What scientists know well, and what's left to learn, about extreme weather and climate change.
MoreThe blistering heat continues in the Southwest, but desert dwellers are in for a little relief soon. The cooling clouds and precipitation that come with annual return of the North American monsoon promise to break the record heat and temper things a bit (relatively speaking anyways) through the rest summer.
The Indian monsoon is the best known monsoon in the world, but the North American monsoon, which affects the southwestern U.S. and northeastern Mexico, becomes an important weather player during the early summer.
Monsoons are known for their heavy rain to the average person, but the reality of what the monsoon is is more complex. More accurately, a monsoon is a seasonal shift in wind direction, so there can be dry monsoon winds and wet monsoon winds. Large scale winds from ocean to land create wet monsoons, while winds from land to ocean create dry monsoons.
The strong sun that comes in early summer heats up the dry plateaus of the Southwest every year, inducing a broad area of low pressure there. In response, the wind flows northward from the Gulf of California into the Southwest, bringing just enough moisture to spawn showers and thunderstorms across the region.
The National Weather Service defines the monsoon season in the Southwest as the period between June 15 and Sept. 30, though the monsoon rains often start after the official date. Until 2008, the arrival of the monsoon was only declared once there were three consecutive days when the average daily dew point temperature (the temperature at which dew forms), was 55°F (13°C) or higher. This occurs most frequently during the first week of July, but it can happen as early as mid-June or as late as mid-July.
Precipitation amounts spike as a result of the monsoon. The average monthly precipitation in June is a paltry 0.02 inches in Phoenix, but jumps to 1.05 inches in July once the monsoon sets in. In any month, it is rare to get more than 2 inches of rain in Phoenix, but it can happen, such as when 5.11 inches fell in September 2014.
While these monsoon thunderstorms do not produce nearly the volume of rain as those that form in the eastern U.S., they can still produce flash flooding.The barren desert landscape means less water is soaked up by the ground while canyons and dry riverbeds quickly funnel water into what can become dangerous torrents. Last year, monsoon-influenced storms caused the deadliest flash floods in Utah history.
Strong winds also accompany these thunderstorms. On Wednesday, a wind gust of 71 mph brought down power lines in the Phoenix suburb of Magma, according to the National Weather Service office in Phoenix. And as these thunderstorms begin to fade in the evening, the winds from the decaying storms can lift dust hundreds of feet into the air, producing a long-lived dust storm called a haboob.
In addition to the onset time of the monsoon, the strength of the monsoon also varies from year to year. Colder than normal ocean temperatures in the central North Pacific ocean and warmer than normal ocean temperatures in the subtropical North Pacific correlate with monsoons that start earlier. The reverse pattern can also cause the monsoon to start later.
There is also a tendency for wet winters to be followed by drier monsoons and dry winters to be followed by wet monsoons.
How climate change will affect the North American monsoon is an area of active research. The most recent Intergovernmental Panel on Climate Change report suggests that the monsoon will arrive later and persist later than it currently does. But it appears that shorter term variability in ocean temperatures will continue to play a dominant role in shaping the monsoon each year.
While these monsoon thunderstorms temper the broiling heat, the threat of flooding and haboobs are a couple of the tradeoffs that come with living in the desert climate of the Southwest.
