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Southern California, USA
Reference
MacDonald, G.M., Kremenetski, K.V. and Hidalgo, H.G. 2008. Southern California and the perfect drought: Simultaneous prolonged drought in Southern California and the Sacramento and Colorado River systems. Quaternary International 188: 11-23.

Description
The authors developed dendrochronological reconstructions of the winter Palmer Drought Severity Index (PDSI) for southern California over the past one thousand years (first figure below), plus concomitant annual discharges of the Sacramento and Colorado Rivers (second figure below). This work revealed, in their words, that "prolonged perfect droughts (~30-60 years), which produced arid conditions in all three regions simultaneously, developed in the mid-11th century and the mid-12th century during the period of the so-called 'Medieval Climate Anomaly'," which is also widely known as the Medieval Warm Period, leading them to conclude that "prolonged perfect droughts due to natural or anthropogenic changes in radiative forcing, are a clear possibility for the near future." Consequently, since the perfect droughts of the 20th century "generally persist[ed] for less than five years," while those of the MWP lasted 5 to 12 times longer, one could reasonably conclude that late 20th-century warmth was significantly less than that of the central portion of the Medieval Warm Period.