National Weather Service Warnings for Flash Floods and Debris Flows - - PowerPoint PPT Presentation

Building a Weather-Ready Nation

National Weather Service Warnings for Flash Floods and Debris Flows

Alex Tardy and Brandt Maxwell - Meteorologists National Weather Service/San Diego

Building a Weather-Ready Nation

Warning Notification

• Flash Flood Watch – potential for

heavy rain exceeding thresholds today or tonight

• Flash Flood Warning – heavy rain

is occurring or imminent and flooding is expected along with possible debris flow for “defined area”

• Threshold for rainfall (rate per hour)

would be met and burn scar would be mentioned in Flash Flood Warning if affected

W th /S Di

FEMA alert app

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Wireless emergency alerts

TV and radio EAS alerts

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Wireless Emergency Alert in polygon

NWS Warning polygon

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Force and depth of flash flood debris flow

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Weak El Niño Expected This Winter

Both wetter and drier than normal winters have occurred in southern California with a weak El Niño. Flash flooding can occur in southern California regardless of if there is an El Niño, La Niña or neutral conditions.

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Atmospheric River

Jet Stream highest moisture in yellow and red

BIG storms

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Rainfall is widespread but localized heavy

Winter Storm

This type of “embedded rainfall” can cause high rain rates and debris flows

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https://www.wrh.noaa.gov/sgx/kml/holyfire.php

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Flash Flood Warning Debris Flow Thresholds for Holy Fire

Rainfall Duration Rainfall Amount

15 minutes 0.20 inches 30 minutes 0.30 inches 60 minutes (1 hour) 0.50 inches

US Geological Survey

Questions?