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Monday, June 13, 2016

Is there such a thing as a typical monsoon season?

Today’s blog comes from Ken Waters, NationalWeather Service’s Warning Coordination Meteorologist with the Phoenix Office. Ken has been with the NWS for 20 years including stints in Guam, Texas, and Hawaii. 

Previously Ken had a career as a Weather Officer for the U.S. Air Force and served much of his career in the Pacific. Ken has a Masters Degree in Meteorology from Florida State University and a Bachelors Degree in Biology from Whittier College. In his spare time Ken is an avid genetic genealogist as well as software and hardware developer of electronic sensors.
Monsoon Sunset
Photo by: Bryan Snider

Every year I look forward to the part of the year where Arizona gets its most active weather. Of course we know it as the monsoon. That's when the prevailing winds shift substantially, bringing north into the state a large amount of humid air. The increase in moisture is undeniable. We all feel it as soon as we step outside. It can be reflected in a jump in dew point temperatures, often from 40 degrees up to 70 degrees.
Dew points are probably the best way to measure moisture and, when combined with temperature, can yield the relative humidity. Meteorologists regularly monitor dew points; specifically, changes in dew points to measure the moisture surge as it typically moves north from the Mexican state of Sonora into first, southern Arizona, and later into the rest of the state. Typically this moisture surge occurs during the latter part of June for the southern portion of the state, and about the first week of July for the Phoenix area.
The primary impact we see from the monsoon is an increase in thunderstorm activity, particularly over the higher mountainous portions of the state, including the Mogollon Rim, the Mazatzal Mountains, the White Mountains, and the Santa Catalina Mountains. The daily ritual starts with puffy cumulus clouds forming over the highest peaks late in the morning. The afternoon sun warms the atmosphere causing these clouds to build into thunderstorms. If upper atmospheric winds are favorable then these thunderstorms can be pushed down into the lower parts of the desert around late afternoon and early evening.
Radar showing Monsoon
Image by: NWS/NOAA
So, what is a typical monsoon season? Well, in fact, there really is no such thing. If anything the monsoon season is characterized by variation. Variation can be measured day-to-day as the monsoon moisture surge comes and goes, typically in about 3-day cycles where the dew points jump up into the 70s for a few days only to return to drier 50s for a few days. Variation also occurs from year-to-year with some years being dominated by severe dust storms such as 2011, and others with large numbers of severe thunderstorms such as 2008. Lastly, there's the spatial variation. It's commonplace to have some locations get a large amount of rain and flooding from a single storm or set of storms, only to have a location just a few miles away stay dry.
From a statistical standpoint, the National Weather Service measures monsoon seasons using a few fixed points - such as Sky Harbor Phoenix airport. This location has been supplying weather data for over 100 years, so it can show some sense of year-to-year variation. However, the airport often may not be representative of the monsoon season for other areas, such as the Valley or State as a whole.  Even measuring the monsoon season can be tricky as it depends on where you are. We do have some other ways  to measure the season by averaging a number of points such as with the National Weather Service's Phoenix Rainfall Index, or PRI.
Bottom line on the monsoon is it will happen every year, and will occur sometime during the pre-declared
Monsoon flood in Wickenburg
Photo by: Rick Delaney
monsoon season of June 15 to September 30. The best we can do is to prepare for it:
  • Monitor media and government weather information on a daily basis during the season.
  • Don't drive across normally dry washes that have water flowing.
  • Avoid dangerous dust storms while driving; but if caught in one, pull over and turn the lights off.
  • Watch out for, and stay away from downed power lines.
  • Heed cell phone alerts for dust storms and flash floods.
Visit for preparedness tips and hazard information.

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