Arctic blast turned US white
19 February 2021 12:00-22:00 UTC
A band of ice from freezing rain over Texas could be clearly seen in RGB imagery on 19 February 2021.
10 June 2022
26 February 2021
By Jochen Kerkmann (EUMETSAT), Ivan Smiljanic (CGI), Bill Line and Daniel Lindsey (CIRA), Scott Bachmeier (CIMSS)
Earlier in mid-February a southward surge of arctic air brought severe sub-zero conditions to a lot of the US (except the east and west coasts) — surface temperatures in Minnesota on 13 February plunged to -45°C (-50ºF). Rare severe winter weather even affected cities such as Houston, Dallas, New Orleans and Jackson (Figure 1). Only the south-eastern States (Georgia, Florida, The Carolinas) were excluded from the snowstorm event.
At least 20 cities in the US registered their lowest surface temperatures ever recorded since the beginning of measurements. In Texas, one of the most southern States, night-time temperatures dropped below -25°C. Even in Houston, close to the Gulf coast, temperatures cooled down to -10°C.
On 19 February, in the wake of those conditions, Terra MODIS True Color RGB and Snow RGB images (Figure 2) show large areas of snow cover (white in the True Color RGB, red in the Snow RGB) across the Great Plains/the Midwest, the South and the North-East (snow hidden beneath clouds).
MODIS RGB comparison
Figure 2: Comparison of Terra MODIS True Color RGB (Bands 1-4-3) and Snow RGB (Bands 3-6-7), 19 February (Credit: NASA)
A closer view of the 19 February GOES-16 Day Snow RGB image (RGB VIS0.8-NIR1.6-IR3.9r) (Figure 3), shows the far southern extent of snow cover across northern Mexico, Texas, Louisiana and Mississippi. The most interesting feature in this Snow RGB image is a dark red band from southern Texas across northern Louisiana to Mississippi. This signal highlights an area where ice accumulated due to freezing rain*. Since ice absorbs more strongly than snow in the near/mid-IR, those areas appear 'darker'. The signal is apparent in the ABI 2.2 and 1.6µm near-IR bands, but is even more evident around 1.2µm, such as from VIIRS.
*Note: There were multiple periods of wintry precipitation during the extended winter event, including two significant winter storms. The first major winter storm impacted the region on 14-15 February, bringing heavy snow and areas of sleet and freezing rain. The second winter storm occurred on 17 February, producing additional heavy snow along with a long and wide corridor of heavy sleet and ice. Cloud cover limited the view of the scene until 19 February. See the links to event summaries (text and maps) from some of the local NWS offices within the path of the freezing rain. These offices received reports up to 1.25cm (0.5 inches) of ice with the 17 February event.
Shreveport, LA: https://www.weather.gov/shv/ArcticOutbreak2021
Houston, TX: https://www.weather.gov/hgx/2021ValentineStorm
Lake Charles, LA: https://www.weather.gov/lch/20210214-17
In this case, having more context (i.e. having more information on the freezing rain in southern Texas) helped to pinpoint the reasoning behind the appearance of the dark band. Otherwise, melting snow, with water ponds that absorbs more (lower reflectance) than fresh/clean snow, could also have been an explanation of the feature.
Figure 4 shows the so-called Cloud Phase RGB (RGB NIR1.6-NIR2.2-VIS0.6) for the same situation. Snow appears as blue in this RGB, low clouds are bright white to yellow and cloud-free ground is brown. As already noted in the case Heavy snow in parts of Spain, this RGB can reveal notable gradients in blue shades of snow and ice (fresh snow, old snow, melting snow, large/small snow grains, black ice etc.). This can be seen in the image — the ice band is much darker (nearly black) than the fresh snow.
Note: The Cloud Phase RGB might also be called 'Snowmelt RGB', following the tradition of the VIIRS Snowmelt RGB (RGB M10-M08-M05, i.e. RGB 1.61-1.24-0.67 microns).
As one can see in the animation of the Cloud Phase RGB (Figure 5), on 19 February a lot of snowmelt occurred in western and central Texas. The pixels with melting snow gradually change their colour from blue (snow colour) to brown (land colour). More details of the rapidly melting snow can be seen in the NIR1.6/VIS0.6 animation from 20 February over Texas/Oklahoma, namely the 'darkening' in the NIR1.6 image (absorption increasing in the near-IR), as liquid content of the snow increases prior to complete melting of the snowpack.
This animation also shows the development of cumulus cloud streets over the ice area. Analysing area surface observations, surface dew points over the ice (and under the cumulus) were a good 3-5°C (5-10°F) higher than adjacent areas, due to recent rain/melting ice, and resulting in a relatively moist boundary layer (see sounding from Jackson, Mississippi). The drier and slightly cooler northerly flow over the warmer/more moist region resulted in the development of the very low-level cumulus (approximately 1,000-2,000ft cloud bases). Processes were likely similar to cumulus developing over an open lake under cold air advection.
For people who are not so familiar with the Snow RGB or the Cloud Phase RGB, Figure 6 shows the well-known Natural Colour RGB. A lot of snowmelt can be seen in western and central Texas, less in Oklahoma and Arkansas. The ice band appears with a darker cyan color.
28 February
The sharp northern boundary of the dark red ice band in Figure 3 posed the question whether land type could have contributed to the feature. As we had no access to detailed geological maps of the area, to answer this question we checked the Snow RGB image four days later, 23 February at 15:00 UTC, when all the ice and snow had melted (Figure 8). When we compared the two images (19 February and 23 February), we could not find a land feature that would explain the dark red band. As one can see, the dark red band covers different land types. In particular it crosses two river valleys, namely the Red River valley and the Mississippi River valley. So, we are quite convinced that the dark red band is due to meteorology (freezing rain/ice) and not to land surface type.
GOES-16 Snow RGB comparison
Figure 8: GOES-16 Day Snow RGB on 19 February 15:00 UTC (left) and on 23 February 15:00 UTC (right). Credit: EUMeTrain
Additional content
Night-time (12:00 UTC, 19 February) Dust RGB image showing the snow cover in Texas
Ice on Lake Superior seen in GOES-16 Snow/Cloud layers RGB, 20 February 2021 18:00 UTC. Credit: CIRA
Ice on Lake Erie seen in GOES-16 Snow/Cloud layers RGB, 21 February 2021 18:00 UTC. Credit: CIRA
Snow-covered Ice on Lake Erie seen in GOES-16 Snow RGB, 21 February 2021 15:00 UTC. Credit: EUMeTrain
Arctic outbreak with snow cover extending to South Texas (CIMSS Satellite Blog)
VIIRS Imagery and Visualization Team Blog (CIRA)
RGB Imagery to Monitor Snow Melt (NASA SPoRT)
Texas Seeks Relief as Winter Storm Damage Piles Up (New York Times)
