![\"Topography](\"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-content\/uploads\/sites\/5\/2018\/10\/181018_topo_surface_pressure_anim.gif\")
<\/a>Topography background with surface pressure analyses at 03, 06 and 09 UTC [click to enlarge]<\/p><\/div>With a large dome of high pressure centered over Iowa\/Missouri\/Illinois (above),<\/strong><\/em> the stage was set for a night of strong radiational cooling across much of the Upper Midwest on 17\/18 October 2018. Minimum temperatures<\/strong><\/a> were generally in the 20-40\u00baF range, with the coldest being 15\u00baF at Champion in the Upper Peninsula of Michigan.<\/p>\n An animation of 5-minute GOES-16 (GOES-East)<\/em> “Clean” Infrared Window (10.3 \u00b5m<\/strong><\/a>) images (below)<\/strong><\/em> suggested a lack of clouds across most of the region, except for some patchy mid\/high-level clouds drifting slowly northward from Nebraska\/Wyoming across western South Dakota and far southeastern Montana. However, a close inspection of the imagery revealed the subtle appearance of “motion” across northern\/eastern North Dakota into eastern South Dakota and far western Minnesota. This was noted by Carl Jones (NWS Grand Forks), who further pointed out “strong winds just above the surface (45 kt at 900ft per KMVX VWP) mixing down a huge warm tongue at 900mb along with downslope influences”. A comparison of the GOES-16 Infrared images with topography showed the slightly higher elevation of the Coteau du Missouri<\/strong><\/a> (elevation around 2000 feet) across southwestern North Dakota and western South Dakota along with the more narrow Coteau des Prairies<\/strong><\/a> in northeastern South Dakota and southwestern Minnesota — and a subtle “downslope warming” effect could be seen on the Infrared images (warmer temperatures are darker shades of blue with the applied enhancement)<\/em>.<\/p>\n 5-minute GOES-16 “Clean” Infrared Window (10.3 \u00b5m)<\/em> images + topography, with hourly plots of surface reports [click to play animation | MP4<\/strong><\/a>]<\/p><\/div>The same effect was evident on hourly images of the GOES-16 Land Surface Temperature product (below)<\/strong><\/em>. However, with the Land Surface Temperature product enhancement, warmer temperatures appear as lighter shades of cyan.<\/p>\n Hourly GOES-16 Land Surface Temperature product + topography, with plots of surface reports [click to play animation | MP4<\/strong><\/a>]<\/p><\/div>Similarly, a sequence of higher spatial resolution Infrared Window images from Terra\/Aqua MODIS (11.0 \u00b5m) and NOAA-20\/Suomi NPP VIIRS (11.45 \u00b5m) (below)<\/strong><\/em> showed the slow propagation of the downslope warming. The lack of fog or low clouds was confirmed via this<\/strong><\/a> comparison of VIIRS Infrared Window and “Fog product” Brightness Temperature Difference at 0903 UTC; no airports were reporting any clouds or a surface visibility less than 10 miles.<\/p>\n Infrared Window images from Terra\/Aqua MODIS (11.0 \u00b5m)<\/em> and NOAA-20\/Suomi NPP VIIRS (11.45 \u00b5m)<\/em> + topography, with plots of surface reports [click to enlarge]<\/p><\/div>12 UTC NAM40 winds and isotachs at a height of 0.5 km above ground level (below)<\/strong><\/em> verified the presence of broad southwesterly flow off the Coteau du Missouri and the Coteaus des Prairies, with subtle warming (darker shades of blue)<\/em> immediately downwind.<\/p>\n GOES-16 “Clean” Infrared Window (10.3 \u00b5m)<\/em> image + topography, with NAM40 winds\/isotachs at 0.5 km above ground level [click to enlarge]<\/p><\/div>Plots of 12 UTC rawinsonde data from Bismarck, North Dakota and Aberdeen, South Dakota (below)<\/strong><\/em> showed strong low-level temperature inversions that morning — and at the top of those temperature inversions, southwesterly winds of 27 knots at a height of 1056 feet over Bismarck and 26 knots at 728 feet over Aberdeen.<\/p>\n<\/a><\/a><\/a><\/a>