Debris Flow and Flash Flood Impact Decision Support Services within a Postwildfire Environment

On July 16th, 2017 the Detwiler Fire, ignited across rural Mariposa County in California. The fire grew explosively from a couple thousand acres within a few days, eventually surpassing the 70,000 acre mark at its peak. During this time, several communities had to be evacuated with several homes threatened. The fire was contained within a month, although it left behind a scorched landscape. The worst of the burn severity, according to the California’s Department of Forestry and Fire Protection’s (CalFire) Watershed Emergency Response Team (WERT), occurred across the Merced River in the northern half of the burn area.

Forecasters at the National Weather Service (NWS) in Hanford were able to utilize GOES-16 fire products, to communicate possible changes of spread and intensity with the Incident Meteorologist on site. One minute data was available from GOES-16 after a mesoscale sector was successfully requested through the Monterrey Weather Forecast Office for an extended period of time.

The worst of the burn severity, according to the California’s Department of Forestry and Fire Protection’s (CalFire) Watershed Emergency Response Team (WERT), occurred across the Merced River in the northern half of the burn area. While most burn areas have some level of hydrophobicity, the intensity of the Detwiler Fire left behind a soil base which had a higher degree of hydrophobicity than usual. This factor, combined with antecedent conditions, resulted in a region which was extremely vulnerable to debris flow, mud slides and flash flooding, which unfortunately eventually occurred on March 22, 2018.

The focus of the presentation will be on the post wildfire environment and how decision making tools such as the High Resolution Rapid Refresh (HRRR), National Weather Service (NWS) Western Region’s (WR) HRRR Post-Fire Debris Flow tool, and NWSChat were used to increase Impact Decision Support Services (IDSS) during this March 20-22nd atmospheric event across Mariposa County, the Detwiler Burn Area, and Yosemite National Park. Specifically, the HRRR Debris Flow tool suggested that 50% of the basins within the scar had a high debris flow potential, if 0.38" fell in 15 min.

A Narrow Cold Frontal Rainband (NCRB) combined with the incoming Atmospheric River to easily exceed these suggested rates. The NCRB was in place during the early morning hours near Sacramento, and was accurately predicted by the HRRR to impact the Detwiler Burn Scar in the afternoon. Many partners were warned of this possibility through NWSChat and by direct contact early in the day, which was a good example of IDSS. This presentation will also focus on the outcomes from this life threatening event, including how the county and National Park Service used the information provided to effectively evacuate the area prior to the greatest streamflows, to ensure minimal impact on the community’s citizens.