Satellite: GOES-10
Channel: Long-Wave Window Infrared (Ch. 4)
Time Span: 1500-2030 UTC on Friday Oct 1, 1999
Time Resolution: 30-minute resolution

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The infrared loop shows an east-west boundary moving southward over NE CO and NW KS. This boundary is associated with a cold front as evidenced by hourly surface observations from Limon, CO (location denoted in loop by northern end of red line) from 1855-2155 UTC:

1855 UTC ---- Temperature: 78 F; dewpoint 20 F, RH 11% ---- Winds: calm
Pressure - 1005.1 mb ---- Skies: clear; visibility 10 miles

1955 UTC ---- Temperature: 80 F; dewpoint 20 F, RH 11% ---- Winds: variable at 6 mph
Pressure - 1004.3 mb ---- Skies: clear; visibility 10 miles

2055 UTC ---- Temperature: 67 F; dewpoint 42 F, RH 40% ---- Winds: NE at 26 mph
Pressure - 1007.2 mb ---- Skies: clear; visibility 10 miles

2155 UTC ---- Temperature: 60 F; dewpoint 38 F, RH 44% ---- Winds: NE at 26 mph
Pressure - 1009.8 mb ---- Skies: clear; visibility 10 miles

These observations indicate a frontal passage at Limon between 1955 to 2055 UTC: temperature decreased 13 F, dewpoint increased 22 F, winds changed from variable at 6 mph to NE at 26 mph, and pressure increased almost 3 mb.

The satellite-observed boundary represents a gradient in surface temperature, which was caused by the cooling of the surface by the cold air behind the front. The satellite-observed boundary may trail the cold front slightly since it takes a finite amount of time to cool the surface.

The detailed orientation of the satellite-observed boundary appears to be control by local topography. At 1930 UTC, the boundary was oriented from west-northwest to east-southeast across NE CO and NW KS ( infrared image from 1930 UTC ). A map of local topography shows that the orientation of the boundary corresponds well with the northern slope of the Palmer Lake Divide, an east-west ridge extending off of the Rocky Mountains into east-central CO. This correspondence suggests cold air damming along the Divide. The development of clouds over the Divide from 1930-2030 UTC is further evidence of the importance of local topography.

The front was also observed with the NEXRAD WSR-88D radars located in Denver, CO and Goodland, KS. At 2059 UTC, low-level scans from the Denver radar and the Goodland radar show an east-west oriented boundary corresponding well with the boundary observed in the satellite loop (note that both satellite and radar show a boundary oriented from southwest to northeast over NW KS). The radar-observed boundary mostly likely represents backscatter off of airborne debris, which was lofted by the leading edge of the front. However, the western extent of the radar-observed boundary may represent both airborne debris and precipitation from clouds over the Palmer Lake Divide.
 

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We welcome your comments and discussion at ramsdis@comet.ucar.edu