Sit down. This is gonna take some explaining.
The monsoon season started yesterday.
And it’ll likely rain on you today.
But that ain’t no monsoon storm.
You following this? Let us explain.
Arizona and New Mexico have long benefited from a unique pattern of summer storms brewed in the Gulf of Mexico and the Gulf of California that deliver about 40 percent of the state’s rainfall in July and August.
Once upon a time, the National Weather Service declared the start of the monsoon season any time the temperature at which water would condense out of of the air in Phoenix fell below 55 degrees for three days running.
But that required explaining the term “dew point” to confused reporters every summer — and putting up with all those anxious questions from the public about when the season had actually started.
So the National Weather Service offices in Phoenix and Tucson decided to just set the start of the monsoon at June 15 — which is to say, yesterday.
Fair enough. Drum roll: Let the monsoons begin.
Sure enough, the National Weather Service predicts rain today and/or tomorrow — with clearing on Thursday and Friday — and renewed showers and thunderstorms this weekend — followed by a warming trend next week that should boost daytime highs by 10 degrees.
Only it ain’t no monsoon, according to National Weather Service meteorologist David Blanchard.
The storm expected to drizzle through Rim Country this week wandered in off the Pacific Ocean, said Blanchard.
Monsoons, by contrast, boil up from the warm waters on both sides of Mexico. Blistering desert temperatures in the lower elevations of Arizona cause hot air to rise rapidly, which creates a low pressure system that sucks in the wet air from twin gulfs.
As that wet, cool air collides with the rising columns of hot desert air, it creates a whole chain of thunderstorms. Those storms sweep over Tucson and Phoenix then break against the ramparts of the Mogollon Rim at more than 7,000 feet. As a result, the cooling, condensing clouds dump most of their remaining rain on the highlands of Central Arizona. That normally produces afternoon thunderstorms, about 8 of our 22 inches of average rainfall and more lightning strikes along the Rim than almost anywhere else in the country.
Blanchard noted that the precursors to the monsoon season have already started building over Mexico.
However, he doesn’t expect the monsoons to arrive here for several weeks, when the rising desert air shifts the prevailing wind directions.
“Everything I see says things look like normal,” said Blanchard, “which means anywhere from the tail end of June to the middle of July for a start date.
But Mr. Weatherman — will it be a wet monsoon season or a dry one?
“Don’t know that. There’s a lot of skill in predicting that yet,” said Blanchard. “Forecasting monsoon seasons is still in its infancy.”
Other media outlets talking to different soothsayers have predicted a wetter than normal monsoon, due to ocean temperature anomalies in the Pacific.
“I haven’t seen those reports,” said Blanchard dryly.
Admittedly, it’s been kind of a freaky year — with near record winter rain and snow, a bizarrely dry spring and then a sodden May.
“It was quite unusual,” said Blanchard of the roller coaster rainfall. “But it was usual all across the country — so it was part of some much larger scale process.”
Oh my word. Global warming?
“If it’s part of some larger process, it would take a lot of investigation to determine what the links are,” said Blanchard cautiously.
On the other hand — he’s pretty sure you’re gonna get rained on this week, break out the cooling fans and maybe click on the air conditioner by week’s end and get your picnic spattered upon this weekend. So far the highs in June have been 10 degrees below normal — but may be slightly above normal next week.
“Then we’ll be back into the waiting mode,” he said.
Waiting on the monsoons.
So if you happen to be leaning over the porch railing watching the rain pitter-pat on the pines this week and some wag says he just loves it when the monsoons start — you can smugly correct him and say it’s just another spring storm — and not a monsoon at all.
Go ahead. People just love that.