National Weather Service Introduction to Flooding Jason Elliott Senior Service Hydrologist

National Weather ServiceIntroduction to Flooding

Jason ElliottSenior Service Hydrologist

NWS Baltimore/Washington Open HouseSeptember 22, 2012

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

Today’s Topics

County WarningArea (CWA)Flash Flood◦ 43 Counties in 3 states◦ District of Columbia◦ 12 Independent Cities◦ MD Chesapeake Bay

Hydrologic ServiceArea (HSA)River Flood◦ Potomac Basin◦ Shenandoah Basin◦ Rappahannock Basin◦ West Chesapeake

Areas of Responsibility

13 RFCs Nationwide Focus on river forecasting & modeling

River Forecast Centers (RFCs)

River Gauges

Precipitation Monitoring

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

How do we forecast flood potential?

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

Pre-event assessment

Slow-moving or Stationary Fronts

Favorable Weather Patterns

High Moisture Content◦ Dewpoints & Precipitable Water

Slow-moving & Training (Repeating) Thunderstorms

Favorable Weather Patterns

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

Observations during heavy rain

Remote Sensing◦ Radar◦ Satellite

Precipitation Estimates

Ground truth◦ A picture really is worth a thousand words!◦ Rainfall measurement / flood report / stream measurement

Observations during heavy rain

US 301 / MD 4Upper 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.

Types of Flooding

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

What makes a flash flood?

Flash Flood debris flownear 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.

When does flash flooding occur?

Berkeley Springs, WV flash flood – September 1, 2012

River Flood Forecast Points

Travel Times (in hours)

26

1632

33 4447 36

56 45

6

17

30

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

Coastal Flooding

Annapolis, MD – 2003

Two distinct peaks for positive anomaliesJanuary/February & September/October

September stands out due to tropical systems

Coastal Flooding

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!!!

Floods – a major weather killer

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

Flood Safety

River Road, Falmouth, VAMarch 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

Turn Around, Don’t Drown!

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

tsunamideclarations, a different typeof flood)

Flood Damages

Great Falls, MD1996

Flood Fatalities

Flood Fatalities

Our ProductsOutlook •Hazardous Weather Outlook

•Days 3 - 7

Watch •Flood/Flash Flood/River Flood Watch•Up to 48 hours (up to 72 for rivers)

Warning •Flood/Flash Flood/River Flood Warning•Occurring or imminent

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.

Watch vs. Warning

Our goal is to warn for:◦Where the rain is going to go◦Where the water is going to flow

This meanswe may warnfor placeswhere it willnot even rain!

Basin-Based Warnings

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

Local River Assessment

http://water.weather.gov

Advanced Hydrologic Prediction Service (AHPS)

Jason ElliottSenior Service HydrologistNWS Sterling, VA

[email protected]

Questions?