1 Hydrometeorlogical Disasters Matt Kelsch, Hydrometeorologist & Instructor University Corporation for Atmospheric Research (UCAR) Cooperative Program for Operational Meteorology, Education and Training (COMET) Boulder, Colorado Wednesday, 5 September 2001

2 Hydrometeorlogical Disasters How are flash floods different from other weather disasters? Tropical Cyclones/Hurricanes Winter Storms Slow rise floods Severe Weather/Tornadoes What exactly IS a flash flood? Why is the public so vulnerable to flash floods? Why is flash flood forecasting so difficult?

3 Tropical Cyclones: what specific problems do tropical cyclones (hurricanes) present? Like Flash Floods: Increasing population in vulnerable areas Even in relatively “easy” forecast events, uncertainty exists about the precise location likely to be hit hardest Public does not understand uncertainty well Unlike Flash Floods: Objective definitions about the phenomenon (> 65 kt wind) General understanding of location of greatest impact of wind and storm surge (along and near coast) Longer life spans allow tracking and detail monitoring

4 Hurricane Floyd 16 Sep 1999

5 What specific problems do tropical cyclones present? Impact larger areas than severe weather or flash floods Most intense winds and storm surge are likely in a relatively small coastal area near eyewall: Uncertainty can be significant for forecast periods > 24h Coastal communities may need more than 24h to prepare Consequently, warnings take uncertainty into account and typically encompass larger areas than necessary Post-landfall inland rainfall can be extreme

6 In the last 30 years, inland flooding has been the most disastrous part of tropical cyclones Hurricane Camille 1969 (Virginia) –Category 5 storm at Mississippi landfall, but does more damage as a historic rainstorm in Virginia after the winds decrease Hurricane Agnes 1972 (Pennsylvania/New York) Hurricane Tico 1983 (Texas/Oklahoma) –Came in from Pacific and set rainfall records as an ex-hurricane Hurricane Fran 1996 (NorthCarolina/Virginia) Hurricane Mitch 1998 (Central America) Hurricane Floyd 1999 (South Carolina to Massachusetts) Tropical Storm Allison (Texas to East Coast) –Weak storm, but potent rainfall producer

Tar River near Tarboro, NC in the aftermath of Floyd

Tarboro after Hurricane Floyd, Sep 1999

9 Winter Storms: an evolving and complex relationship between nature and society Like flash floods: Problem is from precipitation, not wind damage Societal infrastructure is heavily impacted Risk taking puts people in wrong place at wrong time Unlike flash floods: Winter Storms have objective definitions They generally occur over larger space and times scales (with the exception storms near mountain ranges and in the Great Lakes)

10 Front Range Snowstorm: 8-9 March 1992

11 Winter Storms: an evolving and complex relationship between nature and society Prior to the 1880s, snow was often welcome for commerce because it ensured good sleighing & efficient transportation Then came: Power & communications lines –National security threatened by downed lines during 1888 Blizzard from Washington DC to New York City Inflexible work hours –1888 Blizzard killed > 200 people in NYC commuting to work The automobile, and snow became transportation’s enemy –Transportation SNAFUs like Colorado’s October blizzard of ‘97 –Longer commutes, expensive & difficult road maintenance

NOOA/LDAD Display: Eastern Colorado Blizzard, October 1997

13 Main Stem River Floods Like flash floods: Too much water for natural or human-made channels Societal infrastructure heavily impacted, can enhance the problem (dam and levee failures, blockages at bridges) Risk taking puts people in wrong place at wrong time Toxic materials, fires Unlike flash floods: Occur over larger space and times scales—allows preparation time Victims drown in river floods, but may suffer fatal traumatic injury in flash floods

April 1997 Red River Flood, Grand Forks, ND

15 Severe Weather: rapid evolution of dangerous weather phenomena Like Flash Floods: Rapid storm-scale evolution makes for very difficult forecast problem Small scale nature difficult to convey to the public Unlike Flash Floods Objective definitions for threshold and verification –50 kt winds and/or 0.75” diameter hail –Tornado Trained spotters help with field reports Reasonable correlation between meteorological data (radar/satellite) and severe weather

16

Kansas City Brush Creek Flash Flood and Severe Winds 4 October Oct UTC 4 Oct UTC

18 When severe weather turns into a hydrologic disaster Severe weather can evolve into a flash flood situation, especially in urban environments where it does not take as much rainfall Cheyenne 1985 Dallas 1995 Kansas City 1998 Cheyenne, WY, 2 August 1985

19 Flash Flood: what is it? A flood where the rainfall and the subsequent runoff are occurring on the same time and space scales. The problem is the rapid change in floodwater momentum, not the final depth, velocity, or extent. Fort Collins, CO, 28 July 1997

20 Flash Flood: what is it? Wall of Water? These typically only occur when a structural failure is involved Can Define by Velocity It’s really the rapid change in velocity rather than the actual value that’s important Cheyenne,WY, 1 August 1985

21 Flash Flood: why are they so difficult to forecast and warn for? Unlike severe weather, hurricanes, and winter storms: Involves important and complex non- meteorological processes No reliable relationship between meteorological data (rainfall) and flash flood potential Hydrologic response may dominate, causing major flash floods with relatively unimpressive rainfall –Especially small basins, steep basins, urban basins, and altered basins (fire scars deforestation)

22 Flash Flood: why are they so difficult to forecast and warn for? Unlike hurricanes, winter storms, and slow-rise floods: Occurs on very small time and space scales In fast-response basins, the rainfall may not be the most important factor Cheyenne, WY, August 1985

Avg ~5 in/hr Avg ~8 in/hr Accum.: inches Basin Size: 5-50 miles 2, avg=18 miles 2 Rainfall Rates: 3-12 inches/hr

24 Rapidan, VA Big Thompson, CO Las Vegas, NV

26 Flash Flood: Public Vulnerability Rainfall is necessary for life– water just doesn’t seem that threatening Difficult to know when a flood goes from a nuisance to a threat Unrealistic feeling of safety in our vehicles, especially SUVs!

27 What are the typical scales of the flash flood process? Almost always < 75 km 2 (30 mi 2 ), and sometimes less than 25 km 2 (10 mi 2 ). Less than 6 hours, and typically the real intense precipitation burst and subsequent flash flood are occurring in the 1-2 hour time frame. Rainfall rates >150 mm/h (6 in/h), perhaps only ~ 100 mm/h (4 in/h) dry climates.

1-h Accumulation, 12 July 1996 Purples=50-75mm  Fire Scar Buffalo Creek, CO

Omaha, Nebraska 0500 UTC 7 Aug UTC 7 Aug 1999

Fort Collins, CO Flash Flood 28 July 1997

36 Timetable of flash flood event 5:30 8:00 8:30 9:00 9:40 10:30 11:00 1:20 Rain begins EOC Activated Ponds Overflowing, rapid water rescues begin Most intense rain commences NWS Warning Storm begins to dissipate and move northeast Trailer Park Flooding, Fires, Train Derailment Declared City Disaster

37

October 1999 Hydromet class at Spring Creek

43 Hydrometeorlogical Disasters It Doesn’t Have to Rotate to Kill Ya! Flash Floods take more lives in a typical year than Severe Weather, Hurricane Winds & Storm Surge, or Winter Storms Flash Floods involve complex and rapidly evolving meteorological and hydrologic processes– difficult to define with objective quantitative criteria Floods are not viewed by many as being as violent as tornadoes or hurricanes