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The July 8, 1999 Las Vegas Flash Flood

The July 8, 1999 Las Vegas Flash Flood. Case Review: 8 July 1999 Summary of Southwest Monsoon Event in Las Vegas Valley. Heavy convective rains struck the Las Vegas valley around midday on 8 July resulted in damaging flash floods.

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The July 8, 1999 Las Vegas Flash Flood

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  1. The July 8, 1999 Las Vegas Flash Flood

  2. Case Review: 8 July 1999Summary of Southwest Monsoon Event in Las Vegas Valley Heavy convective rains struck the Las Vegas valley around midday on 8 July resulted in damaging flash floods. More than 2 inches fell in about an hour over parts of the urban areas, and rainfall rates likely exceeded 4 in/h More than 3 inches may have fallen in the terrain favored areas to the northwest and northeast of Las Vegas where the low-level moist flow swept up the slopes of the mountains and the durations of the rain was greater than in the Las Vegas valley. (Matt Kelsch, Kim Runk COMET October2001)

  3. Case Review: 8 July 1999Summary of Southwest Monsoon Event in Las Vegas Valley Monsoon Moisture • Tropical connection evident on the Water Vapor Imagery • Very moist low levels as seen in the surface and 850 mb dewpoints • Moist low-level flow is channeled up the Colorado River Valley in to southern Nevada • Orographic effects focus storms to northeast and northwest of LAS • Possible convergence zone in Las Vegas Valley helps focus strong convection. • Efficient precipitation growth with moist environment and weak to moderate updraft and wind shear. • Inverted 500 mb trof helps to destabilize atmosphere

  4. Case Review: 8 July 1999Summary of Southwest Monsoon Event in Las Vegas Valley Desert Drainage • Natural Drainages are primarily dry washes • Sudden and rapid movement of water immediately following heavy rain is natural • Urbanization plays a critical role in enhancing the severity of the flash flooding • Impermeable urban environments result in more rapid and increasingly complex runoff patterns • Urbanization interferes with natural drainages causes water to surge through city streets • Terrain gradient on the west side of the valley is much steeper than the slope on the east side. Alluvial fan deposits show that flash floods are part of the natural cycle here.

  5. Concluding Remarks • The frequency of significant flash floods in Las Vegas is higher than climatology suggests • As the metro area expands, the impact of such storms will continue to increase

  6. The Monsoon Season in Las Vegas

  7. 2.59” (8/21/57) 1.75” (8/10/42) 1.56” (8/12/79) 1.36” (7/28/84) 1.34” (8/17/77) 1.32” (7/24/56) 1.29” (7/24/55) 1.25” (7/26/76) 3.19” (7/8/99) Blue Diamond Ridge 3.13” (8/10/97) Boulder City 2.24” (9/11/98) Meadow Valley Wash 2.05” (7/19/98) Flamingo Wash 1.89” (9/11/98) California Wash Exceptional Storm Totals At McCarran: Within Clark County:

  8. 3-h radar accum 1902 UTC

  9. Desert Southwest Topography and Radar Accumulation (inches) for 8 July 1999

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