From US’s National Weather Service – big precipitation anomalies in US this past year. The purple areas are extra wet, while the red areas are extra dry.
Precipitation anomalies in USA for 2011 in mm
update:
“Normal” precipitation is derived from PRISM climate data, created at Oregon State University. The PRISM gridded climate maps are considered the most detailed, highest-quality spatial climate datasets currently available. The 30 year PRISM normal from 1971-2000 is used for precipitation analysis since 2004. Prior to 2004 the 30 year PRISM normal from 1961-1990 is used.” from http://water.weather.gov/precip/about.php
Extra wet and extra dry? Compared to what? HOw is “normal” precipitation determined for this purpose?
Its explained via the link to the figure source
http://water.weather.gov/precip/about.php
“Normal” precipitation is derived from PRISM climate data, created at Oregon State University. The PRISM gridded climate maps are considered the most detailed, highest-quality spatial climate datasets currently available. The 30 year PRISM normal from 1971-2000 is used for precipitation analysis since 2004. Prior to 2004 the 30 year PRISM normal from 1961-1990 is used.”