June 15th marks the official start of monsoon season 2017, according to the National Weather Service. Meteorologist Lee Born spoke with KNAU’s Aaron Granillo about the start of the monsoon.
Aaron Granillo: So, it sure doesn’t seem like it, but today is the official start of the monsoon. Why today? And, tell us what the monsoon is.
Lee Born: Well, the monsoon is a season, and today, June 15th is the official start date. And, it runs through September 30th. It doesn’t feel like the monsoon season for many of who have experienced it. But, back in 2008, the National Weather Service instituted hard dates for the monsoon season, being June the 15th to September 30th. A lot of folks that have been here for a while remember the old way of defining the monsoon was when it actually started raining, and we tracked the dew point temperatures to track that tropical moisture. And, when it was here, and we had three consecutive days of the average dew point being 55 degrees or greater, we called that the beginning of the monsoon, which is still a couple weeks away.
And, what causes this wind shift?
Well, the monsoon is defined by a wind shift. You’re absolutely right. If you looked it up in the dictionary, that’s the definition of a monsoon – a seasonal wind shift. Getting into this time of year, the jet stream and the winter spring storm track starts to retreat completely out of the lower 48 states. It just starts to retreat more and more northward here as we move through summer. And, as that is happening at the same time, we have a building of high pressure across much of the central United States. It’s known as the Bermuda high pressure cell. It moves all the way from the east coast into the central United States. And, also at the same time as that high pressure cell is building in, we have a big area of low pressure at the surface over the Mohave Desert. Because it’s so hot there and all the hot air is rising and leaving the surface, it causes a vacuum effect there. So that also draws in moisture from the Pacific Ocean and the Gulf Of Mexico into the lower desert locations. So, those two things combined draw in the moisture and also cause the wind shift.
And, what is the outlook for this year’s monsoon? Are there any indicators that give us clues or indicators that can tell us how the monsoon will play out this year?
You know, seasonal monsoon outlooks are a little bit still in their infancy, and not all that accurate. It’s not that well correlated with ocean temperatures, like when we do long-term winter forecasts. And, we say ‘oh, there’s an el niño or a la niña. Warmer or cold ocean temperatures. And that is usually associated with a wet or dry winter. There are a couple indicators as the science is growing and been researched. And, el niño and la niña are just a small bit correlated with monsoon rainfall. So, right now we are in el niño neutral with a slight tilt toward warmer water, el niño, which is usually correlated with a little but of a drier monsoon. That doesn’t mean we’re not going to get rain. It just possibly means we have more breaks in the monsoon because the monsoon does come in bursts and breaks. We get active periods and then we get little, less active periods. It kind of ebbs and flows so we’ll see. None of this is bulletproof.