GJT Monsoonal Info

The Southwest Monsoon

During this time of the year, the word "Monsoon" is heard frequently, but how many people really understand what it is? Most commonly, the term is associated with heavy rain over India and southeast Asia. However, a similar event takes place over the southwestern U.S. and northwest Mexico/gjt.and is called the Southwest (or Mexican, or North American) Monsoon.

Although many associate a monsoon with heavy rain, it is more fundamentally linked to a large-scale, synoptic wind shift rather than precipitation. In fact, the term "monsoon" is derived from the Arabic word "mausim" which means "season" or "wind shift".

As the summer progresses, a large area of High Pressure over the eastern Pacific Ocean tends to move eastward. This results in a shift in winds to a more southerly direction across the southwestern states, allowing increased moisture to stream northward from the Pacific Ocean and Gulf of California. (This moisture is especially enhanced by tropical storm activity in the eastern Pacific).

Monsoon Synoptic Pattern. (Click on image for larger picture)

According to a study by Dr. David Mitchell of the Desert Research Institute , monsoon rains generally do not reach Arizona and New Mexico (where the effects of the monsoon are more pronounced) until the sea surface temperatures (SSTs) of the Gulf of California reach and maintain at least 79°F (26°C).

June is usually the driest month on the Western Slope. Although some locations may receive wetting rains during this time, many of the thunderstorms are high-based and produce rain that evaporates in the drier air underneath. This creates a wispy weather phenomenon below the storm called "virga", which can result in windy conditions at the surface. However, these thunderstorms are not part of the monsoon.

For western Colorado and eastern Utah, the Southwest Monsoon generally begins around the second week of July. An area of High pressure usually "breaks away" from the main Pacific ridge and settles in over the Great Basin by June, bringing hot temperatures during June and early July. As this High center moves eastward across the Continental Divide and into the Central Plains, a slightly cooler but much more moist environment will prevail in the southwest flow behind the High. In most years, the monsoon is over by the end of August/gjt.but can last as late as October. ( In fact, many areas in southwest Colorado and southeast Utah experience a secondary precipitation maximum in October due to late-season tropical storm moisture that's been carried northward by the monsoonal flow ).

One unofficial "rule-of-thumb" for determining the onset of the Monsoon Season in the American Southwest is to watch the dew point temperature in Phoenix, Arizona . When the average daily dew point there reaches, or exceeds, 55° F for 3 or more consecutive days, monsoonal conditions have arrived.

Once the Monsoon Season is underway, the southwesterly circulation does not produce thunderstorms everyday, but rather consists of a pattern that undergoes a series of "bursts" and "breaks": *

Burst	A movement of a weak trough in the upper level westerly wind into the southwest U.S. which spreads upper level cold air into the region. In the lower levels of the atmosphere, strong surface heating and southerly winds will transport moisture into the region. This creates intense atmospheric destabilization and leads to widespread thunderstorm outbreaks.
Break	An enhanced ridging of the Pacific subtropical High Pressure moves inland, effectively cutting off the moisture flow and stabilizing the atmosphere.

*Definitions by climatologist Andrew Carleton

While the monsoon thunderstorms can bring beneficial rains to eastern Utah and western Colorado, they can also result in one of our deadliest weather events: Flash Flooding .

Flash floods often occur in steeper terrain that has experienced recent heavy rains. Saturated or impervious soils allow for little infiltration of the rain, forcing the water to run overland.

Recently-burned areas are also subject to flash flooding. High intensity wildfires often melt the resin out of pine needles and other vegetative growth, leaving a hardened, water-resistant layer on the ground. Fires will also consume much of the ground cover, leaving the area prone to overland water flow. Heavy rains in burned areas can often result in not only flash floods, but also mudslides and debris flows.

Several Factors Contribute to Flash Flooding:

Rainfall Intensity and Duration
Slope and Topography
Soil Conditions
Ground Cover

While outdoors when thunderstorms are present, avoid steep narrow canyons that can fill rapidly with water. Also, never drive into flooded areas as 2 feet of water will carry away most vehicles!

NOAA National Weather Service Weather Forecast Office