Of the 11.59″ rain that Tucson typically receives during the year, over half of it falls during the Monsoon.
As the Super Bowl of Southern Arizona weather approaches, there are a number of influences that may dictate how much rain Tucson receives. My 8th annual Monsoon forecast highlights what to expect this year.
WHAT THE CPC SAYS
For the July-August-September timeframe, the Climate Prediction Center calls for a 50% chance of above normal temperatures across Arizona. This probably shocks very few people, yours truly included. I’ll always bet the over on temperatures during the Summer months.
Regarding rainfall, the CPC’s forecast is as clear as mud. According to their calculations, there’s an equal chance of below, normal or above average rainfall. Not real helpful, right?
There’s four factors that I’m keeping an eye on this Summer. The first? Waters off the Western Mexico coast.
WHAT THE EASTERN PACIFIC OCEAN SAYS
El Niño or not? Models have been flirting with the idea of a developing weak El Niño this Summer. As of June 8th, there was a 41% chance of a weak El Niño, but a 55% chance sea surface temperatures (SST’s) stay normal.
What does history say about weak El Niño events? 10 of the 18 Summer El Niño events have been of the weak variety. Tucson’s average Monsoon rainfall during such events? 4.41″. Well off the average total of 6.08″ and under the normal range of 5.52-6.64″.
While a weak El Niño is a possibility during this Monsoon, the forecast is trending toward SST’s staying fairly close to normal.
WHAT THE EASTERN PACIFIC HURRICANE SEASON SAYS
NOAA’s 2017 Eastern Pacific Hurricane Season outlook predicts a near-normal or above-normal season. There is a 40% chance of an above-normal season, a 40% chance of a near-normal season, and a 20% chance of a below-normal season.
If an above normal hurricane season in the Eastern Pacific materializes, it could boost the Arizona Monsoon. Want to guess what decides that? (Psst…El Niño)
While Eastern Pacific influences seem unclear regarding our Monsoon, there’s two other players I think play a more important role.
WHAT DOES THE ARIZONA DROUGHT SAY
Entering June, moderate drought has been declared for roughly 28% of Arizona. The vast majority of the moderate drought resides in Southern Arizona. This slice of the state missed out on 25-50% of Winter rainfall.
Recent research suggests the worst drought areas can dictate where the Monsoon high ends up. The prime position for this high is the Four Corners, opening up the moisture pipeline from Mexico. But if the Monsoon high parks over Southern Arizona, it would bottle up deep moisture south of the border.
Important to note: this is not a hard & fast rule. But I give it more value this year because of…
WHAT THE ROCKIES SNOWPACK SAYS
Could what happens in the Rockies impact our Monsoon? Studies say it’s possible. The theory is an above average snowpack in the Rockies can slow the Monsoon high’s northward progression from Mexico.
Here’s where the Colorado Rockies snowpack stands in early June. Running over 200% above normal, this massive snowpack may halt the ramp-up of Monsoon storm activity.
WHAT DO I THINK?
Based on my research, I feel a drier Monsoon is more likely than not. Certainly the Eastern Pacific tropical influence can sway this (see: Monsoon 2014, 2015, 2016) and I can’t rule out that possibility. Even if the historically Monsoon-damaging El Niño doesn’t develop, there’s no escaping an ongoing drought and loaded Rockies snowpack.
In short, my lean is for Monsoon 2017 rainfall to come in slightly below the average of 6.08″.
JEFF’S TUCSON MONSOON 2017 RAINFALL PREDICTION: 5.63″