8/4/26 Tuesday 4:30 am
Historically, the onset of the monsoon in Arizona was determined by a simple parameter. Three consecutive days with a dew point at or above 55 degrees. In Phoenix, this usually occurred between late May and early June. It was simple to understand.
As the story goes, NWS offices in Albuquerque, Las Vegas, El Paso, and Flagstaff had concerns because they could still get active storms with a dew point below 55 degrees. Something we are well aware.
For years on this blog, I have championed the saying: “Sometimes, it just snows,” meaning just because it’s snowing, it doesn’t make it a storm. Which makes me scratch my head a couple of times when I hear NWS forecasters struggle to separate a rogue thunderstorm from monsoonal flow. “Sometimes it just rains”.
Anyway, they threw out the three-day dewpoint-above-55-degrees parameter, instead assigning a start and end date to the season, much like hurricane season, which, by the way, has been a dud so far this year, which is not too bad a thing.
Did you notice I almost went down the rabbit hole, but I am course-correcting immediately.
This happened 18 years ago. Now Monsoon Season is officially June 15th to September 30th.
Diehards like myself still like the dewpoint rule. It is also very popular still in Arizona. So I still use it to verify when we are under monsoonal flow.
So let’s use that parameter again to estimate when our rain returns. Here is the dewpoint forecast beginning this Sunday morning.
Now we can correlate dew points with the multi-parameter forecast run, beginning Sunday morning.
Both runs confirm our Monday transition back to afternoon showers with heavier showers arriving by mid-week.
My next post will be on Wednesday.