Podcast Title: Off the Radar
Host: Emily Gracey, The National Weather Desk
Episode: Deadly Debris Flows – The Flood After the Fire
Release Date: May 13, 2025

Introduction to Post-Wildfire Hazards

In the episode titled "Deadly Debris Flows – The Flood After the Fire," Meteorologist Emily Gracey delves into the often-overlooked dangers that follow the containment of wildfires. While communities may breathe a sigh of relief as flames subside, new threats emerge when rainfall interacts with the charred landscape. Emily sets the stage by painting a vivid picture:

"Imagine this... a destructive wall surging downhill at 35 mph. Massive boulders tumble through neighborhoods... all unfolding with terrifying speed." [00:00]

She underscores the critical transition from wildfire to subsequent hazards, emphasizing the need for advanced forecasting tools to mitigate these risks.

Understanding Debris Flows and Their Impact

Emily introduces Dr. JJ Gorley, a research hydrometeorologist with NOAA's National Severe Storms Laboratory (NSSL), as her guest to unpack the complexities of debris flows post-wildfire.

What Are Debris Flows?

Dr. Gorley explains that after a wildfire, the removal of vegetation disrupts the landscape in multiple ways:

1. Increased Runoff Volume:
   "There's going to be a little bit more volume of water that's going to be entering the system." [04:00]

2. Higher Water Velocity:
   Without vegetation to slow down water flow, runoff becomes more rapid.

3. Soil Instability:
   The loss of root structures makes soils prone to mobilization, leading to debris flows.

Additionally, Dr. Gorley highlights the phenomenon of hydrophobic soils, where the intense heat of fires causes soils to repel water, further exacerbating runoff issues:

"So now the soils are not as capable as acting as a normal buffer... it's going to transfer to runoff almost immediately." [04:00]

Geographical Areas at Risk

While mountainous regions are the primary hotspots for significant debris flows, Dr. Gorley points out that flash floods can occur in a variety of terrains:

"Just like in Mother Nature... we have a lot of these situations." [06:21]

He notes that even though large debris flows are more common in elevated terrains, flash floods and sediment-laden flows can affect areas downstream, resembling "chocolate milk" in appearance.

Timeline of Post-Wildfire Hazards

When asked about the critical period for debris flow risks, Dr. Gorley emphasizes that the hazard starts almost immediately after a wildfire:

"It's almost immediate... it triggered a deadly debris flow that was more deadly than the fire itself when the fire was still active." [08:06]

He outlines a general timeline, suggesting that the highest risk persists for up to five years post-fire, with the first few years being the most active. However, factors like climate and ongoing droughts can extend this period significantly.

NSSL's Innovative Use of Mobile Radars

A significant portion of the episode focuses on NSSL's pioneering approach to predicting debris flows using mobile radar technology traditionally employed for severe weather tracking.

Deployment of Mobile Radars

Dr. Gorley describes the utilization of NOAA's X-band Polarimetric Radar (NOXP):

"We set up ... a number of instruments... and then the NOXP radar was about 30 km parked away in a state park." [10:43]

By positioning these radars near burn areas, NSSL achieves high-resolution rainfall data essential for forecasting debris flows. The mobility and adaptability of these radars allow for precise monitoring in areas where fixed radar systems like the NEXRAD network fall short, especially in mountainous terrains.

Testing and Implementation

In a case study partnership with the USGS in Colorado, NSSL established a post-fire hydrometeorological observatory. This setup included various instruments like rain gauges, soil moisture sensors, and non-contact stream radars to monitor vulnerable streams. Real-time data feeds enabled local authorities to receive instant alerts:

"If certain thresholds were exceeded, then the local emergency management official... would receive a text that said hey, the stream has risen..." [16:15]

Advantages Over Traditional Systems

Dr. Gorley highlights the superior spatial resolution of mobile radars compared to the Multisensor Rainfall Estimate (MRMS) system:

"With Innox P and it's just this beautiful depiction of the rainfall and the gradients that it captures it... it's just stunning." [23:08]

This enhanced detail allows for more accurate and timely forecasting, which is critical for issuing warnings and implementing safety measures.

Integration of Drone Technology

Expanding on technological advancements, Dr. Gorley discusses the integration of Unmanned Aerial Systems (UAS), commonly known as drones, into their monitoring toolkit:

"We have the ability to map out the terrain with the drone... get a very high-resolution digital elevation model." [17:45]

Drones equipped with hyperspectral cameras and thermal infrared sensors provide comprehensive data on terrain changes, vegetation health, and water temperatures. This information is invaluable for assessing the ongoing risks and effectiveness of mitigation efforts.

Prevention and Mitigation Strategies

While forecasting remains paramount, Dr. Gorley addresses the importance of proactive measures to prevent or mitigate debris flows:

"They can physically put this fencing up... that'll capture the debris whenever it goes by." [19:56]

He emphasizes that while engineering solutions like debris fences can be effective, community awareness and preparedness are equally crucial. Improving forecast products and educating the public ensures that individuals can take timely actions to protect themselves.

Future Projects and Innovations

Looking ahead, Dr. Gorley shares exciting developments, including the upgrade of mobile radars with new capabilities:

"There will be some major upgrades to these radars... ready in mid September... deploy in January time frame." [21:32]

These upgrades aim to enhance data collection and expand the scope of monitoring to new burn areas, potentially covering regions prone to atmospheric river events and other significant weather phenomena.

Public Awareness and Safety Tips

In his closing remarks, Dr. Gorley cautions the public against complacency post-wildfire:

"Don't let your guard down... It doesn't take a lot of rain... just a really common thunderstorm... enough to trigger debris flow." [24:22]

He advises residents in affected areas to remain vigilant, stay informed through official channels, and heed warnings promptly to ensure their safety during these unpredictable events.

Conclusion

Emily Gracey's episode on "Deadly Debris Flows" provides a comprehensive exploration of the hidden dangers that follow wildfires. Through insightful discussions with Dr. JJ Gorley, listeners gain a deeper understanding of debris flows, the innovative technologies aiding their prediction, and the essential steps communities can take to safeguard against these catastrophic events. The episode underscores the critical intersection of meteorology, hydrology, and community preparedness in addressing the multifaceted challenges posed by post-wildfire environments.

Notable Quotes:

• "Imagine this... a destructive wall surging downhill at 35 mph." — Emily Gracey [00:00]
• "There's going to be a little bit more volume of water that's going to be entering the system." — Dr. JJ Gorley [04:00]
• "Don't let your guard down... It doesn't take a lot of rain... just a really common thunderstorm... enough to trigger debris flow." — Dr. JJ Gorley [24:22]

For more information on debris flows and ongoing research projects, visit the National Severe Storms Laboratory website under NOAA's National Oceanic and Atmospheric Administration.