Each year, when June turns preposterously hot, we pray for rain.

As the thunderstorm season officially arrives on Wednesday, let us consider what we’re asking for: humidity, lightning, flash floods, dangerous downdrafts and dust storms.

It’s the price we must pay.

The arrival of the monsoon and its copious moisture mark the end of the scariest part of the fire season in Arizona, and it reliably provides about half our annual rainfall.

But first, it gets hot. Very hot. The National Weather Service in Tucson issued an “extreme heat watch” for Sunday and Monday, with temperature predictions of 113 Sunday and 112 Monday.

The moisture that came our way Thursday through Saturday was not the start of the monsoon.

Parts of our region were treated to some “pre-monsoon” thunderstorms — some good rain along the border and in the mountains, and mostly sprinkles in Tucson, with a few small, fierce storm cells that brought lightning, hail, rain and blowing dust with them.

“It’s not the monsoon,” said University of Arizona atmospheric scientist Christopher Castro, who concurred with other weather watchers — that shift in weather patterns is still a couple weeks off.

It was “not the monsoon,” according to the National Weather Service, which now officially declares the season to run from June 15 to Sept. 30.

It was “not the monsoon,” according to the traditional means of measurement — three consecutive days with average dewpoint temperatures of 54 or above. Tucson reached average dewpoints of 58 and 56 degrees on Thursday and Friday, but Saturday slipped to 52 degrees.

So, the monsoon officially starts tomorrow, though we’re not yet in a monsoon pattern, and we can expect things to get a lot hotter and drier before that happens.

North American Monsoon researchers, like Castro, define the monsoon as a shift in wind patterns that occurs when the high pressure that brings us our hottest days in June moves north, classically setting up over the Four Corners region and creating a southeasterly circulation that steers moisture from the mountains of Mexico and the Gulf of Mexico.

That monsoon shift is only partly responsible for our summer rain totals. Surges of moisture from the Gulf of California, which usually occur with tropical storms late in the season, often account for September rain.

Last week’s moisture followed a similar pattern, said climatologist Mike Crimmins of the University of Arizona’s extension service.

It wasn’t the start of the monsoon, which he expects by the beginning of next month — because that’s when it always arrives.

“It’s dependable. We get crabby about the monsoon because it doesn’t rain at our house as much as we want it to. But it has never ‘not shown up,’ which is amazing,” Crimmins said.

Tucson weather records dating back to 1895 show that the summer rain has failed to total 2 inches just once, in 1924.

On average, Tucson receives 6.08 inches of rain, half its annual average.

Forecasters predict an average rain total this summer. The National Climate Prediction Center gives the Southwest an equal chance for above-normal, below-normal and normal precipitation for the summer months.

“The official forecast is a complete coin flip,” said Crimmins, “but the forecast models lean a little bit wet.”

Contact reporter Tom Beal at

tbeal@tucson.com

or 520-573-4158. Follow him on Facebook or

@bealagram

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