Monsoon season: The dew point dilemma

Dust storm rolling into Phoenix

Monsoon season officially starts this week in Arizona.

It’s our summer thunderstorm season when the weather goes from “a dry heat” to hot, humid, dusty and sometimes rainy.

Monsoon means a shift in the winds, in this case, from the west and northwest to the south and southeast. That wind shift brings moisture from the Gulf of California and Gulf of Mexico into Arizona.

The monsoon officially runs from June 15 until September 30 each year, but longtime Arizonans may remember when the season wasn’t “set” that way.

In 2008, The National Weather Service in Phoenix decided to make a big change. For decades, the monsoon was said to have started after three consecutive days with a dew point of 55 degrees of higher. The dew point is a measure of moisture in our atmosphere, and when the dew points start to climb, it’s a good indication that our weather pattern has shifted and the monsoon season has arrived. But it’s not always that simple a message to communicate.

MOBILE/APP USERS: Click here to see pic of monsoon storm over Phoenix

The system varied around the state. In Tucson, the NWS was using 53 degrees as the threshold for the monsoon start.

As moisture slowly moves into our atmosphere, storms usually start earlier in the summer in Southern Arizona and Northern Arizona than they do in the Valley. And with varying criteria for dew points with elevation, it was often confusing to residents to say that the monsoon had “arrived” in some parts of our state, but not in the Valley, simply because our dew points weren’t high enough.

We also get frequent dust storms early in the season, well before the “wet” part of our season arrives. But those wind and dust threats are still part of the monsoon. The National Weather Service decided it was time to take the focus off the start and finish dates of the monsoon, in an effort to better educate residents about the dangers they should be prepared for.

So now, similar to hurricane season, monsoon season has a calendar-based system. The former system has been nicknamed the “legacy monsoon definition.”

MOBILE/APP USERS: Click here to see image of dust storm rolling into Phoenix

While the season starts June 15, we typically do see the dew points rise closer to the beginning of July. That’s usually the time we start to see our first dust storms and/or thunderstorms in the Valley.

Usually, those dew points start to drop off again in September as winds shift and drier air returns to Arizona. But sometimes hurricanes in the Pacific push their moisture up the Gulf of California and give us a late-season boost of moisture, fueling storms. That’s one more reason the NWS determined monsoon season won’t officially end until the end of September. It’s another good reminder to be on guard for all the dangers the monsoon season brings.

For more information on monsoon season in Arizona, check out our Power of 2 Monsoon Arizona special:

[SPECIAL SECTION: Monsoon 2017]

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Good Morning Arizona Meteorologist

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