Monsoons in America?

  • Published
  • By 2nd Lt. Tyler Brock
  • 25th OWS
On the evening of July 17, a massive thunderstorm ripped through the Marana-Silverbell Army Helicopter Support Facility complex which lies approximately 15 miles north of Tucson, Ariz. wreaking havoc on operations base-wide. 

Regional forecasters at the 25th Operational Weather Squadron, Davis-Monthan AFB, Ariz. first spotted the cell on radar as it built along an outflow boundary on the northern fringe of Tucson. The thunderstorm became severe within a matter of minutes with radar revealing extremely high reflectivity, (75 DbZ) reaching up to 25,000 feet, in the core of the storm. Without hesitation, Airman First Class Jessica-Nicole Peterson, a 25th OWS weather forecaster, issued a warning for winds greater than 50 knots for Marana because it was directly in the path of this monstrous storm. 

At 7:52 p.m., base operations at Marana called to inform the 25th OWS forecaster that the cell collapsed directly overhead with sustained winds of 52 knots and a one time gust of 75 knots. Wind damage was reported across the complex as several roofs were torn off buildings and more than 25 trees ranging from two to three feet in diameter were completely uprooted. Heavy rainfall also accompanied this storm with rainfall rates estimated at 2 inches per hour on radar. At storms end, nearly $330,000 in damage was reported. 

Typically a storm of this caliber would frequent the southern plains during spring and forecasters might consider such a storm to be an ordinary run of the mill supercell. The rarity is that this storm occurred over an Arizona desert in the middle of July. The culprit: The North American Monsoon. 

When most meteorologists see the word, 'Monsoon', they almost always associate it with the Indian Ocean Summer Monsoon, which accounts for more than 90 precent of total annual precipitation in central and western India, with Calcutta's annual mean precipitation hovering near the 100-inch mark. A common misconception is that the United States is not directly influenced by a monsoonal regime like those seen in the Eastern Hemisphere. This is far from the truth. 

In fact, the North American Monsoon's impacts are felt across seven states with Arizona receiving the brunt of the activity. Forecasters at the 25th OWS are aware of the auspicious months of June through September when the North American Monsoon creeps slowly northward from the Sierra Madre Occidental and hinders flying and ground operations in Arizona, Nevada, and southern California. Because of the dramatic change in weather that is associated with the monsoon, forecasters are concerned with its onset; however, there is an ongoing debate about when the monsoon officially begins in the southwestern United States. 

As a general guideline the National Weather Service codifies the start of the monsoon season when the average daily dew-point hits 54 degrees Fahrenheit for three consecutive days. Applying this rule, the average start date for the North American Monsoon is July 3 and this year's monsoon season start date was June 28. While this method of determining the onset of the monsoon may be "the standard," many meteorologists believe that synoptic scale circulation is the best indicator. 

To help settle this debate and to better prepare 25th OWS forecasters for the monsoon season, the squadron enlists the help of Mr. Erik Pytlak, the Science and Operations Officer of the Tucson National Weather Service Office. Considered by many to be the North American Monsoon expert within the NOAA organization, Mr. Pytlak firmly believes in the notion that the synoptics pattern is the best indicator of the onset of the monsoon, and he has no problem stating this during the annual North American Monsoon Seminar that he conducts at the 25th OWS each May. 

Southwest regional operations Non-Comissioned Officer in Charge, Master Sergeant Rubi Tornero has experienced numerous monsoon seasons and knows first hand the importance of recognizing this circulation when it develops. 

"The training given by Mr. Pytlak prior to the onset of the monsoon is invaluable," said Sergeant Tornero, "It is imperative that our forecasters understand the impact of the monsoon circulation and its interaction with the local terrain. Failure to do so could severely impact flying and ground operations in our area of responsibility."
While the southwestern United States is typically considered an ideal location for military operations and training, the North American Monsoon serves to counter that assumption. During the eight months preceding the onset of the monsoon, October through May, weather remains fairly quiet. During this period squadron forecasters supporting locations in southern Arizona issue approximately 70-100 watches, warnings, and advisories per month. This number increases nearly five-fold during the monsoon to approximately 400-500 per month in June through September. This summer was no exception as Tucson saw one of the most active monsoon seasons on record. 

Through Sept. 18, Tucson International Airport recorded 10.20 inches of precipitation ranking this season as the sixth wettest monsoon season on record. The highest occurred back in 1964 when an astonishing 13.84 inches of rain fell at Tucson International Airport in a 90 day period. Tucson's unique topography makes forecasting extremely difficult during the monsoon season as seen by the large disparity in precipitation between the valley floor and higher elevations. The city is surrounded by four mountain chains with the largest being the Santa Catalina Mountains to the north and the Rincon Mountains outlining the eastern edge of the city. These mountains have a significant impact on the development and movement of thunderstorms in the region since they often serve to prevent thunderstorms from moving into the valley. For example: Mount Lemmon which sits at 8,940 ft near the top of the Santa Catalina's received an incredible 24.57 inches of rain during the monsoon while the airport in the valley below received only 10.20 inches during the same three month period. Considering that the average annual rainfall for the airport is 12.17 inches; it is easy to see why flooding is a major concerns for forecasters in southern Arizona. 

Along with the destruction seen at Marana-Silverbell AHSF, two major storms impacted Davis-Monthan in July, bringing about a base-wide power outage, damage to a "tent-city" and damage to three sunshades covering A-10 aircraft on the flight line. In both cases OWS - Weather Flight teamwork was extraordinary and decision makers on base were provided ample lead-time to protect aircraft and other valuable assets. In both situations the extent of the damage was less than $20,000, but these events serves as a reminder of how important coordination is between the OWS and supported units to better prepare the base for rapidly changing conditions. 

"Increased vigilance is an absolute necessity when dealing with convection in a desert climate due to the potentially severe effects associated with downbursts. Destructive winds can have a relentless effect on operations; therefore, coordination with our Weather Flights must be timely and accurate in order to enable our supported units to effectively protect their assets," said Sergeant Tornero. 

With such an expansive area of responsibility, the monsoon presents unique and difficult forecast challenges every year for both seasoned and new forecasters at the 25th OWS. A desert climate hardly seems like it would present such difficult forecasting challenges, but in such an unstable environment, conditions can change from tranquil to severe in a matter of minutes. The key to staying on top of the situation is to understand every aspect of the monsoon from its signature synoptic circulation, to its interaction with the local terrain, to its impact on flying and ground operations. There is no doubt that the North American Monsoon will continue to be one of the most operationally significant weather phenomena affecting the United States. With the hard work of the 25th OWS and its supported Weather Flights, military installations across the southwestern US can rest assured they are receiving the best weather support possible.