River valley fog in Wisconsin, Minnesota and Iowa

July 26th, 2016 |

Suomi NPP VIIRS 11.45 µm - 3.74 µm Infrared brightness temperature difference ("fog product") at 0735 UTC [click to enlarge]

Suomi NPP VIIRS 11.45 µm – 3.74 µm Infrared brightness temperature difference (“fog product”) at 0735 UTC [click to enlarge]

A nighttime image (above) of the Suomi NPP VIIRS 11.45 µm – 3.74 µm Infrared brightness temperature difference (often referred to as the “fog/stratus product”) showed the development of narrow fingers of river valley fog in parts of southwestern Wisconsin, southeastern Minnesota and northeastern Iowa at 0735 UTC or 2:35 am local time on 26 July 2016. At that time the surface visibility was reduced to 1/4 mile at Boscobel, Wisconsin (station identifier KOVS).

During the subsequent daylight hours, GOES-13 Visible (0.63 µm) images (below) revealed the extent of the valley fog which had formed (the yellow symbols denote stations reporting fog). However, this fog quickly dissipated quickly with strong heating from the July sun.

GOES-13 Visible (0.63 µm) images [click to play animation]

GOES-13 Visible (0.63 µm) images [click to play animation]

This region frequently experiences such episodes of river valley fog, but they are most common during the Autumn months as nights grow longer and nighttime temperatures get colder. In this late July event, the primary ingredient favoring fog formation was high soil moisture due to recent heavy rainfall (below), much of which occurred on 24 July.

7-day precipitation, departure from normal, and percent of normal {click to enlarge]

7-day precipitation, departure from normal, and percent of normal {click to enlarge]