monsoon arizona
A 'dry' monsoon? They're more common than you think.
01:20 - Source: CNN
CNN  — 

Amid a worsening drought across the southwestern United States, residents are hoping for rain from the upcoming monsoon season. But there may not be much luck this year.

“A drier than normal Southwest monsoon season is slightly favored for 2021,” Scott Handel, lead meteorologist at the National Oceanic and Atmospheric Administration’s (NOAA) Climate Prediction Center (CPC), told CNN.

“Confidence in below-normal precipitation is generally higher for eastern areas of the monsoon region,” Handel said, which includes New Mexico.

The forecast is less uncertain further west into Arizona and the desert Southwest. But for meteorologists, it’s still not clear if the monsoon season will feature below average, above average or normal rainfall.

Climate Prediction Center's precipitation outlook for June through August.

There is a “large divergence in solutions within the seasonal forecast models CPC uses to make their forecast. Typically, the CPC has more confidence toward a wet or dry monsoon forecast if the majority of the seasonal forecast models are in agreement,” said Christopher Castro, professor of Hydrology and Atmospheric Sciences at the University of Arizona.

Since the weather forecast models disagree on the rainfall outcome, the CPC is unsure how this season will play out for some locations in the Southwest.

Pacific Ocean water temperatures can affect monsoon forecasts

The role of ocean temperatures in the Pacific Ocean near the equator can also play a role in this forecast. Water temperatures in this part of the world are currently near average, which is known as neutral state.

When the average water temperatures reach a certain threshold of above or below average, it is designated as an El Niño or La Niña, respectively. These water temperatures affect weather forecasts such as hurricane season outlooks.

“The Southwest monsoon does have a relationship to ENSO, such that La Niña conditions tend to weakly favor wetter and earlier monsoons,” said Castro, referring to the cycle of changing water temperatures, or El Niño-Southern Oscillation.

El Niño, on the other hand, can delay the onset of the monsoon season, bringing drier conditions to the region early in the season.

Since ENSO is in a neutral state and will remain so through at least the summer, it often becomes more difficult for meteorologists to predict a below average or above average rainy season.

Drought concerns will remain high

The Southwest is in the middle of a drought, according to the US Drought Monitor, which began monitoring US water shortages in 2000.

The most recent monitor update from this week indicates that more than half of Arizona and Utah are in an exceptional drought, the most severe category on the monitor.

Even if it does rain during the next few months, it’s not expected to make a significant dent in the drought.

“Even a good monsoon will not alleviate much the large shortfalls in reservoir storage here in the Colorado River Basin, as stream flow is mostly dependent on winter snow pack in the upper portion of the basin,” Castro said.

With below average rainfall forecast going into this summer for at least eastern portions of the Southwest, the drought will likely worsen.

This time of the year is also when it starts to get very hot, with heat alerts in effect for next week.

This combination of dry ground, hot air and windy conditions make the environment favorable for wildfires to rapidly grow once ignited.

In their monsoon outlook, the National Weather Service said the greatest potential for wildfires in Arizona will be from late June into early July.

Monsoon season explained

The monsoon phenomenon takes place every year in the Southwest from June 15 through September 30. It is essentially the rainy season for this region, as moisture often surges north from the Pacific Ocean and Gulf of Mexico, with the occasional help with tropical storms and hurricanes in the eastern Pacific.

Anywhere from 30% to 60% of rainfall each year in the Southwest comes from the North American monsoon season.

In Arizona, between 2 and 16 inches of rain is measured from the monsoon, according to the National Weather Service. The rest of the year’s rain often comes during the winter.

That was not the case this winter, however.

Arizona witnessed its driest past 12 months (May 2020 through April 2021) on record, with average rainfall across the state nearly 8 inches below normal, according to NOAA’s National Centers for Environmental Information.

And that comes after a record dry monsoon last year. High pressure was parked directly over the Southwest, preventing rain from falling and moisture from surging northward.

The North American monsoon season is typically driven by low pressure over the desert Southwest and high pressure further east, directing moisture north from the Pacific Ocean and Gulf of Mexico.

A typical monsoon pattern in the US involves low pressure centered over the desert region, which spins counter-clockwise, and high pressure to its east spinning clockwise.

These two pressure differences act as a funnel for the ocean moisture to move north and over the Southwest, leading to rain and thunderstorms.