Alluvial Fan Flooding Workshop Barry Pierce National Weather Service, Las Vegas Death Valley NP, August 15, 2004
Outline National Weather Service Alluvial Fan/Flash Flooding Weather Regimes Conducive For Flash Flooding in Southern Nevada Review Local Events National Water Center
Mission and Vision Provide weather, water, and climate data, forecasts and warnings for the protection of life and property and enhancement of the national economy. A Weather-Ready Nation: Society is Prepared for and Responds to Weather-Dependent Events
Who We Are 122 Weather Forecast Offices located in the US and its territories 9 NCEP offices which provide special services on a national level (Storm Prediction Center, Climate Prediction Center, Tropical Prediction Center) 13 River Forecast Centers 21 Central Weather Service Units 6 Regional Headquarters Federal government Department of Commerce National Oceanic and Atmospheric Administration
Las Vegas Area of Responsibility CWA ~81,000 square miles Incredibly diverse terrain Highest point in the CONUS (Mt. Whitney) Lowest point in the US (Death Valley) Population served: approximately 2.5 million
Weather Forecast Office (WFO) Issue all Local Warnings, Watches and Advisories Build and Maintain Relationships With Local, State, and Federal Partners Provide Expert Advice to Emergency Operations Centers Solicit Customer Feedback on Products and Services Conduct Community Awareness and Education Programs Provide 7 Day, Aviation, Fire Weather and Marine Forecasts
Perfect Storm Southern Nevada s topography, geology and weather regime align to occasionally create intense flash floods. Depending on the specific location these flash floods can cause millions of dollars in damage and be killers.
Alluvial Fans Common land forms in the southern Great Basin and Mojave Desert, result of flash flooding or water erosion. Sediment and debris deposits form in a fan shape as floods and gravity carry debris down slope. Distance the material is carried down slope is related to the carrying capacity of the flood water.
Flash Floods A flash flood is caused by heavy or excessive rainfall in a short period of time, generally less than six hours. Flash floods are usually characterized by raging torrents after heavy rains that rip through river beds, urban streets, or mountain canyons sweeping everything before them. They can occur within minutes or a few hours of excessive rainfall. In the U.S. floods kill more people each year than tornadoes, hurricanes or lightning.
Flash Floods Densely populated areas are at a high risk for flash floods. The construction of buildings, highways, driveways, and parking lots increases runoff by reducing the amount of rain absorbed by the ground. This runoff increases the flash flood potential. Normally dry washes through cities and towns are routed underground into storm drains. During heavy rain, the storm drains can become overwhelmed and flood roads and buildings. Low spots, such as underpasses, underground parking garages, and basements can become death traps.
Weather Regimes Conducive for Flash Floods North American Monsoon (Warm Season) Atmospheric River (Cool Season)
North American Monsoon
North American Monsoon Over our area, the monsoon circulation tends to move northward from Mexico in late June into Arizona and New Mexico eventually spreading into our area in early July. However, the full effects of the monsoon can start in the middle of June or hold off until the end of July due to year-to-year variability. The monsoon season is not a continuous period of thunderstorm activity in our region. Instead, there will be active periods and also periods with no thunderstorm activity. Typically, July through September are the months most likely to have flash flooding due to abundant moisture in the atmosphere and fully established monsoon high.
Thunderstorm Ingredients Moisture Surface Dewpoint >55 o F; Precipitable Water > 1 Instability Convective Available Potential Energy (CAPE), Lifted Indices Trigger upper level disturbance, jet streak Storm Motion Rainfall Thresholds 0.5 in 30 minutes 1.0 in 30 minutes
Typical Monsoon Start Dates and Importance to Climatology Seasonal distribution of precipitation. Broken line represents area of 50%+ of annual precipitation during July, August, and September.
Rainfall Characteristics of the North American Monsoon This chart shows in percentage the contribution of rainfall from the months of July through September to the annual rainfall total. Southern Nevada typically sees between 20% to 35% of the annual rainfall during this three month period.
Typical Moisture Sources for the Monsoon Low-Level Moisture Sources
Thunderstorm outflows Outflow boundaries from previous day s thunderstorms can be a significant source of low-level moisture. Over a series of days this additional moisture from outflows can moisten up the low-levels considerably This moisture typically will move in from the east or southeast especially overnight.
Gulf of California Surges A Gulf of California moisture surge is a low-level flow of moist, relatively cool air that moves northward over the Gulf of California and into the Southwest. It is induced when higher pressure is located in the southern Gulf of California and lower pressure is over our area. This is a primary source of lowlevel moisture for monsoonal convection over the Southwest U.S.
A less frequently observed pattern for a Gulf Surge is when a tropical storm or hurricane is located near the southern tip of Baja California. Strong flow around the tropical system pushes moist air northward into the Gulf and continues toward the thermal low in the deserts of the Southwest. Gulf Surge (Cont.)
2014 Tropical Cyclone Tracks
Convective Initiation Areas
Atmospheric Rivers
Atmospheric River (ARs) Facts ARs are relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapor outside of the tropics. ARs tied closely to both water supply and flood risks, particularly in the Western U.S. In the strongest cases ARs can create major flooding when they make landfall and stall over an area. On average ARs are 400-600 km wide. A strong AR transports an amount of water vapor roughly equivalent to 7.5 15 times the average flow of liquid water at the mouth of the Mississippi River. On average, about 30-50% of annual precipitation in the west coast states occurs in just a few AR events a season.
AR Climatology Jon Rutz, Jim Steenburgh, Marty Ralph and Michael Alexander
AR Climatology Jon Rutz, Jim Steenburgh, Marty Ralph and Michael Alexander
Local Events August 13, 2003 August 25, 2013
August 19, 2003 Gulf Surge moved rapidly northward into southern Nevada that morning The increase in low level moisture enhance the Convective Available Potential Energy (CAPE) across Las Vegas Thunderstorms over the Spring Mountains propagated southeast into NW Las Vegas intensifying as they moved into the more unstable air mass
Northwest Las Vegas 1990 (Storm Total Rainfall)
Northwest Las Vegas 2003 (Storm Total Rainfall)
August 19, 2003 One station reported 1.02 in fifteen minutes, several stations in excess of 2 in 2 hours. Those rainfall intensities easily exceeded the 100-year average recurrence intervals for Las Vegas (Source: NOAA Atlas 14 Point Precipitation Frequency Estimates) Las Vegas Metro Police, 9 swift water rescues by air; with nearly 60 rescues over 3 hour period Small Business Administration estimated total damage at $4.8 million. 251 homes and 173 businesses sustaining damage City of Las Vegas estimated $1.2 $1.3 million in damage to public facilities, Clark County estimated $330,000 in roadway damages
August 19, 2003
August 25, 2013
August 25, 2013 Third and most severe rainfall event in 6 week span following the Carpenter 1 Wildfire Highway 157 (Kyle Canyon Rd) damaged, Highway 95 closed during the event, and many other roads in the area sustained damage Storm runoff and debris along with outflow from the Kyle Canyon Detention Basin caused significant inconvenience to residents in the area for days as streets were largely impassable
August 25, 2013
National Water Center Facility that will enable NOAA, in partnership with other federal agencies, to deliver a new generation of water information and services to the nation National Water Center includes employees from NOAA, USGS, FEMA, Visiting Scientists and Contractors Some Activities Water resources forecasting operations center- National Water Model Applied water resources research and development center Proving ground for transitioning research into operations Geo-intelligence facility Airborne snow and soil moisture observation analysis facility
Questions?