When thunderstorm outflows collide...

An intense band of thunderstorms and heavy rain impacted northern Louisiana (7/6/2023) during the evening hours after colliding outflow boundaries converged on the area from the north and south. Radar and the surge in satellite lightning detects along the enhanced convective band help show how outflows can be a rapid and significant trigger for new storms! How does this happen?

Thunderstorm precipitation downdrafts lead to cooler and denser outflow air near the surface that spread out in all directions. The edges of growing "cold pool" air masses under thunderstorms behave similar to a passing cold frontal boundary by lifting warm/moist air upwards along the way. Sometimes this is all that's required to initiate sufficient updrafts for a new round of thunderstorm development as the outflow boundary passes by or collides with other outflows, even if only briefly but intensely as was the case in the Louisiana example showcased in this post.

Not all outflows will trigger more storms if the environment is not ideal. A couple examples include outflow boundaries moving into a drier air mass or into a region with a strong capping inversion aloft that prevents deep cloud growth.

During the North American Monsoon under these non-ideal convective environments, it is quite common for mountain thunderstorm outflows when dropping into the lower valleys to fail in developing new cumulonimbus cloud formation. An ongoing monsoon forecasting challenge is determining if mountain favored thunderstorms can "survive" the journey into the lower elevations, especially to impact large population centers like Phoenix and Tucson. Abundant moisture and stronger midlevel thunderstorm steering winds (e.g., around the 500mb pressure level) are key parameters checked on sounding data that assist a mountain to valley thunderstorm transfer.

Should daytime mountain convection successfully transition to the lower elevations, thunderstorm activity may carry on well into the overnight hours as outflow boundaries crisscross back and forth across the landscape colliding with each other. You can see how thunderstorms may propagate long distances much like a chain-link fence in this manner acting as a positive feedback cycle (thunderstorm downdraft, spreading surface cold pool, a new source of local lift, additional thunderstorms, repeat).

Data Source: College of DuPage