El Niño could lead to a drier monsoon - Tucson News Now

El Niño could lead to a drier monsoon

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El Niño is developing in the waters off the west coast of Mexico and Central America. When this appears, it can influence the summer rain pattern, including the monsoon right here in Arizona. Recently the Tucson National Weather Service crunched the numbers for various locations in southern Arizona, including Tucson. A wet monsoon isn't necessarily in the forecast this year. 

For details on the developing El Niño click here.

El Niño is when a pool of warmer-than-average water appears in the equatorial East Pacific Ocean. This pool of warmer water can weaken the summer temperature contrast between the hot land and cooler water. The greater the difference in temperature between the two, the better the onshore flow of tropical moisture needed for the monsoon. Since El Niño weakens this gradient, at the very least there is generally a delay in the start of the monsoon. There are other weather pattern influences but this temperature gradient is one of the most important for the monsoon to begin in the Southwest.

However, warmer waters associated with El Niño favor an active hurricane season. Tropical storms and hurricanes off the west coast of Mexico can trigger moisture surges into Arizona. These surges rev up monsoon action, which means potential flooding downpours. If a series of storms track northward along the west coast of Mexico, southern Arizona could still see some impressive monsoon rain totals. 

However, the data suggests a drier monsoon overall for southeast Arizona with an El Niño in place. Below is a list of numbers for Tucson. The El Niño data stretches back to the mid 1950s. During this time, there were 17 summers when an El Niño was present; 10 of the 17 led to a drier monsoon. Plus 51 percent of the time June was drier than average, whereas the rest of the monsoon months ranged from 14 percent to 25 percent drier than average. 

This year's El Niño is developing similar to one that formed in 1997. In 1997 the monsoon started off slow but ended with near-average rain totals in August and September. 

Click here to see the numbers in your town or city.

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