The tragic flash flooding in Grand Canyon National Park this past weekend was related to an energized Southwest Monsoon, producing torrential downpours throughout the Four Corners region. A normal, annual weather pattern, generally stretching from July into September, the Monsoon sweeps moisture in from the Pacific Ocean, the Sea of Cortez and the Gulf of Mexico.
This flow is driven by high pressure over the Southern Plains and by low pressure over the Desert Southwest. When these atmospheric "pumps" are especially strong, the moisture is enhanced; in addition, storm systems in the Pacific and/or the Gulf of Mexico may inject excessive moisture into the Monsoon "river," increasing the risk of flooding.
As a tropical storm comes ashore near Houston and a tropical system moves up the west coast of Mexico, they may supercharge the Monsoon, augmenting the risk of flash floods from Southern California to Colorado.
