CW3E Launches Public Subseasonal-to-Seasonal (S2S) Experimental Forecast Webpage for Winter 2020-2021

November 3, 2020

Demand for subseasonal-to-seasonal (S2S; 2-week to 6-month lead time) forecasts of precipitation and other meteorological variables is historically high in the stakeholder and applications communities. In particular, water managers, energy and insurance companies, agriculture producers, and other end users are keenly interested in improved prediction of precipitation, atmospheric river (AR) activity, and circulation patterns that may serve as precursor patterns for future wet or dry conditions relative to normal.

In response to this demand, California Department of Water Resources (DWR) has funded a partnership between the Center for Western Weather and Water Extremes (CW3E) and the National Aeronautics and Space Administration Jet Propulsion Laboratory (NASA JPL) to create S2S experimental forecast products supported by peer-reviewed literature assessing the skill of model(s) used to make a prediction. This effort is focused on precipitation over California, but also includes predictions of ARs and ridging events (extended periods of high atmospheric pressure) that influence precipitation amount during wintertime. These S2S experimental forecast products and their associated research supports CW3E’s Strategic Plan by producing experimental S2S outlooks of ARs, total precipitation, and ridges and by improving understanding of S2S predictability of these quantities through research studies.

The URL for the newly-launched CW3E S2S experimental forecast product website is

Support from California DWR for this effort has spawned the creation of a number of experimental S2S forecast products and their associated peer reviewed journal articles providing hindcast skill assessments of the prediction systems used to make the experimental forecasts. There are currently four experimental S2S outlooks displayed on the public S2S website: weeks 1-3 AR activity outlooks; weeks 1-6 ridging outlooks; seasonal precipitation outlooks based on Canonical Correlation Analysis (CCA); and Odds of Water Year Normal precipitation outlooks. The following papers provide hindcast skill assessments for each of the products:

DeFlorio et al. 2019b (weeks 1-3 AR activity outlooks)

Gibson et al. 2020b (weeks 1-6 ridging outlooks)

Gershunov and Cayan 2003 (seasonal CCA outlooks)

The Odds of Water Year Normal Precipitation outlooks are calculated based on only historical data and are not considered a dynamic prediction system. Future S2S experimental prediction products will be added to the public S2S website, contingent upon the publication of their associated hindcast skill assessment (if needed) and the approval of CW3E Director F. Martin Ralph.

DeFlorio, M. J., D. E. Waliser, F. M. Ralph, B. Guan, A. Goodman, P. B. Gibson, S. Kumar (2019b), Experimental subseasonal-to-seasonal (S2S) forecasting of atmospheric rivers over the Western United States. Journal of Geophysical Research – Atmospheres (S2S Special Issue), 124, 11,242-11,265.

Gershunov, A., and D. R. Cayan (2003): Heavy daily precipitation frequency over the contiguous United States: sources of climatic variability and seasonal predictability. Journal of Climate, 16, 2752–2765,<2752:HDPFOT>2.0.CO;2.

Gibson, P. B., D. E. Waliser, A. Goodman, M. J. DeFlorio, L. Delle Monache, and A. Molod (2020b), Subseasonal-to-seasonal hindcast skill assessment of ridging events related to drought over the Western United States. Journal of Geophysical Research – Atmospheres, in press.