Drought deepens in the Southwest, driven by little rain and extreme heat

Drought is deepening in the Southwestern United States, because of a lack of rain and persistent heat over the past several months.

The parched conditions are most pronounced in Arizona, Nevada and southeastern California, where severe to extreme drought expanded in recent weeks.

As of Wednesday, Las Vegas has not had measurable rain in 158 days - the second-longest such streak for the city on record, according to the Brian Planz, a meteorologist with the National Weather Service in Las Vegas. The longest rain-free run was 240 days in 2020. “Measurable” rain is defined as .01 inches or greater.

“The last six months or so have been really unusually dry,” said Curtis Riganti, a climatologist with the National Drought Mitigation Center in Lincoln, Nebraska. That has combined with high evaporation from heat waves, especially in southern Nevada, Arizona and central and southeastern California, he said.

The rainfall deficits started to mount in many areas when the monsoon fizzled out this summer and have continued into December.

“We saw an early start to monsoon precipitation in late June, but thunderstorm activity really slowed down by late July,” Michael Crimmins, a climate scientist at the University of Arizona, said in an email. “By the end of September, most of the Southwest was behind in summer precipitation and short-term drought conditions crept back in.”

Extreme heat this summer and fall has accelerated the descent into drought, as the atmosphere draws more moisture from soil and plants.

Both Phoenix and Las Vegas saw their hottest summers on record, as did other parts of the Southwest.

An extraordinary heat wave in October brought record temperatures to Phoenix for 21 straight days.

“The heat definitely had an exacerbating effect - driving up evapotranspiration levels and quickly drying out any soil moisture that was gained with summer rain,” he said.

Although the latest drought conditions are considered short term, they are happening on top of the broader Southwest megadrought that has seen precipitation deficits over more than two decades, combined with warming temperatures.

Early-season mountain snowpack has been mixed, with much of the story this winter still to be written. It’s far below normal in much of the Southwest, including in Arizona and New Mexico. The picture is a bit brighter in the Upper Colorado River Basin - a region that is key to water supply for two major freshwater reservoirs, Lake Powell and Lake Mead - and where the snowpack is near to above normal.

A weak and short-lived La Niña is expected to develop over the next couple of months, and by some measures has already arrived. The climate pattern, which features cooler than normal water in the eastern tropical Pacific, can portend a dry winter for the Southwest.

“The weak La Niña outlook may give us some wiggle room for storms to sneak at some point in the upcoming season, but the climate models suggest otherwise and are bullish on continuing dry conditions,” Crimmins said.

So far this season, most storms are targeting the Pacific Northwest and Northern California, but there are growing signs that the storm track could dip south before the end of December.

If the winter ends up dry, however, that could quickly spell trouble for wildfire risk and water supply next year. The latest outlooks show drought expanding in the Southwest in the coming months.

“Couple that with the previous dry summer and ongoing above average temperatures and we could be in a really tough spot by the spring,” Crimmins said.