Forecasting severe weather along the Mogollon Rim Convergence Zone

With the nation-wide deployment of the WSR-88D Doppler radar, it has become apparent that there are many regions of the country with persistent, terrain-enhanced surface wind flows. Some of these flows have been shown to be associated with higher frequencies of severe thunderstorms producing both large hail and non-supercell tornadoes (NST).

The Flagstaff, Arizona, Doppler radar has been instrumental in documenting the presence of a persistent convergence zone located in the lee of the Mogollon Rim. The Rim is a crescent-shaped feature that arcs from the north-central part of Arizona southward through the Flagstaff area, thence eastward to the New Mexico border. Leeward of the higher terrain of the Rim is the Little Colorado River valley. This valley drains southeast to northwest from the higher mountain terrain along the Arizona-New Mexico border to the Colorado River in northern Arizona. It often produces southeast surface drainage winds overnight that persist into the morning and early afternoon. Days with light to moderate southwest flow are often associated with the genesis of the Mogollon Rim Convergence Zone (MRCZ). The MRCZ typically forms on the east side of the Rim as the terrain drops into the river valley and the synoptic-scale southwest winds interact with the southeast drainage winds of the valley.

Observations have shown that this convergence zone is a preferred region for the genesis of thunderstorms during the late spring and early summer months. As has been shown with other terrain-induced convergence zones, there is often a zone of enhanced vertical vorticity that may be tapped by the rapidly growing updrafts of thunderstorms. This vorticity zone may then quickly collapse from a scale of a few kilometers to one of a few hundreds of meters and can be responsible for the rapid genesis of non-supercell tornadoes.

There have been a few recently documented NSTs along the MRCZ and there are probably a larger number of events that go unreported or even undetected. Nonetheless, the capabilities of the WSR-88D have been instumental in documenting this feature and alerting forecasters to the possibilities of NSTs and other severe weather occuring in this region.