If El Niño had a relationship status on Facebook during the Monsoon, it would read “it’s complicated”.
El Niño’s definition is warmer than normal sea surface temperatures in the eastern Pacific Ocean. During the Winter months, El Niño is synonymous with above average rain & mountain snow in southern Arizona. Its impacts during the Summer is more boom or bust. Let’s start with the bust.
The above charts courtesy of the National Weather Service delve into Monsoon rainfall for select Arizona cities during El Niño conditions. There’s a fairly obvious trend that sticks out from Flagstaff to Tucson. Since 1951, the vast majority of Summer El Niño events correspond with below average rainfall. In Tucson, climatology says the Monsoon is typically 17% drier during El Niño. So what gives?
It all has to do with the temperature difference between the eastern Pacific and Mexico mainland. During a typical Monsoon, the eastern Pacific waters are relatively cooler compared to the scorching land temperatures in Mexico. Tick ocean temperatures up a few tenths of a degree and the temperature difference is smaller, thus leading to more stable atmospheric conditions. The net result is fewer storms & not as much Monsoon moisture surging into the state.
To summarize: odds tilt toward a dry Monsoon. But the odds don’t always come to fruition.
Warmer than normal waters can also be the catalyst for an active eastern Pacific hurricane season. In 2014, 20 named storms developed off the Mexico coast. Two of those storms directly impacted the southern half of Arizona. Phoenix saw its all-time daily rainfall record shattered on September 8th by remnant moisture from Norbert (3.29″ at Sky Harbor). Days later, Odile’s remnants soaked Santa Cruz and Cochise Counties with 2-4″ of rain. The irony: El Niño never truly developed in 2014.
The chances of an eastern Pacific tropical system impacting Arizona are slim each Summer. With El Niño in the mix, those chances get a little better.
So what does this mean for Monsoon 2015? NOAA’s latest forecast calls for a 90% chance of El Niño sticking around this Summer & a slight chance for above normal precipitation between July and September.
I’m still leaning toward a drier than normal Monsoon, barring a lack of tropical influences on the Grand Canyon State. The optimists (and storm chasers) can hang their hat on the small chance I have for some flavor of an eastern Pacific tropical system impacting Arizona.
Look for my official Monsoon 2015 forecast next month on the TV side & blog.