national weather service introduction to flooding jason elliott senior service hydrologist nws...
Post on 22-Dec-2015
215 Views
Preview:
TRANSCRIPT
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)
2
616
32
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
jason.elliott@noaa.gov
Questions?
top related