NHC-led Study Identifies Promising New Approach for Forecasting Winter Precipitation in California

A previously unexplored climate variable has recently been shown to offer information critical to accurately forecast precipitation in California. The work, led by NHC Senior Scientist Dr. Mariza Costa-Cabral, is described in a manuscript now in press at Journal of Climate entitled Projecting and Forecasting Winter Precipitation Extremes and Meteorological Drought in California Using the North Pacific High Sea-Level Pressure Anomaly (Costa-Cabral et al., 2016). The project was funded by NASA.

Many studies have used large-scale climatic indices to predict precipitation totals and extremes. Indices are also used by weather forecasters, in addition to computerized climate models. The NHC-led team showed that the sea-level pressure at a location known as the North Pacific High (NPH) correlates very strongly with winter precipitation in California, as shown in the image below, and that the NPH index provides a much better predictor of precipitation than the more commonly used El Niño indices. The study also used the NPH index to explore future precipitation projections for California associated with changes in climate, indicating a higher risk of very wet years in future but no evidence for higher risk of extremely low precipitation years.

Precipitation forecasting can provide substantial benefits for water resources planning, agriculture, energy, insurance, and flood risk management in California. For water management, many reservoirs in California capture snowmelt in spring for later use in summer. Under current operations, some reservoir storage is reserved to store flood flows during the wet season – with flood control volume often varying by month – and excess flow is released downstream. The NHC-led study offers a better tool to predict precipitation and may allow flood managers to fine-tune dam operations to safely increase water storage earlier in the year.

NHC can adapt these precipitation forecasting tools to evaluate precipitation and runoff within individual watersheds, as we are doing currently for a major water agency in California. The tool can be of great value in the future under changing climate to help water managers make the most of potentially more variable precipitation seasons, as well as declining snowpack and greater peaks in runoff in wet years.

This image (reproduced from Costa-Cabral et al. (2016)) shows the linear correlation value between the proposed NPH index and historical precipitation since 1948. Very negative (purple) or positive (red) values correspond to the strongest correlations. The purple regions over the Pacific Ocean and the US Southwest Region indicate strong correlation values. Winter precipitation tends to be highest when the North Pacific High has unusually low atmospheric pressure.

Contact:  Mariza Costa-Cabral; 206.241.6000