The Monsoon Begins

the-monsoon-begins-1-613x380When most people think of monsoons, they think about torrential tropical rains advancing northward through the Indian subcontinent. While that’s the prototypical example of a monsoon, North America has its own version of a monsoon. In meteorology a monsoon is defined as a prevailing wind that changes direction seasonally. During the warm season the land gets very hot and is at a lower atmospheric pressure than the air mass over nearby ocean areas. This pressure difference drives moist oceanic air over the land and triggers thunderstorms and heavy rain; during the winter the situation reverses and high pressure over the land mass causes the prevailing wind to switch and blow from the land to the ocean, shutting off the precipitation.
While the monsoon in North America is not as spectacular as its Indian cousin, it does cause a significant amount of storminess in Arizona, New Mexico and the other southwestern states. The area around Tucson, Arizona is known for its particularly photogenic storms and spectacular lightning displays. The infrared satellite image shown above shows large thunderstorm complexes in southern Arizona and New and old Mexico. They’re easily recognized in the infrared image because thunderstorms have the highest, coldest clouds and in this image the coldest clouds are shaded red and green.
The monsoon typically begins in late June or early July and lasts until mid-September. More than half of the annual rainfall in Tucson and other cities in the region falls during the monsoon period.
Exactly where the monsoon moisture comes from has long been a matter of debate. The original thought was that the moisture originated in the Gulf of Mexico. One problem with this idea was that the humid air is often present at low levels, and it was hard to see how it could have made it across the high elevations of central Mexico. Now it is believed that the source of most of the monsoon moisture is the Gulf of California and the tropical Eastern Pacific, and the monsoon thunderstorms “turn on” when these bodies of water get warmer than about 80°F. The Gulf of California is only about 100 miles from the U.S. border, and the water there can approach 90°F by mid-summer. When strong winds blow from the gulf to California’s Imperial Valley the desert region can become the most humid area in all of the United States, with dew points exceeding those of places more traditionally thought of as muggy, like Key West, Florida and Corpus, Christie, Texas.

Warm Water Everywhere

noaa-1-680x380The picture above, courtesy of NOAA/NESDIS, gives the sea surface temperature (SST) “anomaly”, or difference from the normal temperature for the date. In this color scheme, yellows and red are warmer than normal and blues are cooler. It is obvious to even the casual observer that the contiguous U.S. is currently surrounded by water that is warmer than normal. Only far southern California is cooler than normal, and that is connected with some recent upwelling that brought it down from very warm temperatures during the winter.
The band of warm water along the equator in the Pacific is connected to the incipient El Niño, but the connection between all the other warm water and El Niño is less clear. Whatever the cause, these warm oceans evaporate a lot more water into the atmosphere and can fuel hurricanes and other storms. The Eastern Pacific hurricane season got off to one of its fastest starts ever, with Hurricane Andres followed closely by Hurricane Blanca, both of which became category 4 major hurricanes. When Andres struck Baja California June 8th it became the earliest tropical cyclone to strike the peninsula, beating the old record by a full month. Hurricane Blanca moved very slowly at one stage of its life cycle, and may be the reason for the blobs of cooler than normal water seen west of Mexico in the image above. Intense, slow moving tropical cyclones churn up cold water from great depths, and at least temporarily lower the temperature of the surface water.
As the moist air from Blanca pushed northward into the U.S. it caused light rains to fall in Los Angeles and even caused severe thunderstorms with large hail near WeatherExtreme offices in Incline Village. Unfortunately it did little to alleviate the extreme drought in California. Somewhat ironically, San Diego experienced its highest “precipitable water” ever in June, as a result of that moist air, but received almost no rain. Precipitable water measures how much water is present as vapor between the surface and the top of the atmosphere, and is related to the potential for heavy rains. It’s only a potential, though, and if there is no lifting mechanism present then no rain will fall.
In contrast, May brought extremely heavy rains to Texas and Oklahoma and made the month became the rainiest on record in the contiguous U.S. There is a good chance that the unusually warm waters encircling North American contributed to this record rainfall. Unfortunately a new (as yet unnamed) tropical system may once again bring heavy rain to Texas, with 10 inches or more expected in some areas.