In the last three weeks, Flagstaff’s rain and snow from a series of storms increased Upper Lake Mary’s drinking water capacity from 25 percent to 73 percent.
With more rain forecast for this weekend, Brad Hill, director of Flagstaff Water Services, said they expect the lake to fill and even spill this year. The last time the dam spilled was in 2017. Upper Lake Mary has a capacity of 5.3 billion gallons of water, which can provide water to the city for three to four years.
Record-breaking snowfall dropped over 40.8 inches at Flagstaff Pulliam Airport between Feb. 21 and 22, triggering states of emergency from both the city and Coconino County. The week before, warmer rain in the northern part of the state combined with colder snowmelt flooded parts of Coconino and Yavapai County and led to water in Oak Creek cresting over 12 feet and damaging many parts of the counties.
The record-breaking precipitation has also led to a significant amount of water flowing into Upper Lake Mary.
“We have seen peak flows entering Upper Lake Mary this month in Newman Canyon as high as 359,000 gallons per minute,” Hill said.
The increase in water is welcome news after last year’s dry winter did little to assist the lake’s water levels. Hill said that at the end of February last year, the lake was filled to 53 percent.
Winter is an important time to increase water reserves in the lake as the city’s average water use ramps up in the summer.
“Our winter average [use] this past month is approximately 6 million gallons per day, as compared to our summer average use last July [that] was 9.2 million gallons per day,” Hill said.
Summer tends to bring rain through monsoon, which has led Flagstaff Water Services to begin ramping up the amount of Upper Lake Mary surface water they use during the hot dry months of May and June.
“Given that we anticipate Upper Lake Mary to fill and spill this year, we have already increased surface water usage,” Hill said. “Today it’s approximately 36 percent of our total water supply.”
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Increasing the city’s dependency on Upper Lake Mary allows the city’s underground water resources to collect more water, Hill said.
Emily Thornton, meteorologist at the National Weather Service, said the weather service is not expecting rain and snow flooding events this weekend that would be similar to the Oak Creek flooding that happened earlier this month.
Despite the deep snowpack, Thornton said that the weather leading up to the rain will help mitigate rain and snow flooding risks.
“We did have a lot of heavy snow, but we’re having several warm days between the snow and rain event with temperatures in the mid-60s to 50s,” Thornton said. “With the sun being out, the snow will melt more gradually instead of a quick melt.”
Additionally, early models suggest that Flagstaff will only get a quarter to a half an inch of rain this weekend. Thornton said they expect the rain to start near noon on Saturday and end near noon on Sunday.
“By the time we get to the next rain event, the amounts are minimal,” Thornton said. “Certainly we won’t be seeing the same rainfall intensity. Any melt will be gradual and any runoff will be manageable.”
Thornton added that their understanding of the amount of expected rain could change as the storms get closer.
“If things were to change, which is unlikely, we would communicate that,” Thornton said.