Abstract
To assist water managers in south-central Oklahoma prepare for future
drought, reliable place-based drought forecasts are produced. Past,
present, and future-forecasted climate indices (Multivariate ENSO Index,
Pacific Decadal Oscillation index, and Atlantic Multidecadal Oscillation
index) and past and present Palmer Drought Severity Index (PDSI) are
employed as predictor variables to forecast PDSI using a multivariate
regression technique. PDSI is forecasted 18 months in advance with
sufficient skill to provide water managers early warning of drought.
Using a training dataset from January 1901 to November 2021, a
second-order model equation that contains, without any restriction, all
the predictors and interaction terms is built to predict drought
intensity. Significant predictors are selected through stepwise
regression, with cross-validation producing the simplest restricted
model that describes the data well. PDSI values are predicted using 1000
fitted restricted models produced from bootstrapping, then averaged
monthly. The technique found the best-fitting model and estimated the
model coefficients that minimized the sum of squared deviations between
the fitted model and the predictor variables. The adjusted R-squared
value of the restricted model is large enough to explain the strength of
the model, and relatively low values of error measures point to good
predictive ability of the model. Although the model slightly
overestimates the PDSI forecast maxima and minima, it necessarily
captures the timing of the periods of severe to exceptional drought.