Updated

Updated · USA TODAY · Sep 28

Hybrid Storm Threatens 8 Inches of Rain Across Central U.S. as Hurricane Remnants Merge

Updated

Updated · USA TODAY · Sep 28

Hybrid Storm Threatens 8 Inches of Rain Across Central U.S. as Hurricane Remnants Merge

Q: Why are forecasters especially worried about a predecessor rain event in New Mexico and Texas before the main flood threat fully arrives?

Forecasters are especially concerned because a predecessor rain event can deliver very heavy rain 24 to 48 hours before the main storm core or broader flood shield arrives, meaning New Mexico and western Texas could start flooding well before people expect the worst conditions. In this case, tropical moisture from the remnants of Polo and Odalys is already interacting with an upper-level low moving in from the West. That setup can create a focused band of intense rain far ahead of the main system. Meteorologists say those bands can produce repeated downpours over the same areas, quickly overwhelming dry washes, roads and drainage systems. The worry is heightened by local terrain and ground conditions. Parts of New Mexico and West Texas include burn scars, canyons, low-water crossings and complex terrain that speed runoff and make flash flooding develop fast. Some areas are also already wet, so soils will absorb less additional rain. Forecasters have recent examples in mind as well: predecessor rain events have caused some of the worst flooding in past tropical-related disasters because the early rain saturates the ground before the main event arrives. If that happens again, the later rounds of rain this week could produce more dangerous and widespread flooding than the main system alone would cause. That is why watches are already up: the early rain is not a minor prelude, but a potentially life-threatening part of the event itself.

3 articles · Updated · USA TODAY · Sep 28

Flood watches through Sept. 30 cover parts of the Southwest as a newly forming hybrid storm spreads heavy to excessive rain from Arizona toward the central U.S., with flash-flood risk rising from Sept. 28.
Tropical moisture from the remnants of Hurricanes Polo and Odalys is colliding with an upper-level low moving in from the West, while Gulf moisture is being funneled north to widen the rain shield.
Up to 8 inches of rain could fall in parts of Texas, eastern Arizona, Oklahoma, southwestern Colorado, Kansas and Nebraska, with rain also expected as far east as Chicago.
NOAA warned of considerable flash flooding in parts of New Mexico and western Texas into midweek, with eastern Nebraska and western Iowa facing possible flash, urban and river flooding by Sept. 29-30.
Forecasters also flagged a predecessor rain event over the Four Corners, a setup that can trigger serious flooding well ahead of a tropical system's core, especially near burn scars and saturated ground.