When does the Arizona monsoon season start, and how much rain will the Phoenix area see?

Yahoo

02-06-2025

Arizona's monsoon season starts in June and goes through the end of September, but that doesn't guarantee storms will pop up immediately.
Most experts agree that there's no way to know exactly when black clouds, streaks of lightning and walls of dust will appear. Forecasting the monsoon is notoriously difficult. The June date is more for public awareness than a definitive arrival time for monsoon storms.
'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.
After a disappointing 2024 monsoon and an abnormally dry winter that deepened drought conditions, experts say Arizona needs rain.
Here's what to expect for hopefully a more fruitful 2025 monsoon.
The monsoon officially starts June 15 and goes until Sept. 30.
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 Niña can trigger an earlier start.
'What I've seen in the climate models so far is they're shrugging,' Crimmins said. 'They're not leaning in either direction, so we're going to be week-by-week looking through June looking for a start.'
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 State's 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.
In terms of actual rainfall, Arizona could see roughly two inches in lower deserts, including the Phoenix area, and up to 16 inches in mountain ranges. Metro Phoenix typically experiences about 10 days with measurable rainfall during the monsoon, while the higher mountainous areas see roughly 30 to 50 days of rainfall.
The weather service projected Phoenix with a 43% chance of above normal precipitation during the monsoon, a 33% chance of near normal precipitation and a 24% chance of having below normal precipitation. For context, Phoenix had an average of 2.43 inches of rainfall during monsoon seasons from 1991 through 2020, according to the weather service.
Other cities that leaned toward having above normal rain included Tucson, Flagstaff, Prescott, Kingman and Show Low, among others.
The weather service recommended people check their local forecasts before going out and ensure they're signed up to receive weather warnings.
This article originally appeared on Arizona Republic: Arizona monsoon season starts in June and goes through September