Experimental Subseasonal‐to‐Seasonal (S2S) Forecasting of Atmospheric Rivers Over the Western United States

Abstract

A multimodel evaluation of subseasonal‐to‐seasonal (S2S) hindcast skill of atmospheric rivers (ARs) out to 4‐week lead over the western United States is presented for three operational hindcast systems: European Centre for Medium‐Range Weather Forecasts (ECMWF; Europe), National Centers for Environmental Prediction (NCEP; U.S.), and Environment and Canada Climate Change (ECCC; Canada). Ensemble mean biases and Brier Skill Scores are examined for no, moderate, and high levels of AR activity (0, 1–2, and 3–7 AR days/week, respectively). All hindcast systems are more skillful in predicting no and high AR activity relative to moderate activity. There are isolated regions of skill at week‐3 over 150–125°W, 25–35°N for the no and high AR activity levels, with larger magnitude and spatial extent of the skill in ECMWF and ECCC compared to NCEP. The spatial pattern of this skill suggests that for high AR activity, a southwest‐to‐northeast orientation is more predictable at subseasonal lead times than other orientations, and for no AR activity, more skill exists in the subtropical North Pacific, upstream of central and southern California. AR hindcast skill along the western U.S. is most strongly increased in hindcasts initialized during Madden‐Julian Oscillation (MJO) Phases 1 and 8, and hindcast skill is substantially decreased over California in hindcasts initialized during MJO Phase 4. Skill modulations in the ECMWF hindcast system conditioned on El Niño‐Southern Oscillation phase are weaker than those conditioned on particular MJO phases. This work provides hindcast skill benchmarks and uncertainty quantification for experimental real‐time forecasts of AR activity during winters 2019–2021 as part of the S2S Prediction Project Real‐time Pilot Initiative in collaboration with the California Department of Water Resources.

Windows of Opportunity for Skillful Forecasts Subseasonal to Seasonal and Beyond

“…Relationships between weekly AR occurrence, ridging, and certain phases of ENSO, the Arctic Oscillation, the PNA, and the MJO (DeFlorio et al 2018;Guirguis et al 2018) have formed the basis for predictability considerations. An experimental prediction framework is being developed to predict weekly AR frequency (DeFlorio et al 2018(DeFlorio et al , 2019 based on hindcast and forecast information from the S2S Prediction Project (Vitart et al 2017). Results indicate conditional useful skill 1-3 weeks ahead.…”

Subseasonal‐to‐Seasonal Hindcast Skill Assessment of Ridging Events Related to Drought Over the Western United States

“…The pattern correlation scores are also assessed with respect to the particular phase of the MJO at forecast initialization. This is carried out to assess whether the MJO is capable of modulating forecast skill such that certain phases of the MJO may offer extended forecasts of opportunity (e.g., DeFlorio, Waliser, Guan, et al, 2019; DeFlorio, Waliser, Ralph, et al, 2019; Ferranti et al, 1990; Mariotti et al, 2020). The commonly used MJO real‐time multivariate RMM index (Wheeler & Hendon, 2004) is used to define strong and persistent MJO events, based on an RMM amplitude greater than 1 and persisting for at least five consecutive days.…”

“…Collectively, these studies highlight a barrier for the direct use of precipitation forecasts in early drought warning systems at S2S lead times. This limited skill for forecasting precipitation has also motivated subsequent research on examining AR prediction skill, given the critical importance of AR frequency on precipitation variability across the western United States and the potential for longer lead time prediction skill (e.g., DeFlorio, Waliser, Guan, et al, 2019; DeFlorio, Waliser, Ralph, et al, 2019; Nardi et al, 2018).…”

Introduction to Special Collection: “Bridging Weather and Climate: Subseasonal‐to‐Seasonal (S2S) Prediction”

“…The landfall of atmospheric rivers is known to be associated with periods of persistent precipitation that can lead to devastating flooding (e.g., Mundhenk et al, ). In the western United States skill is modulated by the phase of ENSO and the MJO (DeFlorio et al, ), while over the North Atlantic, established European weather regimes are important for assessing potential atmospheric river impacts at S2S lead times (Pasquier et al, ).…”