The July 8, 1999 Las Vegas Flash The July 8, 1999 Las Vegas Flash FloodFlood

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)

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

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.

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

The Monsoon Season in Las Vegas

Exceptional Storm Totals

• 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

At McCarran: Within Clark County:

3-h radar accum 1902 UTC

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