It’s the most wonderful time of the year for meteorologists in the Grand Canyon State.
June 15th marks the official start of the Monsoon, a 108 day span that accounts for over half of Tucson’s annual rainfall. El Niño’s officially over. La Niña’s likely to settle in. What could it mean for the Super Bowl of southern Arizona weather? Here’s my 7th annual take on what to expect during Monsoon.
WHAT THE CPC SAYS
The latest Climate Prediction Center (CPC) outlook for the July-September timeframe calls for equal chances of above, normal or below average rainfall for Arizona. As I touched on last year, the CPC tends to paint with a rather broad brush. There are local effects that can sway seasonal precipitation forecasts one way or the other. Here are a couple of factors I’m focusing on for Monsoon 2016.
WORKING IN OUR FAVOR
After a record-setting El Niño, waters off Mexico’s west coast are starting to cool off. Models are in agreement that this trend continues throughout the Summer. The latest forecast for sea surface temperatures in the eastern Pacific show waters roughly 0.5-0.7° cooler than average, indicating a weak La Niña.
How does this benefit southern Arizona? What El Niño is to the Winter, La Niña is to the Monsoon. The working theory is cooler than normal Pacific waters up against hot temperatures on Mexico’s mainland drives a more unstable atmosphere. That increased temperature difference between land and sea could develop more storms in Mexico, allowing subsequent moisture to enter southern Arizona.
Since record keeping began in 1950, a weak Summer La Niña has been present 10 times. Of those 10 La Niña events, Tucson recorded above normal Monsoon rainfall 9 times. In all, Tucson’s average Monsoon rainfall during a weak La Niña computes to 8.36″, which is well above the seasonal normal of 6.08″. Tucson’s two wettest Monsoons on record (13.84″ in 1964, 13.08″ in 1955) occurred during a weak La Niña event. It’s important to note that a weak La Niña does not guarantee above average Monsoon rainfall. It increases the odds that it’ll happen.
Another potential factor that may sway Monsoon 2016 toward a wetter side is the eastern Pacific hurricane season. Mexico’s meteorological government agency is forecasting near normal activity, which may include 17 named storms. 9 of those storms are expected to become hurricanes.
WORKING AGAINST MONSOON RAINFALL
The biggest factor that could shut down a stormy Summer is where the Monsoon high sets up shop. Quick refresher: the Monsoon is defined by a seasonal shift in winds aloft, changing from the west to the east or southeast. This flow funnels in moisture from Mexico and the Sierra Madres, fueling our daily rounds of thunderstorm activity. The “sweet spot” for the Monsoon high is the Four Corners area.
Recent research has shown this dome of high pressure likes to settle into areas which were dry during the Winter & Spring. Thanks to El Niño’s meager influence this past Winter, much of Arizona and New Mexico saw the drought come back with a vengeance. As of June 7th, a good chunk of the southwest was under moderate drought. Severe drought was present in much of Santa Cruz and Cochise Counties.
If high pressure sets up in these areas, it could shunt moisture south of the border & leave southern Arizona with below normal Monsoon rainfall. Not a good sign, considering a lingering La Niña into Winter historically leads to less rain & mountain snow.
Two key trends I’ll be watching closely this Monsoon: first – will the expanding drought lead to a higher frequency of dust storms? I’m leaning toward yes; at the very least, more than last year. Recall that we went into last Summer running a rainfall surplus, hence the lack of haboobs. As of this writing, Tucson sits roughly 0.40″ below average for rainfall for 2016.
The other trend is whether storms will linger well into September, like the last few years. Again, the initial indication is no. Climate models (posted above) lean toward our daily rounds of thunderstorm activity fizzling out after Labor Day. Historically speaking, this is when we expect the Monsoon to start winding down.
MY FINAL CALL
Numbers don’t lie. More often than not, a weak La Niña has historically delivered above average Monsoon rainfall to the Old Pueblo. A huge plus – one that I think outweighs the biggest factor working against a wet Summer: where high pressure sets up. Last year’s Monsoon was pretty decent. I expect more of the same this time around.
Tucson’s average Monsoon rainfall: 6.08″
My Monsoon 2016 rainfall forecast: 6.85″ (+0.77″ above normal)