A very shallow cold airmass had established itself across much of the high plains by the day on 9 Feb 2021. Analyzing the 12z Denver sounding, a strong temperature inversion is observed with a surface temperature of around -12C below a max temp of around 3C (Fig 1) less than 1 km AGL.
Viewing GOES-East Day Cloud Phase Distinction RGB imagery, the low clouds (light blue) trace the lowest elevation areas in the eastern valleys, representing areas of cool/saturated air below the inversion (Fig 2). Flurries were observed under these very shallow clouds. The higher elevation areas are cloud free as they lie within the warmer/drier air in the warm nose of the inversion.
A comparison between the cloud features in the imagery and the underlying topography is shown in Figure 3.
Within the river valleys and under the low clouds, temperatures were generally in the teens by the afternoon (Fig 4). In the clear sky, higher elevation areas of eastern Colorado, temperatures had risen into the 40s. Similarly warm temperatures were observed in the high mountain valleys to the west.
Other factors were influencing surface temperatures along the front range during the day. For example, at Cheyenne, a period of breezy westerly winds (subsidence) during the late morning quickly and briefly boosted temperatures into the mid 40s, which quickly moderated back into the upper 30s after a southerly wind shift (Fig 5 and 6).
Bill Line, NESDIS and CIRA
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