Large-scale Statistically Meaningful Patterns (LSMPs) associated with
precipitation extremes over Northern California
Abstract
We analyze the large-scale statistically meaningful patterns (LSMPs),
also called large-scale meteorological patterns, that precede extreme
precipitation (PEx) events over Northern California (NorCal). We find
LSMPs by applying k-means clustering to the two leading principal
components of daily 500hPa geopotential height anomalies persisting two
days before the onset. A statistical significance test based on the
Monte Carlo simulations suggests the existence of a minimum of four
statistically distinguished LSMP clusters. The four LSMP clusters are
characterized as the NW continental negative height anomaly, the
Eastward positive “PNA”, the Westward negative “PNA”, and the
Prominent Alaskan ridge. These four clusters, shown in multiple
atmospheric and oceanic variables, evolve very differently and have
distant links to the Arctic and tropical Pacific regions. Using binary
forecast skill measures and a new copula-based framework for predicting
PEx events, we show that the LSMP indices are useful predictors of
NorCal PEx events, with the moisture-based variables being the best
predictors of PEx events at least six days before the onset, and the
lower atmospheric variables being better than their upper atmospheric
counterparts any day in advance.