LAS VEGAS (KTNV) — A research paper examining flooding trends in Las Vegas determined that winter is the most likely time for the worst flooding, and not the summer monsoon.
The research found that since 1950, daily rainfall records in Las Vegas have trended up in the winter and down in the summer. Dr. Guo Yu of the Desert Research Institute led the study, published in January 2023 in the Journal of Hydrometeorology.
Meteorologist Chris Outler, a Lead Forecaster at the National Weather Service office in Las Vegas, says that while flooding happens during both seasons, potent winter weather systems present a unique threat.
"When we get those really strong storm systems where it rains for hours and hours, and it's more than just the sprinkles that we sometimes get for hours, and it comes down at a heavier rate, that's when we start to have more flooding issues," he told KTNV. "We do tend to see that at least once every few years, when we'll have one of those types of storm systems."
In recent memory, the sixth wettest day in Las Vegas weather history happened in winter on January 9, 2018. Officially, 1.33" fell at what is now known as Harry Reid International Airport, drenching the city during the morning commute.
Wet winter storms are typically able to tap into atmospheric rivers, where enhanced streams of moisture are steered from the Pacific Ocean into the Western United States. Southern Nevada usually sees the most rainfall when storm systems swing across Southern California as opposed to when weather-makers approach from Northern California and much of the moisture is intercepted by the Sierra Nevada Mountains.
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The winter of 2022-2023 saw exceptional snow in the Spring Mountains, with a record-breaking 265" of snow at Lee Canyon. This was tied to several atmospheric rivers sending heavy rain across California and heavy snow across much of the Western United States.
Scientists say this uptick in atmospheric rivers is partly due to warmer Pacific Ocean temperatures, driven by climate change.
The findings of Dr. Yu's research suggest that Las Vegas will need to monitor the threat of widespread flooding when sizable weather systems cross the Southwest United States.
When heavy rain does hit Clark County, the Regional Flood Control District says they're prepared. With the Las Vegas valley's population exploding from 35,000 people in 1950 to 2,600,000 people as of the 2020 Census, flood mitigation is a top priority.
Steve Parrish, General Manager and Chief Engineer of the Regional Flood Control District, tells Channel 13 that his agency plans for development that has yet to happen when it comes to building washes, flood channels, and retention basins.
"We realize that the valley has essentially been doubling in population every decade for the last fifty years, and so with that comes development. The more pavement you have, the more rooftops you have, the higher your runoff, so when we design a facility, we design it assuming that everything in that watershed that's develop-able is developed. So we're not designing something that's too small for future conditions," he added. "We want to make sure that as things develop out, things are sized adequately."
On the flip side, the "nonsoon-monsoon" of 2020 is also fresh in our minds, when Las Vegas tallied 240 consecutive days without measurable rain at Harry Reid International Airport. Research says that since 1950, daily rainfall records in summer have trended down.
To be clear: the North American Monsoon from mid-June through September can still deliver heavy rain and dangerous floods to Southern Nevada. Just last summer, nearly 1" of rain on July 28 caused flooding at some Downtown and Strip properties, and necessitated emergency water rescues as people attempted to drive through low-lying flooded areas.
According to the National Weather Service, a variety of atmospheric conditions can influence our summer thunderstorm season in Southern Nevada. "The monsoon can be really fickle, kind of feast or famine," Chris Outler reminds us.
"Especially here in Vegas, we're really on the western edge of things, which tend to favor Arizona more than Southern Nevada. Each monsoon has its own personality, and we can see some summers where we get a lot of flood situations, and others summers go by where we hardly get a drop," Outler adds.
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