Monsoon 2019: When will the heat, rain begin?

The pleasant spring weather we’ve enjoyed in the Phoenix area and throughout Arizona could push back the start of the 2019 Arizona monsoon.

Monsoon season, as declared by the National Weather Service since 2008, begins June 15 and runs through Sept. 30. Monsoon conditions and storms don’t follow a set schedule, however.

The National Weather Service in Tucson released its 2019 monsoon outlook video on May 30. In short, condi

tions lean toward a later-than-normal start to seasonal storms and below-normal rain from June through August. There’s a better chance for rain to pick up in August and September.

And say goodbye to the lower temperatures we’ve benefited from this spring. The Climate Prediction Center outlook calls for above-normal heat during the 2019 monsoon.

“That’s how things appear to look at this point, in terms of the trends,” said Glenn Lader, the National Weather Service meteorologist who produced the video.

Several factors figure into the monsoon outlook. El Niño conditions in the equatorial eastern Pacific Ocean (signaled by above-normal temperatures in those waters) often mean a drier-than-average summer in Arizona. The correlation between El Niño and the monsoon is kind of weak, but other indicators also point toward a slow start, Lader said.

The low-pressure areas that combined with El Niño to keep temperatures below normal for most of May can also delay the shift in winds needed to spark monsoon storms.

“More (low pressure) in the West will keep the flow a bit more westerly, as opposed to southerly or southeasterly,” Lader said. “That may keep things drier a little longer and keep the deep moisture further south of our area.”

That low-pressure area is starting to weaken, though. As a result, the weather will start to heat up, which should start the shift toward monsoon conditions.

But another remnant of our pleasant spring — not to mention the wet winter — can also play a role in delaying the beginning of the monsoon.

The subtropical high-pressure area that is a catalyst for the wind shift that causes our monsoon conditions needs heat to move into position.

That high-pressure area starts in Mexico. When it moves up toward the Four Corners region, winds in our area shift from a westerly direction to coming out of the south or southeast. Those winds carry moisture from the Gulf of California, which can help fuel summer storms.

Hot, dry conditions in the desert usually cause the shift that allows that area of high pressure to travel north. This usually starts to happen in late June and early July, but that may not be the case this year.

State climatologist Nancy Selover explained that the wet winter and relatively cool spring may slow down that process.

“In years with a really heavy snow pack and a lot of moisture in the soil from winter activity, it sometimes suppresses the development of the high pressure we need to spin around and bring moisture up for the monsoon,” Selover said.

Selover said that doesn’t mean it won’t happen eventually, it just might take a little longer. Dry soil heats up more quickly than moist soil.

The El Niño conditions that are helping delay the onset of the monsoon might contribute to our rainfall totals late in the season.

Sometimes the warmer Pacific waters associated with El Niño can strengthen tropical storms in that part of the world. Those storms, particularly in late August and September, can boost the moisture that our monsoon storms feed on. Remnants of those storms can even make their way into the state, in some cases.

Heavy rains in Arizona last October were the result of the remnants of hurricanes Rosa and Sergio reaching Arizona.

Computer models aren’t predicting a repeat of last year’s heavy rains, particularly this far in advance, but they are leaning toward above-normal rainfall levels in August and September.

Speaking of computer models, the Climate Prediction Center’s summer outlook calls for an end to the cooler weather we’ve enjoyed this spring.

The outlook for June through August shows a 40 to 50 percent probability of above-normal temperatures for Arizona. The rainfall outlook features normal to below-normal chances for precipitation for June through August, but the forecasts improves later in August into September.

We’ll see how this year’s monsoon plays out. The 2018 season got off to a slow start, with just a trace of rain in June before things picked up in July and slowed down again in August. Last year’s monsoon season wound up just 0.08 inches below normal with 2.63 inches of rain from June 15 to Sept. 30, according to National Weather Service numbers.