Tropical Storm Rosa hit Mexico and brought flooding to ArizonaSatellite images show then-Hurricane Rosa churning toward Mexico's Baja California and scientists say climate change is a big reason storms are taking unusual paths. Photo: Wikimedia

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By Ucilia Wang

As Tropical Storm Rosa rolls through Arizona, dousing desert towns with heavy rains and filling reservoirs nearly depleted by a lengthy drought, the storm’s arrival is also a message from the sky about the impact of climate change: it can produce weird weather that in ancient times might have been attributed to angry gods.

Now, however, the unusual sight of flooded streets in Arizona can be explained by science.

“There’s a climate change component to it and we can say that with confidence,” said Kevin Trenberth, senior scientist at the National Center for Atmospheric Research.

Global warming has raised ocean temperatures and warmed the air above it, supercharging storms with moisture that they unleash in bigger rainfalls. Researchers now have enough data to connect the dots between increasing greenhouse gas emissions, greater moisture in the air and outsized rainstorms, Trenberth said.

The air above the ocean can hold 4 percent more moisture for every degree Fahrenheit and scientists have found about 5-20 percent more moisture gathered in the air since the 1970s, Trenberth said. The result played out last month when Hurricane Florence doused parts of the Carolinas with 30 inches of rain.

“The main effects are the storms are more intense and last longer, and the rain is heavier. In Hurricane Florence, we have all three effects in play,” he said.

Climate change isn’t just leading to more damaging storms. It’s also affecting the path they travel.

Researchers believe rising temperatures in the ocean could also weaken a high-pressure system called anticyclone, which has historically caused storms to lose their strength as they hurtle toward California, Trenberth said.

He noted that a weakened anticyclone near Hawaii caused the island state to receive three big storms this year, including one in Kauai that led to 49 inches of rainfall during a 24-hour period.

“Hurricanes usually lose strength once they hit California. But the risk to the southern part of the state may increase a little bit because water is warmer, so hurricanes won’t lose strength as quickly,” Trenberth said.

One of the issues facing people in the path of these wetter hurricanes is that traditional home insurance does not cover damage from flooding, leaving people facing huge financial losses. Insurance experts have estimated that Hurricane Florence will produce insured losses of up to $5 billion. But according to risk modeling firm RMS, 70 percent of losses will be uninsured because much of the flooding happened inland, where people do not have flood coverage.

Florence sent tens of thousands of people into shelters across the Carolinas, causing major power outages and leading to the dozens of deaths, including 37 in North Carolina.

A computer model of Florence, based on data taken before it made landfall, showed that climate change was likely to boost its rainfall by 50 percent.

Rosa began as a Category 4 hurricane but lost its power before it made landfall in Baja California, Mexico as a tropical storm on Monday night.

The rating system refers to wind speed and not rainfall, however,  prompting debates over whether it can cause people to underestimate the true impact of a storm. Florence was downgraded from Category 4 to Category 1 before landfall, but the water it brought inflicted far more pain than the wind.

While Rosa wowed desert dwellers with rain, meteorologists pointed to another unusual phenomenon Tuesday: two Category 5 hurricanes simultaneously crossing the Pacific.

“Simultaneous Cat 5s are very rare, and this is the first time in the historical record that Cat 5s have existed simultaneously in the Northwest Pacific and Northeast Pacific,” wrote Jeff Masters, co-founder of Weather Underground.          

The rain unleashed by Rosa was creating rarely seen sights of flooded streets in Phoenix and other cities in central Arizona. The weather service said Phoenix, with 2.24 inches of rain on Tuesday, experienced the second wettest October day and the “ninth wettest day ever (so far).”

Flat and dry landscapes don’t need much rain to create floods, Trenberth said, because the super dry soil of the Southwest doesn’t soak up rain well.

The upside is that the rain will bring some relief to water managers who have watched  reservoirs along the Colorado River system fall to worrisome levels. The Colorado River delivers water to farms and homes from Wyoming to California.

A recent report by the Colorado River Research Group, which is made up of 10 scientists, painted a bleak picture of the low water levels at two of the big reservoirs, Lake Mead in Nevada and Lake Powell, which straddles Arizona and Utah.

But the group warned in a separate report that even if climate change brings more rain, it won’t offset the prolonged drier conditions.