This week has seen plenty of dense fog over Iowa as a shallow layer of cooler and humid air near the surface has been difficult to mix out. A significant portion of the country has experienced dense fog this week with the National Weather Service issuing many Dense Fog Advisories. The IEM attempts to curate an archive of the NWS alerts and provides a download interface for the data. For Dense Fog Advisories, the IEM archive dates back to 1 October 2005 (there's mechanistic reasons for that date). A query of this archive indicates that at one point Thursday morning, 1,517 NWS Forecast Zones were simultaneously under a Dense Fog Advisory, which far exceeds the previous maximum value set the prior day and the day before that. You may be wondering about the difference between "Forecast Zones" vs "Counties" as the NWS issues some alerts for zones and others for counties. In Iowa and other states without much topography, these geographic areas generally overlap with identical names, but the NWS as a whole issues some alerts for zones and others for counties NWS-wide.

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Tags:   densefog