Wry Heat » Monsoon

Monsoon forecast 2012

Jonathan DuHamel — Tue, 26 Jun 2012 15:11:42 +0000

The University of Arizona is predicting our summer monsoon rains “to be more vigorous than average, with a strong beginning and end.” See full story here. The Old Farmer’s Almanac is predicting “near normal rainfall.” See here.

Arizona’s monsoon in July, August, and September occurs because a high-pressure area perches over the Four-Corners area and draws moist air from the Gulf of Mexico and the Gulf of California. The mountains and summer heat cause updrafts that condense the moisture-laden air to produce rain from thunderstorms.

To better understand thunderstorms, see my post Anatomy of a Thunderstorm.

Haboobs and the anatomy of a thunderstorm

Jonathan DuHamel — Thu, 07 Jul 2011 14:59:31 +0000

Since we are now in the monsoon, and the haboob that ate Phoenix is the talk of the day, it is well to review the anatomy of thunderstorms and some safety tips.

The monsoon will bring much-needed rain, but it also brings lightning and destructive winds as well as the dust clouds called haboobs. The term “monsoon” does not mean rain or storms, but a seasonal shift in wind patterns. During the winter, Arizona air flow is usually from the west and we receive the remnants of Pacific storms. During the monsoon, the air flows from a southerly direction bringing us moist air from the Gulf of Mexico, the Gulf of California, and the tropical Pacific. The desert heat and orographic uplift from mountain ranges turn that moist air into thunderstorms.

The graphic below from the National Weather Service shows the parts of a monsoon thunderstorm. The storm is moving left to right.

The first stage of thunderstorm development is the updraft where warm, moist air is lifted by columns of hot air rising in desert valleys or by air passing over a mountain range. This uplift causes water to condense to form cumulus clouds. Falling water droplets pull the air down with them to form the downdraft and rainy part of the storm. The strong downdraft causes a dust cloud, the Haboob, in front of the storm. Frequently dust devils dance in front of the Haboob. As the storm progresses, the downdraft can produce destructive microbursts of high wind. “Gustnadoes” are tornado-like vortex, similar to dust devils, but stronger, that appear to develop on the ground and extend several hundred feet upward. These vortices generally develop along the leading edge of an outflow boundary from a thunderstorm cell. Although generally of limited duration, the winds of gustnadoes can be strong enough to cause damage.

Some safety tips about lightning:

Lightning often strikes outside of heavy rain and may occur as far as 10 miles away from any rainfall. As a rule of thumb, if the time between lightning flash and thunder is 30 seconds or less, the lightning is about 6 miles away. Research has shown that the most successive flashes are within 6 miles of the first one, which means that you should have reached a safe place if lightning is less than 6 miles away.

If possible stay indoors or in an automobile. If outside, avoid isolated tall structures such as trees and power poles. Get off and away from open water. Avoid open metal vehicles such as tractors, farm equipment, motorcycles, and golf. Avoid power lines, wire fences, metal pipes and railings. If you are caught in an open area far from shelter, and you feel your hair stand on end, lightning may be about to strike you. Drop to your knees and bend forward putting your hands on your knees. DO NOT lie flat on the ground, that makes you a bigger target and a better conductor of electricity. Stay away from running water inside the house; avoid washing your hands or taking a bath or shower. Electricity from lightning has been known to come inside through plumbing. When inside, stay away from TV sets, electrical appliances, bathtubs and sinks, do not talk on the telephone, or play on the computer, don’t touch an electrical cord or outlet.

Be aware and be safe.

Anatomy of a Thunderstorm

Jonathan DuHamel — Tue, 22 Jun 2010 14:51:39 +0000

The summer monsoon is upon us and hopefully it will bring much needed rain. But it will also bring lightning and destructive winds. The term “monsoon” does not mean rain or storms, but a seasonal shift in wind patterns. During the winter, Arizona air flow is usually from the west and we receive the remnants of Pacific storms. During the monsoon, the air flows from a southerly direction bringing us moist air from the Gulf of Mexico, the Gulf of California, and the tropical Pacific. The desert heat and orographic uplift from mountain ranges turn that moist air into thunderstorms.

The graphic below from the National Weather Service shows the parts of a monsoon thunderstorm. The storm is moving left to right.

Some safety tips about lightning:

Be aware and be safe.