A significant winter storm moved through the Northern Plains and Upper Midwest April 1-3, 2020, bringing over a foot of heavy snow, more than a quarter inch of icing, thunder–sleet and -snow, gusty winds, and moderate rainfall over an area already experiencing seasonal snowmelt flooding. The combination of ice, snow, and flooding made for particularly hazardous travel conditions as well.
It was known ahead of the event that mixed precipitation of sleet and freezing rain would cause ice accumulation on the “warm” side of a slow moving front and eventual surface low. Regional NWS offices messaged this threat ahead of time, although there remained some uncertainty of how much and exactly where ice accretion would take place. The FRAM (Freezing Rain Accumulation Model) from SPC’s HREF as well as NBM’s ice accretion percentiles offered useful guidance to forecasters, although painted a very broad threat area including the entire state of Minnesota.
So how did it pan out?
Well, we can turn to the ABI on board GOES-East to help fulfill this answer, specifically the 1.61 um channel known as the Snow/Ice Band. At this wavelength, solar radiation energy is strongly absorbed by snow and ice with little reflected energy travelling back towards the sensor. This is why snow and ice show up relatively dark in the 1.61 um channel compared to snow-free ground and liquid phased clouds. While both are efficient at absorbing in this wavelength, ice still more strongly absorbs radiation than snow, allowing ice to appear even darker than snow. Additionally, wide areas of liquid water are even more strongly absorbed in the 1.61 um. This makes flood waters including major flooding along the Red River of the North and other ice-free lakes very dark. It is possible that darker swaths in southern Minnesota into Iowa may be enhanced from higher soil moisture from rain prior to a wintry mix producing ice.
With a mostly clear sky in place on April 4, we can use these properties to get an idea of where ice accretion occurred. While we are at it, let’s go down the multispectral imagery path and look at the Day Snow-Fog RGB which includes the 1.61 um channel.
Notice a dark red swath extending down the spine of Minnesota, into eastern South Dakota and into Iowa. There is also another area in southeastern Minnesota, again into Iowa, and western Wisconsin. These are areas where ice from freezing rain and sleet accrued. The streak-like nature of this signature within Iowa and southern Minnesota points to convective elements and showery activity leading to widely varied accretions and likely associated impacts telling us this was a very difficult forecast to pinpoint. Overall, guidance from HREF and NBM did well highlighting the general area, although they likely smoothed out these high spatially varied accretions within their ensemble systems.
The Day Snow-Fog RGB utilizes the 1.61 um channel as it’s green component. Less reflectance of snow and ice in this channel leads to lesser green values added to the overall combination. Within the RGB’s red component, the visible-like 0.87 um channel, snow is much more reflective with little or no reflectance from ice accrued areas. These differences make for easily noticeable contrasts between snow and ice, making it easier for forecasters to diagnose areas that experienced overall more ice than snow.
While the darker signature of icing helped forecasters see where icing may have been more prevalent than snowfall, it does not mean this was the only area of icing. Just to the west of the dark red strip in western Minnesota significant icing still occurred as depicted in this USGS photo of ice accrual on a river gage near Fargo disrupting data transmission. However, a transition from wintry mix later to accumulating snowfall lead to accumulated snow hiding the icing signature. The same could be said for patch around southeastern Minnesota.
As the power of an April sun warmed ground temperatures in the Upper Midwest, ice and thinner snowpack quickly disappears on the Day Snow/Fog RGB and CIRA’s Snow/Cloud Layers RGB.
So how is this information useful after the storm? Not only does it increase a forecaster’s situational awareness of potentially highest impacted areas, it can be used as an Impact Decision Support Service tool for illustrating to partners where exactly these conditions occurred. Forecasters at the National Weather Service in Grand Forks, North Dakota, and hydrologists at the North Central River Forecast Center used this imagery to gauge important spatiotemporal characteristics of the associated liquid water equivalent leading to conclusions of which locations would first see this water move into area rivers already undergoing flooding from winter snowpack melt. Additionally, satellite imagery including the Day Snow Fog RGB image above was used in a hydrologic briefing to core partners within the Red River of the North basin, giving them an idea of these important characteristics while using imagery to justify some aspect of updated river forecasts.
NWS Grand Forks