1. Introduction

Drought is a fundamental feature of the climate of western North America, where several regions of western United States (U.S.) have experienced protracted decadal-scale dry periods in the past centuries   \citep*{Griffin_2014}. Hydrologic droughts in western U.S. were already widespread and persistent during the so-callled Medieval Climate Anomaly, roughly in the period 900–1300 AD  (Meko et al., 2007), with megadrought in southern California during 832-1074 AD and 1122-1299 AD (Raab and Larson, 1997;  Heusser et al., 2015). Multi-year droughts have also recurred in more recent times, e.g. in 1818-1824, 1827-1829, 1841-1848 and 1855-1865, causing tremendous disruption on social, agricultural, ecological and economic fronts (Cole et al., 2002, Fig. \ref{div-544081} ).
In particular, the extreme drought which occurred in 1827-1829 caused death in livestock with thousands of animals died in plains (Guin, 1890). It followed five major droughts, which ended in 1924, 1935, 1950, 1960 and 1977. As well, the one started in 2012 \citep*{Griffin2014} has generated a large pulse of tree mortality (USFS, 2015). How drought may change in future is of great concern as global warming continues (Dai and Zhao, 2016). Yet, how has an extreme drought occurrence over California shifted as a result of the change in climate since historical times? How can we see droughts coming? If we are dry during one drought year, will we likely be dry for other drought years, and then for a decade or more? How cyclical will these patterns be and how are they predictable over multidecadal time-scales? To answer these questions, we examined (with focus on California) the uncertainties in estimating future ramifications of drought years, and how drought changes can replicate over a near-future time-horizon using the Palmer Drought Severity Index (PDSI).