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National Weather Service Policy on Floods, Debris Flows, and Landslides - PowerPoint PPT Presentation

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National Weather Service Policy on Floods, Debris Flows, and Landslides. Patrick D. Moore National Weather Service Greenville-Spartanburg SC. NWS Policy is Set By…. National Headquarters Regional Headquarters Local Weather Forecast Offices.

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National Weather Service Policy on Floods, Debris Flows, and Landslides

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National Weather Service Policy on Floods, Debris Flows, and Landslides

Patrick D. Moore

National Weather Service Greenville-Spartanburg SC

NWS Policy is Set By…

  • National Headquarters

  • Regional Headquarters

  • Local Weather Forecast Offices

The NWS Informs the Public of Weather Hazards Using a Multi-tiered Concept

Outlook – Watch – Warning


NWS Multi-tiered Concept

  • Outlook (Hazardous Weather Outlook – HWO)

    • Issued when an event may develop

    • Typically more than 48 hours before onset

    • Confidence 30-50%

  • Watch (Flash Flood Watch – FFA)

    • Issued when the expectation of an event has increased, but the occurrence, location, and/or timing is still uncertain

    • Typically 6-48 hours before onset

    • Confidence 50-80%

  • Warning (Flash Flood Warning – FFW)

    • Issued when an event has a very high probability of occurrence, is imminent, or already occurring

    • Out to 6 hours

    • Confidence > 80%

Flood Types

  • Flash Flood *

    • Occurs within six hours of the onset of precipitation.

    • Rapid rises, life-threatening

  • Flood

    • Occurs more than six hours after the onset of precipitation.

    • Relatively slow rises, threatens mainly property

  • River Flood

    • Occurs more than six hours after onset of precipitation, sometimes days after precipitation ends

    • Used for main stem rivers with gaging stations

Top image Pickens Sentinel, Bottom image Asheville Citizen-Times

*For tropical systems, GSP will issue Flash Flood Warnings

Other Flood Products

  • Advisory

    • Issued for an event which will cause inconvenience but not necessarily threaten life

  • Statement

    • Issued as a follow-up to a Warning or Advisory to provide more information

John D. Simmons, Charlotte Observer

The National Weather Service DOES NOT Issue Warnings for Debris Flows and Landslides!!!

Photo by Jonathan Lamb, NWS

The Problem With Debris Flows…

  • The NWS does not have knowledge of the underlying geological conditions or geomorphic processes related to debris flows

  • The NWS has no way of knowing what slopes are in danger of failing or if a slope has already failed, with the exception of…

    • Burn scars

    • Identified unstable slopes

Top photo Jonathan Lamb, NWS, bottom photo Patrick Parton, Haywood County

Debris Flows, Landslides and NWS Products

  • The expectation or occurrence of a debris flow may form the basis for a Flash Flood Warning

  • NWS Flood products may contain information about possible landslide activity

  • However, the expectation of landslide activity will not, in general, trigger a Flood or Flash Flood Warning.

Flash Floods Depend On…

  • Rainfall intensity and duration

  • Antecedent conditions

    • stream flow

    • soil moisture

  • Shape of stream channel

  • Topography and land use

John D. Simmons, The Charlotte Observer


  • Are related to periods of heavy rain

  • Do not necessarily occur in stream beds or flood plains…

Jonathan Austin, Yancey County News

The Knoxville Journal

Intense Rainfall and Landslides

  • Most of what we know has come from the Geological community (NCGS, USGS)

  • Research has determined a general relationship between rainfall amounts and landslide activity

Rick Wooten, NCGS

General Forecast Considerations for Landslides in Western North Carolina

Five Inches of rain in 24 hrs

Ten inches of rain in 24 hrs

Widespread slope movement events

High antecedent moisture conditions followed by high intensity rainfall

  • Localized slope movement events

Rainfall rates of 3.5 inches to 4.0 inches per hour dramatically increase the chance of landslides

The Problem With Slope Failures…

  • The rule of thumb between rainfall amounts and landslides sometimes breaks down in the case of modified slopes

  • The NWS has no knowledge of modified slopes across the mountains of NC

Landslides and NWS Products

  • “Calls-to-action” will be inserted if forecast or observed rainfall exceeds ~ 5 inches in 24 hours

  • Flood/Flash Flood Watches

    • If antecedent conditions are moist

  • Flood/Flash Flood Warnings

    • High rainfall rate

  • Flood Statements and Advisories

    • If only minor flooding is expected

    • Updates to ongoing flood situations

Landslide “Calls-To-Action”

  • For a Watch

  • For a Warning


Image by Arthur Raynolds

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