National Weather Service Introduction to Flooding Jason Elliott Senior Service Hydrologist NWS Baltimore/Washington Open House September 22, 2012

 Area of Responsibility  Flood Forecasting / Meteorology  Types of Flooding  Flooding Facts / Safety  NWS Flood Products

 County Warning Area (CWA) Flash Flood ◦ 43 Counties in 3 states ◦ District of Columbia ◦ 12 Independent Cities ◦ MD Chesapeake Bay  Hydrologic Service Area (HSA) River Flood ◦ Potomac Basin ◦ Shenandoah Basin ◦ Rappahannock Basin ◦ West Chesapeake

 13 RFCs Nationwide  Focus on river forecasting & modeling

 Pre-event assessment  Forecast expected rainfall  Consider areas more prone to flooding  Put it all together!

Items to consider:  Wet ground  Current water levels  Rainfall intensity and duration  Topography  Soil (type, moisture, frozen)  Time of year (vegetation, snow cover)

 Slow-moving or Stationary Fronts

 High Moisture Content ◦ Dewpoints & Precipitable Water  Slow-moving & Training (Repeating) Thunderstorms

 Automated observations ◦ Near-real time, but…  Not perfect (could clog or freeze, etc.)  No measurement of how much of the fallen rain is being soaked into the ground, and how much is running into streams

 Remote Sensing ◦ Radar ◦ Satellite

 Ground truth ◦ A picture really is worth a thousand words! ◦ Rainfall measurement / flood report / stream measurement US 301 / MD 4 Upper Marlboro, MD Photo courtesy Prince Georges County EMS

 Flash Flood – A flood which follows within 6 hours of heavy or excessive rainfall, a dam or levee failure, or ice jam break.  (Areal) Flood – A flood which lasts beyond 6 hours after the heavy or excessive rainfall event. Includes small stream flooding.  Coastal Flooding – the inundation of land areas caused by sea waters over and above the level of normal tidal action.

 Typical Flash Flood Characteristics ◦ Caused by convection (thunderstorms or intense showers) ◦ More isolated compared to flooding ◦ Response time is short, usually 1-2 hours, but as little as 15 mins. in urban areas (compared to 6-12 hours for flooding) ◦ Swift Water Rescues, road closures (especially major roads), creeks and streams flood within an hour or two of the causative event ◦ Occurs more frequently in urban areas and in terrain Flash Flood debris flow near Front Royal, VA – 2011

 Can occur anytime – day or night.  Approximately two-thirds of all flash flooding occurs at the night.  Most flash flood fatalities occur at night!  Flash Flooding is NOT restricted to only mountainous areas. Berkeley Springs, WV flash flood – September 1, 2012

 Prolonged onshore flow  Storm Surge from Tropical Systems  Spring Tides: the highest tides in a lunar month, around new moon when the Earth, Moon and Sun are aligned Annapolis, MD – 2003

 Two distinct peaks for positive anomalies January/February & September/October  September stands out due to tropical systems Coastal Flooding at the US Naval Academy June 2005

 Other than heat, floods are the #1 weather killer!  Most flood fatalities occur in cars  Most flood fatalities occur at night Most flood fatalities can be avoided!!!

 Take Appropriate Action!  Better forecasts and earlier warnings won’t help prevent loss of life and property unless people act.  Know your area’s flood risk!  Plan ahead – Identify where to go if told to evacuate. Choose several places (a friend’s home or a motel, or a designated shelter)  Never try to drive, swim, walk, or run through a flooded area  Children should not play in flooded areas River Road, Falmouth, VA March 2011

 If you come upon flood waters, STOP! TURN AROUND AND GO ANOTHER WAY!  Only 6 inches of fast-moving water can knock you off your feet  2 feet of water will float an average size car…less water for smaller cars  Bottom line: never assume there is a safe way to drive through water

 Average: $7.6 billion per year!  2010: $5 billion  In 2011, 75% of all Presidentially-declared disasters were at least in part flood-related ◦ (plus three tsunami declarations, a different type of flood) Great Falls, MD 1996

 WATCH: Conditions are favorable for flooding to occur (>50% chance). Be alert for possible flooding during the watch period.  WARNING: Flooding is occurring or is extremely likely to occur (>75% chance). Stay out of low areas and seek higher ground if necessary!  Note: A watch does not necessarily precede a warning, especially for river flooding.

 Our goal is to warn for: ◦ Where the rain is going to go ◦ Where the water is going to flow  This means we may warn for places where it will not even rain!

 Assessing flood threat at all local stream gauges  Began in September 2011  About 70% complete

 Jason Elliott Senior Service Hydrologist NWS Sterling, VA 