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Southwest Monsoon | Benefits and Threats

Southwest Monsoon brings beneficial rains, but risks as well

Dr. Mario Picazo
Meteorologist, PhD

Tuesday, July 10, 2018, 19:12 - As we move through the month of July, heavier rains and thunderstorms are expected across many areas of the southwest U.S. It's the U.S. Summer Monsoon, and this week, expect it to be fairly active in areas of Arizona, New, Mexico, Colorado, Utah, and even southern Wyoming and southeast California. Weather forecasts are anticipating an off and on situation of afternoon and evening thunderstorms rolling through the region during most of the week. Some areas of eastern Arizona, western and northern New Mexico, and southern Colorado could see anywhere from 3 to 5 inches during the week although some of that precipitation will fall during short periods of time.

(See also: Firefighters gain ground on California fires)

While much of the Southwest U.S. and portions of northwest Mexico continue to experience extreme to exceptional drought, those managing and consuming water resources in the region keep their fingers crossed for a plentiful Summer Monsoon season. July and August are the main months when large thunderstorms roll across the Southwest US and Northwest Mexico, bringing most of the precipitation accounted for during the year. This monsoon type circulation accounts for at least half of the total precipitation received in Arizona and New Mexico.

The entire region has been hard hit by an intense persistent drought since 2017. Despite an essential relief of the dry conditions, the coming rains will also mean localized flooding, mudslides and dust storms in some areas. Last week, was already a fairly wet one for some counties of Arizona and New Mexico, with flash flooding occurring for instance in Oro Valley, Arizona.

Two main synoptic features drive this Southwest Monsoon, a thermal low developed by intense daytime heating over the southwestern US and Northwestern Mexico deserts, and an upper level high pressure ridge normally located east of this low. With this pressure pattern set-up, moisture from both the Gulfs of Mexico and California flows into the region, which, in conjunction with the local orography, can lead to vertical cloud development. The evening and nighttime thunderstorms are capable of producing spotty but intense precipitation accompanied by an incredible lighting display.

Monsoonal thunderstorm reflects colorful light at sunset. Image: Getty Images.

For some, the monsoon is a period of relief as it suppresses much of the intense summer heat that affects the southwest deserts. It also replenishes water resources in the region and nourishes the vegetation. The other side of the coin is that this hit and miss type of precipitation can also be dangerous, causing frequent flash flooding, crop damaging hail and intense gusty winds with occasional dust storms in the Four Corner states.

During the monsoon months, another important phenomenon associated with the frantic thunderstorm activity is lightning. In the Southwest, lightning on average ignited more than 2,300 fires annually since 2001, burning close to 277,000 acres per year.  

Impact of a potential El Niño on the Southwest Monsoon?

How strong and effective the North American Monsoon is on a year to year basis depends greatly on complex interactions between the Pacific and Atlantic Oceans, the North American continent and the atmosphere. As of May 2018, most computer models agree that a weak El Niño (0.5 to 1.0 ºC) could form during the summer and continue to gradually intensify later in the Fall. But the complex interaction between synoptic and mesoscale mechanisms, can overshadow the larger scale El Niño signal, specially when it is a weak one as expected in 2018. 

Sea surface temperatures in the Pacific and Gulf of Mexico near the region are also very important. The main effect of El Niño on the quality of the summer monsoon rains appears to be related to the moisture availability and transport into the region. Research connecting the onset of the Southwest Monsoon and El Niño suggests that only during strong El Niño events, the subtropical ridge struggles to form early on, and delays the onset of the season. With the ongoing transition between a now gone La Niña and an emerging El Niño, near neutral conditions would imply a near normal North American Monsoon season this year.

It is also relevant to follow-up and on the activity of the Julian-Madden Oscillation (MJO) during the summer months, as it could affect the impact of the El Niño phenomenon when present in the Pacific. 

The MJO is an atmospheric disturbance that modulates tropical patterns of atmospheric circulation and thus precipitation. It has a pronounced effect on the winter time jet stream, the upper level winds that drive storms among other areas into the west coast, but it can also affect significantly regions that experience rainy seasons during the summer. 

With a high risk off flash flooding this week motorists should be especially aware of crossing local roads specially when covered with water. Stop on the shoulder during heavy downpours and when low visibility due to dust storms occurs.

Hikers should also be very aware of flash flood areas when venturing into closed canyons. Gusty thunderstorm winds can also be a threat when moving near trees or powerlines in some areas. 

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