The calendar says monsoon season starts June 15, but don't expect to look up and find black clouds and streaks of lightning that night. Most experts say there's no way to know what's coming -- or when -- until the storms arrive.
Forecasting the monsoon is notoriously difficult. The June date is more for public awareness than a definitive arrival time for monsoon storms.
After a disappointing 2024 monsoon and an abnormally dry winter that deepened drought conditions, experts say Arizona needs rain.
But when, or even if, that relief arrives is a puzzle for meteorologists that begins months before the first storm cloud builds over the mountains, and often remains unsolved until the season starts.
"We're really flying blind," said Michael Crimmins, a climatologist at the University of Arizona. "This feels like we could be two weeks out from the monsoon and still have no idea how it's going to turn out."
Every monsoon is different and forecasting can be hit or miss.
Phoenix posted its driest monsoon on record in 2023 with 0.15 of an inch of rain recorded at Sky Harbor International Airport compared to a 2.43-inch average, according to the National Weather Service. The wettest season was in 1984 with 9.56 inches of rain.
Tucson usually has a more active monsoon, with an average of 5.69 inches of rain at Tucson International Airport. The driest monsoon was in 1924 with 1.59 inches, and the wettest in 1964 with 13.84 inches.
An active season can ease drought, wildfire danger and extreme heat, but it's nearly impossible to know what kind of monsoon is coming until it's already underway.
From long-range climate models to day-to-day storm development, predicting the Southwest's monsoon remains challenging.
Why predicting the monsoon is 'really hard'
The National Oceanic and Atmospheric Administration's Climate Prediction Center is charged with the difficult task of issuing seasonal outlooks for weather across the country -- whether certain regions will have temperatures and rainfall above, below or near normal in the months to come.
It issues seasonal forecasts every month up to 13 months out. Despite the unpredictable nature of the Southwest's monsoon, meteorologists have to include it in their outlooks.
"Whether they like it or not, they have to issue a forecast for the Southwest," Crimmins said. "It's really hard, especially several months out ahead of the season. There isn't a lot to go on."
NOAA's July through September outlook shows above normal precipitation and temperatures. But these extended forecasts are just predictions of how conditions can lean and are nowhere nearly as accurate as a seven-day forecast.
Climatologists have studied the monsoon and variables that can influence a season's severity for decades, but correlations in the research are weak or inconsistent, according to Crimmins.
For example, some research shows dry winters can be followed by wet summers.
"It's a weak correlation," Crimmins said. "We're not sure of the mechanism, and it's not stable over the last hundred years. There have been decades where that worked, decades where it hasn't."
There are also weak correlations between the monsoon and El Niño-Southern Oscillation, or ENSO, which is a natural cycle that reflects the relationship between ocean temperatures and atmospheric conditions stemming from the Pacific Ocean.
El Niño, La Niña and a neutral period are ENSO cycles that can have a big effect on the weather, and these events provide some of the best insights for seasonal outlooks for the U.S.
La Niña was present over the winter, which was partly responsible for abnormally warm and dry weather in the Southwest, but conditions have transitioned back to neutral heading into the summer.
"When you don't have either El Niño or La Niña with a neutral pattern, which is what we're at right now, we don't have much to go on," Crimmins said.
There is some evidence that El Niño can delay the onset of the monsoon, while a strong La Niño can trigger an earlier start.
"What I've seen in the climate models so far is they're shrugging," Crimmins continued. "They're not leaning in either direction, so we're going to be week-by-week looking through June looking for a start."
How do monsoon storms form?
Even when the monsoon arrives, predicting day-to-day storms and overall activity throughout the season is tricky.
The season starts when a ridge of high pressure settles over the Four Corners region. The winds reverse, transitioning from westerly to southeasterly winds, pulling moisture into the Southwest from the Gulf of California, the eastern Pacific Ocean and sometimes the Gulf of Mexico.
This high-pressure subtropical ridge slowly travels north during May and June, bringing heat and humidity as it moves toward the Four Corners.
Meteorologists also watch hurricane and tropical storm activity in the eastern Pacific and warming surface temperatures in the Gulf of California, which can provide moisture for monsoon storms.
Even when the monsoon ridge is positioned over the Four Corners, it doesn't guarantee storms, and forecasting is still complex.
"The monsoon doesn't mean we're going to get rain every day," said Erinanne Saffell, Arizona's state climatologist. "We can look a few days ahead, but it's a better understanding looking at the morning forecast and getting a sense of whether we have warm air rising, is there enough moisture in the atmosphere to create clouds and will the clouds get big enough to drop moisture?"
Thunderstorms are created from warm air rising, or convection, while there is moisture in the atmosphere. Meteorologists will look at dew point temperatures to gauge how much water is in the air.
If there is enough water in the atmosphere, clouds will begin to form. Moisture, warm air rising and instability in the atmosphere are the three main ingredients of a monsoon storm.
But even if those conditions are present, storms may not leave the mountains.
For monsoon storms to reach places like Phoenix or Tucson, weather systems in the mountains must push out air, forming new thunderstorms. Some storms die out before they hit the metro areas.
What do experts hope for this monsoon?
Forecasters can't promise a stormy monsoon, but they're certainly rooting for it.
"I'm hoping for an on-time start to cut the fire season down," Crimmins said. "And boy, if we could just pull off average for July, August and September, I'd be thrilled. If we got above-average, we could make some progress in reversing the short-term drought."
Monsoon showers are key to helping cool the state during the summer, interrupting streaks of 110-degree-plus days.
Phoenix saw 70 days at or above 110 degrees, with a 113 consecutive day streak at or over 100 degrees in 2024, a new record. A sporadic monsoon without consistent, widespread rainfall kept Phoenix abnormally hot all summer.
Arizona is also in the middle of its peak wildfire season, which usually starts in April and spans through June until monsoon moisture ends fire risk. Following months of severe short-term drought starting last summer, the vegetation is parched and ready to burn.
While uncommon spring rains have helped delay the wildfire season, they were not enough to end the short-term drought and remove fire risk. Fire managers and meteorologists wait for the monsoon to arrive to end the season.
An active monsoon won't fix the drought either, but it can help until Arizona gets another wet winter.
"It would be very beneficial to put water into the soil, and that's helpful for our winter precipitation," Saffell said. "We need a wet winter for our water supply, but that wet summer helps us too."
Hayleigh Evans writes about extreme weather and related topics for The Arizona Republic and azcentral.com. Email her with story tips at hayleigh.evans@arizonarepublic.com.