These forces drove the deadly Grand Canyon flash flooding
- Arizona's deadly flash flooding this weekend was fueled by a combination of the monsoon season, record-breaking El Nino, a marine heat wave in the Pacific Ocean, and climate change.
This weekend's deadly flash flooding wasn't an isolated weather event.
The torrential rain was part of Arizona's monsoon season, which runs from June 15 through Sept. 30.
But the monsoon alone doesn't tell the whole story. A record-breaking El Niño and a marine heat wave in the Pacific Ocean have combined to load the atmosphere with moisture, helping fuel the deadly flooding.
These factors, driven in part by climate change , have driven atmospheric moisture levels 5 to 15 percent higher than average. That has loaded the dice for downpours this summer, such as those that hit the Grand Canyon on Saturday.
And although the level of moisture flowing through the sky doesn't have a one-to-one relationship with rainfall amounts, having more moisture can boost the odds for rainfall extremes, if there's a force to squeeze that moisture out, such as the cluster of thunderstorms that hit the Grand Canyon on Saturday afternoon.
This weekend, there was an additional force at play. Moisture from the remnants of Tropical Storm Iselle was flowing across the West, siphoned inland by a heat dome and further waterlogging the atmosphere.
Near the epicenter of this weekend's flash floods in the Grand Canyon, 1.24 inches of rain fell in a three-hour period on Saturday, including 0.84 inches in an hour.
High intensity rainfall on steep, rocky and drought-affected terrain where a wildfire from last year had left a burn scar meant that the water wasn't absorbed into the ground and mostly ran off into the canyons and creeks below.
Most Read From The Washington Post
From Arizona to Utah and western Colorado, flood watches remained in effect as the threat for heavy rain will continue Monday.
"Flash flooding will be possible in creeks, normally dry washes, swimming holes, and over recently burned areas. Raging killer currents will be possible in side canyons and narrow slot canyons. Low-water crossings may be flooded," wrote the National Weather Service in Flagstaff, Arizona.
And now, there's another threat to watch in the West.
A few weather models show the remnants of a tropical storm or hurricane approaching California in about a week, with the potential for heavy rain, wind and dangerous sea conditions.
However, more models show the storm pushing out into the open waters of the Pacific, sparing California of a direct impact. Even in this scenario, distant moisture from the storm could contribute to increased downpours across the Intermountain West.
Although the forecast remains uncertain, even the possibility of such an unusual storm underscores how this pattern of Pacific warmth can set the stage for weather extremes.
