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‘Mixed blessing’: Fire-plagued New Mexico faces excessive monsoon rain

The rain should help the fire situation but could lead to dangerous flash flooding and landslides

A map of where moisture is riding north over the Desert Southwest. (WeatherBell) (WeatherBell)
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First it was fires that plagued New Mexico. Now it is potential floods as the summer monsoon season starts with a bang.

A plume of deep tropical moisture wafting over the Southwest, driven by the monsoon, could unleash “excessive rainfall” through Tuesday night, according to the National Weather Service.

While the rainfall is welcome news in a landscape parched by widespread drought and charred by historically large fires, it may be too much of a good thing.

“Excessive runoff may result in flooding of rivers, creeks, streams, and other low-lying and flood-prone locations,” wrote the National Weather Service in Albuquerque, which issued a flash flood watch for much of central and western New Mexico.

The flash flood watch zones include the areas where New Mexico’s two largest blazes on record still rage — the Calf Canyon/Hermits Peak and Black fires.

The rain is “a mixed blessing,” said Andrew Mangham, the senior service hydrologist at the Weather Service’s Albuquerque office. He said the downpours will help the fire situation and that there have already been “dramatic improvements just from the rise in humidity.”

The concern, Mangham said, is “if this rain falls too hard too fast,” particularly over burn scars.

The scars “leave behind very hydrophobic soil conditions,” Mangham said. “Instead of water infiltrating into the ground, it tends to run off more quickly. You tend to get a faster flash flood response, causing debris flows and landslides.”

The possible fire-drenching rain comes on the day the U.S. Forest Service released a report acknowledging its role in setting off the Hermits Peak Fire with a prescribed burn on April 6. The report said the agency miscalculated the risk posed by abnormally dry conditions when the prescribed burn “escaped.” “Unfortunately, the effects of climate change are narrowing the windows where this tool can be used safely,” Forest Service Chief Randy Moore wrote in a forward to the report.

Inside the monsoon

Floods are a frequent hazard in the Southwest during monsoon season, which runs from June 15 to Sept. 30.

The term “monsoon” doesn’t describe a flood, deluge or drenching, as is a common misconception. Instead, a monsoon is simply a seasonal wind shift that occurs with relative predictability. In the Desert Southwest, prevailing winds are usually out of the west or southwest — hence the arid desert landscape.

How the Weather Service cleared the air about Southwest monsoon season

But during the summertime, the flow switches to be out of the south, introducing moisture from the Pacific Ocean, Gulf of California and Gulf of Mexico, and storms bubble up during the heat of the day. They are often pursued by photographers because of their beauty — high cloud bases that crackle with errant bolts of electricity juxtaposed against a sandy desert backdrop — but the sudden torrential downpours can be problematic.

That’s the case this week as the monsoon kicks in a bit early. The Weather Service’s Weather Prediction Center has highlighted most of New Mexico within a level 2 out of 4 risk zone for excessive rainfall and flash flooding.

“Stream flows are much above normal to high in places across much of New Mexico, southeast Arizona and far Southwest Texas from recent rainfall,” the agency wrote. “The potential for additional heavy rainfall across these high stream flow regions and over recent burn scars will continue the threat of runoff issues this period.”

The cool conditions and elevated threats of torrential rainfall and flooding contrast sharply with the effects of the “heat dome” sprawled across most of the eastern two-thirds of the Lower 48, but the opposing phenomena are related. Moisture entrained northwest and wrapping northward on the backside of the clockwise-spinning high is being drawn over New Mexico. That, coupled with an unstable atmosphere — one that fosters rising pockets of air — will gel into sporadic thunderstorms.

Excessive heat swelling from Midwest toward the South

Light rain began to arrive in waves Saturday, with renewed intermittent showers returning Tuesday morning. Rain will increase in coverage and intensity throughout the remainder of the day, with activity peaking during the evening hours. Models suggest the likelihood of at least an inch of rain averaged across the region from extreme southeastern Arizona into central and northern New Mexico. That encompasses much of the Interstate 25 corridor, including Taos, Santa Fe and Albuquerque.

Heavy rain will linger into Wednesday morning before a break ensues much of Thursday. Storms on Wednesday might also slip into eastern Arizona as the moisture plume shifts slightly west. A few downpours return Friday.

An inch or two of rain probably doesn’t sound like a lot, but in New Mexico or Arizona, that could translate to months’ worth of water. In fact, Albuquerque averages only about 8.6 inches of rain annually, roughly half of which comes down in about three months’ time. Any place that sees fewer than 10 inches a year meets the definition of a desert.

The sandy soils are poor absorbers of excess rainfall, so excess runoff can quickly turn dry riverbeds into roaring rapids. Empty arroyos will quickly fill, and low water crossings can become perilous.

Any rainfall will certainly help efforts to put out the 341,471-acre Calf Canyon and Hermits Peak Fire — which is still only 72 percent contained. More than 2,500 personnel are actively involved in combating the blaze, which is raging in the Santa Fe National Forest east of the city.

Several other wildfires, including the enormous Black Fire, continue to burn. So far it’s charred 325,115 acres, and is about two-thirds contained.

Scant rainfall, gusty winds and anomalously warm temperatures during the months of April and May — following a winter with little snow — proved pernicious, helping the fires to fester and grow uncontrollably. As it stands, a third of New Mexico is in the midst of a top-tier “exceptional” drought, and another 40 percent of the state is experiencing a “severe” drought. While any rainfall won’t erase the years-long shortage, anything that falls will make a dent in the deficit.

While the fire risk wanes in New Mexico, more blazes are possible in central California, particularly in the San Joaquin Valley, as isolated “dry thunderstorms” — or thunderstorms from which rainfall evaporates before hitting the ground — unleash lightning strikes with ignition potential.

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