Charts show ice extent and concentration three times weekly during the ice season, for all lakes except Ontario, from the 1973/74 ice season through the 2001/2002 ice season. Most charts are annotated to indicate data sources. Straits of Mackinac, Green Bay and Sault Ste. Marie are enlarged as insets, and in some cases, provided as separate charts. From 1973/74 through 1986/87, charts were prepared by the National Weather Service Forecast Office, Ann Arbor, Michigan. Beginning with the 1987/88 season, the National Ice Center (formerly known as the Navy/NOAA Joint Ice Center) in Suitland, Maryland prepared the charts. Charts are available as GIF files.