The July 8, 1999 Las Vegas Flash Flood

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The July 8, 1999 Las Vegas Flash Flood
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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)

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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

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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.

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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

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The Monsoon Season in Las Vegas
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Exceptional Storm Totals
At McCarran
Within Clark County

• 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

• 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)

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3-h radar accum 1902 UTC
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Desert Southwest Topography and Radar
Accumulation (inches) for 8 July 1999
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