There have been numerous large wildfires across the western US in recent years. After the fire reaches 100% containment, the danger is not over. The wildfire completely changes the hydrology of the landscape due to loss of groundcover and altered soil chemistry resulting in hydrophobicity (Fig 1). Therefore, flash flooding and debris flow are major concerns to life and property over, around, and downstream of burn scars in the years following a wildfire. The characteristics of the burn scar are monitored closely, and rainfall thresholds for the possible occurrence of flash flooding are established and modified over time. Given the considerable threat, forecasters in NWS offices must monitor shower and thunderstorm development closely around burn scars. Not only are they issuing Flash Flood products, but they are in constant contact with core partners regarding the latest developments with regard to the flash flood threat.
Satellite imagery is a vital tool utilized by forecasters when monitoring convective development near burn scars. Considering many of the western fires occur in remote, high-terrain regions, radar coverage is often degraded or not available at all to forecasters (beam blockage, distance from radar), making satellite data even more important to forecasters during such burn scar flash flooding situations. One-minute imagery has particular value in these rapidly evolving situations, allowing forecasters to diagnose boundary interactions and convective trends as early as possible. The example in Figure 2 is from a 2018 Hayden Pass burn scar flash flood event in which 1-min GOES-East VIS was leveraged for tracking convective growth, minute-by-minute, under high level cloud debris near a burn scar in a radar poor region. Forecasters are strongly encouraged to request a mesoscale sector when showers and thunderstorms are forecast in the vicinity of burn scars.
Prior to the development of the first cloud, forecasters monitor water vapor imagery for deep moisture availability and for subtle shortwave troughs that may force downstream convective initiation. Using a 30 July 2021 BOU burn scar flash flood event as an example (East Troublesome and Cameron Peak burn areas), subtle shortwaves could be diagnosed pushing north along the western periphery of a broad central US Ridge and within a stream of enhanced southerly moisture feed (Fig 3). The shortwaves advanced north across CO and eventually into the N CO Rockies, aiding in afternoon/evening convective initiation along the far eastern portion of the moisture stream and within weak mid-upper level flow.
Satellite products, specifically the Day Cloud Phase Distinction (DCPD) RGB, have particular value in the pre-convective environment, when forecasters are using it to monitor cumulus cloud growth and signs for convective initiation. The DCPD RGB captures initial cumulus cloud development (aqua colors), their initial vertical growth and eventual glaciation (aqua > green), and finally convective initiation and rapid vertical growth (green > yellow). By observing the early signs of near-future convective initiation near a burn scar (prior to any radar echoes), forecasters can provide very early alerts to their core partners detailing their latest thoughts on flash flooding risks over the burn scars. Figure 4 from the BOU example shows DCPD RGB evolution prior to the first Flash Flood Warnings. Cu are observed developing along the high terrain through the morning including over the burn scars, with several areas of glaciation developing by 1731 UTC over/near the burn scars. This would be a decision point for forecasters to alert partners of the growing convective threat near the burn scars.
Playing the animation through the afternoon, areas of the cu field continued to glaciate, with heavier rain showers and thunderstorms eventually developing over the burn scars resulting in the issuance of Flash Flood Warnings (Fig 5).
Another DCPD RGB example comes from the Grizzly Creek Burn scar on 03 July 2021. Forecasters monitoring trends in the 1-min RGB imagery across the area acknowledged the increasing potential for near-future development just upstream of the burn scar by 1951 UTC (Fig 6). Initiation had occurred west and east of the burn scar, while more immature cu were beginning to develop just to the northwest (upstream) of the burn scar in an area of untapped instability. Based on the favorable environment, likely storm motion, and trends in satellite imagery, a Flash Flood Watch was issued for the burn area at 2004 UTC.
Glaciation would occur shortly thereafter, followed by the development of thunderstorms and their advancement over the burn scar, per the 1-min DCPD RGB imagery. A Flash Flood Warning was issued for the burn scar at 2104 UTC, primarily based on the development of a lofted thunderstorm core diagnosed in radar imagery. The end result was numerous debris flows over interstate 70 around 2130 UTC.
Similarly and as convection continues to grow, VIS+IR Sandwich imagery is utilized by forecasters to monitor for growing convective threats to burn scars. The imagery maintains the high detail from the VIS, which is important for diagnosing cumulus cloud and storm top trends and details. At the same time, the imagery incorporates quantitative information (IR Brightness temperatures) for forecasters to further assess trends in convective development and heavy rain rate potential. Cooling convective tops represent strengthening thunderstorms, while, just as important, warming tops (and losing texture) signal a weakening trend. The example in Figure 8 occurs during the same time period as that in Figure 5. Areas of rapid cooling and coldest convective cloud tops are easily found in the sandwich imagery, and correlate to greatest heavy rain potential (and FFWs when over burn scars).
Another important use of these GOES imagery products, particularly far from the satellite subpoint, is to track the movement of the thunderstorm updraft. Oftentimes, these summertime thunderstorms over the high terrain develop in areas of weak steering flow, with anvil motion deviating considerably from actual storm movement. Therefore, it is important for forecasters to analyze the movement of the updraft in satellite imagery if possible, as slow moving storms anchored to the terrain pose an elevated flash flood threat. Finally, one must account for parallax when considering the location of thunderstorms with respect to the burn scar.
Bill Line (NESDIS and CIRA)
Input from Klint Skelly (NWS/PUB), Paul Schlatter (NWS/BOU), Michael Charnick (NWS/CYS, previously NWS/GJT)