Has Monsoon 2015 already started?

Three straight days of dew points at or above 54°. That was the old method of determining the start date of the Monsoon. Viewer John Harrison remembers the old method. He writes:

“We have had several days of high dew point days. I know that the “official” start of the monsoon as determined by NOAA is June 15th. However, if we used the “old method” of three days of high dew points, are we in the monsoon? If so, when did it start?”

Average daily dew point in Tucson
Average daily dew point in Tucson

It’s awfully close, John. Thanks to remnant moisture from Hurricane Blanca, Tucson’s average dew point has been in the 60s the last few days. If today’s average dew point sits at or above that magic 54° mark, the old method would say that Tuesday was the start of Monsoon 2015.

However, it would be considered a “false start”. Dew points will drop off significantly heading into the weekend and next week as high pressure starts to build in. Because of “false starts” like this, the National Weather Service decided on a start and end date for the Monsoon (June 15 – September 30).

While Monday marks the official start of our Summer rainy season, we’re still expecting the true onset of the Monsoon to arrive around the 4th of July.


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