About this analysis, from the Advanced Hydrologic Prediction Service of the National Weather Service
:"Observed" data is a byproduct of National Weather Service (NWS) operations at the 12 CONUS River Forecast Centers (RFCs), and is displayed as a gridded field with a spatial resolution of roughly 4x4 km. "Observed" data is expressed as a 24-hour total ending at 1200 Z (same as Greenwich Mean Time, or GMT), with longer periods simply being a summation of multiple 24-hour periods. 1200 GMT is used as the ending time for a 24-hour total, because it is the end of the "hydrologic day", a standard used in river modeling. Additionally, 1200 GMT closely coincides with the reporting time for most of the National Weather Service's cooperative observers, whose data are used as a quality control on the dataset. 1200 GMT coincides with 8 AM EDT, 7 AM EST, 7 AM CDT, 6 AM CST, etc.
The map shows Michigan: Current 30-Day Departure from Normal Precipitation Valid at 6/12/2010 1200 UTC - Created 6/12/10 22:16 UTC. The color palette below provides a key.
