August in the Sonoran Desert: Monsoon Season

The very large anvil portion of a large cumulus cloud from the Arizona Monsoon season.  Often huge anvils like this mean a strong monsoonal storm is coming.  Picture from:http://www.wrh.noaa.gov/fgz/science/monsoon.php?wfo=fgz 

So far this year monsoon season has been a flop.  We are towards the end of August with only an inch of rain or so in Phoenix, only 0.25 inches in the mountains west of Phoenix, and a fortunate 2 inches in the mountains east of Phoenix.  Other areas of the Sonoran Desert are also below the average summer rainfall totals, which are about twice or more than what we have received.  This is quite disappointing considering we haven't had good summer rainfall for several years now.  Us desert dwellers look forward to the break in the summer heat that rainfall provides.  But as is normal for the desert, there really isn't such thing as "normal" rainfall in the desert.  Fortunately, these poor rainfall reports could change to above average with a single large thunderstorm.

What is the Monsoon?
Monsoon season in the Sonoran Desert technically begins June 15th and ends September 30th.  During this time period, the normal dry heat of the desert is replaced with humidity from the Pacific Ocean and Gulf of Mexico.  Extreme summer heat in the desert causes rising air, creating a low pressure system pulling in this tropical humidity.  Wind patterns change as a result of this low pressure, shifting the normal dry western winds to humid winds out of the southeast.  This shifting wind pattern is where the monsoon gets its name, and means "wind-shift" in Arabic.  In order for rain to fall, high level humidity must be in place from the Gulf of Mexico (called the Bramuda High) and low level humidity must be in place from the Pacific Ocean and Gulf of California.

An anvil cloud, a type of cumulus cloud that results from hot rising humid air.  The flat top is a result of cooler air higher in the atmosphere.  There are no obvious cirrus clouds surrounding this cloud.    Picture from Wikipedia.

How to predict a monsoonal thunderstorm without the weatherman
1. High temperature around 100 to 108 degrees are optimal.  These temperatures heat moist air causing it to rise and form rain clouds.  This heated rising air is called convention and the resulting clouds are known as convection clouds.
2. High due points above 50 to 55 degrees, the higher the better.  This supplies the moisture to form clouds and rain due to rising air.
3. Winds generally coming out of anywhere from the south to east to north.  These winds are blowing humid air into low pressure systems and rise to form clouds.
4. Low level cumulus clouds, another indication of lower level atmospheric humidity.  These clouds should form with convection causing moist air to rise due to afternoon heat.  And of course, you need clouds for rain.
5. Large cumulus clouds that form flat-tops called anvils.  The larger the anvil generally the larger the storm.  Clouds wider then they are tall bring the most rain. Smaller, or tall skinny clouds with or without anvils and may appear to be leaning to one side, may bring small amounts of rain, or rain may never hit the ground.
6. High level cirrus clouds.  These clouds may indicate no rain if scattered.  Cirrus clouds are not always present, but a good storm will almost always have cirrus clouds proceeding it around the anvil.

Monsoonal thunderstorms in the Sonoran Desert are quite complex, it takes a lot of different things to be in place for a good storm to happen.  For example, if low level moisture is in place but high level moisture is not in place, clouds will form and some rain may fall from these clouds but may never reach the ground.  Clouds that form under these conditions often will be small, bent in one direction.  If however high and low level moisture are in place, rising humid air will form cumulus clouds lower in the atmosphere that continue to rise until they hit higher level cooler air.  This will result in an anvil cloud, which is a type of cumulus cloud with a flat top like an anvil.  This rising humid air hitting cooler air can form some pretty amazing storm clouds and at times some pretty amazing rainfall.

Monsoon Thunderstorms in the Desert
The drenching rain, spectacular lightning, and violent winds are something to behold.  Experiencing this type of storm will most definitely leave an impact on anyone.  Everything in the desert waits longingly for these storms.  In the dry hot months prior to monsoon season, the land becomes painfully dry, causing plants to shrivel up and die or go dormant.  Watering holes also dry-up, forcing many animals such as javelina and mule deer to stay near more permanent water holes in the mountains.  Mountain lions take advantage of these times by stalking prey near these watering holes, making life even worse for these thirsty animals.  Other animals and birds simply hide away from the heat, only coming out at night or limiting their activity all together.  Everything hides from the heat and dryness, waiting for rain.

An Ocotillo leafing out after some modest summer rainfall.   Most of the year this plant remains leafless in order to conserve water.  When it rains however, the plant quickly grows leafs.  These leafs remain on the plant until the soil drys out.  Once dry the leafs turn yellow and fall off, waiting to leaf out until the next rainstorm.

However, when a drenching rain hits, temperatures can at times drop from the 100's to the 70's or lower.  The ground becomes soaked. If rain comes hard and fast enough the washes fill with water, sending flash floods through the mountain canyons and into basins below, creating new and filling old waterholes to the brim.  With this water, life and energy return to the desert once again.  If you go hiking after a major rain, one of the first things you will notice is the new flurry of insect activity.  Ants will dig out highways in the washes.  Swarms of tiny mosquitoes will form (the one time of year you will want bug spray).  All sorts of gnats will also come out.  This also seems to awaken bird life feeding on these insects.  Animals that were limited to the areas around waterholes in the mountains the day before will once again venture out.  Even in the city, the most non-outdoorsy people will venture outside to experience the break in heat and dryness.  Several days after rainfall, plant life begins to awaken.  Dried out Triangle Leaf Bursage, Brittlebrush, and Creosotebush green-up and begin to grow again forming new leaves.  Ocotillos, which spend most of the year leafless form new leaves.  The Palo Verdes will also grow new leaves.  Annual plants like Prickly Poppies and Sacred Daturas will germinate and grow.

All this activity will slowly wane after days to weeks of time if rain doesn't return.  With the decline of monsoon season towards the end of September though, life does not return to the dull seemingly lifeless heat.  Rather, temperatures have cooled considerably and often more water remains across the landscape as we head into fall.

More information on the Arizona Monsoon: http://geoplan.asu.edu/monsoon.html