areas of the Wiregrass region. However, the drier air will likely take its hold this week as limited cloud cover assists high temperatures in rising to the upper 90s.

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