A new methodology to produce more skillful United States cool season
precipitation forecasts
Abstract
The water resources of the western United States have enormous
agricultural and municipal demands. At the same time, droughts like the
one enveloping the West in the summer of 2021 have disrupted supply of
this strained and precious resource. Historically, seasonal forecasts of
cool season (November-March) precipitation from dynamical models such as
North American Multi-Model Ensemble (NMME) and the SEAS5 from the
European Centre for Medium-Range Weather Forecasts have lacked
sufficient skill to aid in Western stakeholders’ and water managers’
decision making. Here, we propose a new empirical-statistical framework
to improve cool season precipitation forecasts across the contiguous
United States (CONUS). This newly developed framework is called the
Statistical Climate Ensemble Forecast (SCEF) model. The SCEF framework
applies a principal component regression model to predictors and
predictands that have undergone dimensionality reduction, where the
predictors are large-scale meteorological variables that have been
prefiltered in space. The forecasts of the SCEF model captures 12.0% of
the total CONUS-wide standardized observed variance over the period
1982/1983-2019/2020, while NMME captures 7.2%. Over the more recent
period 2000/2001-2019/2020, the SCEF, NMME and SEAS5 models respectively
capture 11.8%, 4.0% and 4.1% of the total CONUS-wide standardized
observed variance. Importantly, much of the improved skill in the SCEF,
with respect to models such as NMME and SEAS5, can be attributed to
better forecasts across most of the western United States.