Tucson experienced the wettest Monsoon in 6 years. And yet, it still didn't feel like it.
Let's go back to early June & my original forecast for the Summer storm season. Based on Colorado Rockies snowpack & the growing drought, I favored slightly below average rainfall for the Monsoon. As I always do, I took an educated stab at a final rainfall tally & came up with 5.63".
Boy was I off, at least for the Old Pueblo.
The final Monsoon rain total came in at a whopping 8.57", marking Tucson's 11th wettest Summer storm season on record. That total is a bit misleading though. 79% of Monsoon 2017 rainfall happened in the last three weeks of July.
Tucson's first half of Monsoon 2017 was the 4th wettest. Compare that to the second half, which was the 30th driest on record.
So what went right & wrong with my original forecast? Let's explore.
WHAT WENT RIGHT
About the only call I nailed was the slow start to the Monsoon. The onset of storms was roughly a week late, with the first measurable rainfall in Tucson arriving on July 10th (then kept arriving, almost daily through the rest of the month). What was the culprit? Snowpack in the Colorado Rockies.
Studies say an above average snowpack in the Rockies can slow the Monsoon high’s northward progression from Mexico. No doubt that happened.
WHAT WENT WRONG
Remember when I mentioned the ongoing drought may play a role in where high pressure sets up? Let's just say it had zero impact on the Monsoon.
July's 6.80" rain effectively brought the drought to its knees & eliminated its wide reach in Southeastern Arizona. By the end of September, Tucson was in a yearly rainfall surplus of just under an inch.
Left: Drought status on July 4th. Right: Drought status on August 29th.
July's staggering storm pace was fueled by abnormally high levels of atmospheric moisture over Southern Arizona. With high pressure in a favorable position much of the month, the pipeline was wide open for a near daily occurrence of storms. So my original forecast was busted by late July.
WHY DID THE MONSOON FIZZLE SO QUICKLY?
In short, the favorable pattern dissolved in August.
High pressure ended up parking over Reno in early August, then sliding too far east in mid August. This allowed westerly winds aloft to set up earlier than usual, cleaning out Monsoon moisture. A trough around August 13th ultimately proved to be the death blow.
By late August, the high was in a favorable position near Las Vegas. It ended up being too little, too late as dry air was being pulled in from New Mexico.
HOW DID SOUTHERN ARIZONA FARE THIS MONSOON?
While the Old Pueblo etched Monsoon 2017 as the 11th wettest on record, much of Southern Arizona saw normal or below normal rainfall.
The above map from the University of Arizona's CLIMAS department shows a lot of brown for the Grand Canyon State. This indicates areas estimated to have only received around 50% of the typical Monsoon rainfall.
For the most part, the ground truth matches up with these estimates.
Five of the nine National Weather Service reporting sites came in with below average Monsoon rainfall this year, including the usual leaders of the pack (Sierra Vista, Douglas, Nogales). Oracle hit the jackpot during Monsoon 2017, registering an astonishing 13.73" rain.
What about Metro Tucson?
As usual, it varies greatly. While the airport came in with 8.57" rain, Tucson Estates picked up over 10" of Monsoon rainfall. In nearby Drexel Heights, a Pima County rain gauge recorded only 3.81" rain between June 15th and September 30th.
The driest spots in the metro area for Monsoon 2017 included Marana & Corona de Tucson.
I'll fully admit: my original forecast was way off for Tucson International Airport. However, the call for slightly below average Monsoon rain held up for many portions of Southern Arizona. Goes to show how finicky our Summer storm season can be. One place gets drenched, another doesn't.
Here's to hoping your backyard hits the weather lottery big next Summer!