Arizona gang – WEDNESDAY, OCTOBER 21 will feature a potentially substantial severe weather event regionally. Scattered rotating supercell thunderstorms are expected, and a few tornadoes are probable across the Copper State. Several hailstorms are also likely, and the strongest supercells could produce damaging hail up to hen egg size – or larger. Please let your friends know that Wednesday is a day to stay weather aware – and have a plan should warnings be issued for your location. "A climatologically rare and a concerning/well-above-typical risk for severe storms is expected, potentially including all severe hazards with large hail, damaging winds and tornadoes all possible," wrote the Storm Prediction Center. The impetus is a strong upper-level low offshore of California. That's kinking the jet stream into a potent dip known as a trough. Ahead of that disturbance, warm, humid air is being scooped north, introducing instability, or thunderstorm fuel. And our disturbance is lifting the air ahead of it, which will generate widespread showers/thunderstorms to feed off that storm fuel. Meanwhile, the approaching jet stream dip is inducing wind shear, or changing winds with height. Surface winds will be south-southeasterly, but at 10,000 feet we have more southerly/south-southwesterly winds and then STRONG southwesterlies aloft. Any thunderstorms that grow tall will feel these changing winds with height and rotate, hence the tornado risk. Across the deserts between Phoenix and Tucson, it'll be a bit drier, meaning cloud bases will be higher – reducing the tornado threat. But for Prescott, Sedona, Kingman, Flagstaff, the Grand Canyon area and even into southern Utah, it'll be a little cooler and more relatively humid, leading to lower cloud bases. That will make it easier for rotation to snake to the surface, perhaps resulting in a couple tornadoes. We're also noticing the upper-air disturbance is taking on a "negative tilt," meaning it's shaped like a backslash, or \. Think of a soccer player kicking a ball. The kick is most powerful when the soccer player "follows through." This system's shape gives it more "kick", meaning we really have to watch it carefully. Finally, much of Arizona is under a flood watch. A broad 1-2 inches is likely, and a couple 3-4 inch rainfall totals are possible where storms pass over the same areas repeatedly. That could lead to flooding in spots. Please have multiple ways to be notified of warnings.