Understanding the Atmospheric Conditions Triggering Severe Weather
On September 28, significant weather risks emerged in Arizona and Southern California. The National Weather Service Storm Prediction Center highlighted a marginal risk, Level 1 out of 5, affecting Phoenix and Tucson. Additionally, a brief, isolated tornado might occur along Southern California’s immediate coastline due to rotating cells that could cause waterspouts. As waterspouts reach the shore, brief tornado damage could ensue before they dissipate.
The culprit behind this weather threat is an upper-level low, characterized by cold air, low pressure, and a spinning motion at high altitudes. This system was positioned offshore from Southern California on Tuesday, driving warm, moist air northward in advance.
Weather System’s Impact on Arizona
Accompanying this upper-level low is an abundant supply of Pacific moisture, prompting a flood watch across much of Arizona. The enriched air mass sets the scene for showers, storms, and possibly severe thunderstorms Wednesday afternoon. Such storms could unleash an inch or two of rainfall rapidly, leading to localized flash flooding on Arizona’s sandy soils. These storms may evolve into supercells, or rotating thunderstorms.
Wind shear, introduced by the upper-level low’s impact on the jet stream results in a trough formation, alters wind direction with height. This affects tall thunderstorm clouds, prompting rotation and elevating tornado risks, an unusual phenomenon for this region.