Flooding within the Red River of the North basin is practically a yearly rite of passage for those living there signaling the exit of winter and coming of summer. This area straddling the North Dakota and Minnesota border is well prepared for river and overland flooding because of its frequency. Spring of 2020 is proving to be no different with major flooding occurring over much of the Red River and its tributaries due to seasonal snow melt.
It can be difficult to keep a pulse on how impactful flooding really is with a lack of comprehensive reports in an ever-evolving and fluid situation. Plus with extensive flood mitigation systems in place, the majority of floods that occur in this area are nothing more than just an inconvenience for most people. Still for the minority that have property impacted by flooding, it can be quite notable and our services can be tailored to them, or at least towards the emergency services aiding them. Additionally, a threat to life can come to fruition if one finds themselves in very cold floodwaters (an example could be a car sliding off the road into floodwaters). This is ultimately our mission as National Weather Service meteorologists: to protect life and property.
This post will explore the satellite imagery that aided NWS Grand Forks in flood operations in mid-April 2020. In this case, flooding was due to springtime snow melt, but the following procedures and workflows may be applicable to warm season convection as well.
On April 10, 2020, moderate to major flooding was occurring along the Red River and its tributaries within the central and northern basin. Wondering if there were any areas experiencing impactful flooding outside of current flooding warnings, forecasters initially turned to GOES-R ABI River Flood Products for help in highlighting areas of observed floodwater coverage. Overlaying flood warning polygons, forecasters then looked for areas highlighted by the ABI River Flood Extent product not encompassed by polygons. This was the case in western Polk and Marshall counties, Minnesota, as noted in Figure 1. Values higher than 60% (orange and red coloring) were of particular interest as lower values into the 30-50% range were believed to be that of non-impactful standing meltwater in agricultural fields. While the ABI River Flood Products are updated every hour, the ABI’s spatial resolution of 1 km can smooth out the spatial extent of potentially impactful flood waters. VIIRS offers this same imagery at a finer resolution of 375 m, but at the expense of one or two images per day which require a daytime, cloud-free sky to provide useful information.
Luckily this area of interest was mostly cloud-free during one of the VIIRS passes. It confirmed higher percentage values in the same areas of interest, particularly in northwest Polk County, Minnesota (Fig 2).
There is additional satellite imagery available to further provide details on floodwaters and its potential impacts. Higher resolution satellite imagery down to 10 m from Sentinel-2 and Landsat 8 has recently become available on the web and already processed for quick viewing at sites like Sentinel-Hub and Remote Pixel. A timely, cloud-free pass from the Sentinel-2 satellite over the area of interest was available to forecasters for interrogation (Fig 3). It revealed extensive break out water from the Red River north of Grand Forks, North Dakota, and surrounding Oslo, Minnesota, although these areas were well within the flood warning polygons. What about the other areas of expansive meltwater in agricultural plots between Alvarado, East Grand Forks, and Tabor, Minnesota? Sentinel-2 imagery hinted that some of these floodwaters might be over roads and surrounded farmsteads. These areas of adjacent flooded plots of land correlated nicely with higher percentages within the ABI and to an extent VIIRS River Flood Extent products, thus confirming impactful flooding would possible here.
Forecasters took this approach of using satellite imagery to hone in on targeted areas for intel gathering of flood impacts. Looking for road closures on state Department of Transportation and county websites, data mining on social media, and calling emergency managers confirmed impactful flooding in these areas. Thus, these areas could warrant a flood headline. Ultimately the decision was made to not issue an Aerial Flood Warning as the majority of these areas fell just within current flood warning polygons.
So does the application of this imagery stop here? Not quite. ABI and VIIRS River Flood Extent products highlight nicely the extent of observed floodwater. In this case it was used as a source for the graphic in Figure 4. The imagery answers the “where” and “when” of flooding within the graphic while the photo depicts the impact, capped off with safety messaging. This imagery also yielded high confidence in overland flooding impacts which was messaged via the Hydro section within the Area Forecast Discussion from NWS Grand Forks. Additional imagery analysis was conveyed to the North-Central River Forecast Center which was useful in assessing the current state of snowpack, river and ditch ice, and floodwater expanse.
In summation, satellite imagery from ABI and VIIRS River Flood Products as well as from Sentinel-2 provided excellent details in gauging flooding impacts from river and overland flooding. Starting with the most coarse spatial resolution, yet highest temporal resolution imagery like ABI was a good starting point in searching for floodwaters over a broad area. Then, incrementally honing in on highest flood extent signals at higher spatial resolution imagery proved to be a good workflow in pinpointing areas of interest to target intel gathering of current impacts from flooding. Furthermore, this imagery was useful in messaging through graphics, discussions, and collaboration.
Did you know Nighttime Microphysics RGB can by used to diagnose floodwaters (Fig 5)? The highest contribution of sensed floodwater comes from the Split Window Difference product. In this case, there was high contrast between lower values of liquid bodies of water and higher values of land. However, due to dependence on sensed infrared energy, seasonal and air mass differences can change the appearance of floodwaters within the RGB. You can adjust the RGB to help draw out this floodwater signal from the Split Window Difference as well.
Besides high resolution imagery from Sentinel-2 and Landsat 7/8, cloud penetrating Sentinel-1 offers similar resolution imagery while capable of detecting liquid water versus snow and bare ground (Fig 6). Although, it does take a more careful eye in interpretation of the imagery, especially when looking at “waterlogged” snowpack which can give a false signal for liquid water. This type of imagery can be extremely useful when pesky clouds prevent imaging radiometers from sensing the ground.
NWS Grand Forks
Bill Line, NESDIS and CIRA