Hurricane Polo, at 115 mph, heads for Baja. Its moisture could bring major flash flooding to the Southwest and nearly a dozen states.

Let's call the science straight. A hurricane doesn't have to come ashore in your state to flood it. Hurricane Polo was about 125 miles west of Cabo San Lazaro, Mexico, with sustained winds of 115 mph, and was forecast to make landfall on the afternoon of Monday, Sept. 28, along the Baja California coast, weather.com reports, bringing as much as a foot of rain there along with flash flooding and mudslides. Then the deep tropical moisture Polo carries will funnel into the southwestern U.S. during the first half of the week, and The Weather Company expects major flash flooding. On Monday, the area most at risk covers parts of Arizona, New Mexico, Utah and Colorado, with the threat reaching from parts of California to Texas. On Tuesday it expands into the Plains: nearly a dozen states are at risk, with Arizona, New Mexico and Texas under the greatest risk, and tropical moisture reaching as far north as Minnesota. On Wednesday the threat shifts east into Texas, Oklahoma and Iowa, with Dallas under the greatest threat and moisture extending into Wisconsin. Regions could see as much as 3 to 5 inches of rain across the week, over a month's worth in some places. For much of the Southwest, it doesn't take a lot of rain to cause major flash flooding. Preparation checklist: know whether your home, campsite or commute runs through a low spot or a wash, and sign up for local alerts. A flash flood warning means flooding is already happening or expected to develop, so act then. A flash flood emergency means the situation is dire: avoid travel and get to higher ground. Never drive into a flooded road, because water can stall or submerge a vehicle in minutes or even seconds. As the National Weather Service says, "Turn around. Don't drown." Forecasts will shift, so check your local NWS office before you go. Desert rain doesn't come often. When it comes, it comes all at once.