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Seminar Abstract

Nate Mantua and David Battisti - December 10, 2002


Regional impacts of day-to-day changes in the large-scale Pacific North America (PNA) pattern: Observations and prospects for skillful 7-14 day lead-time weather risk forecasts

Craig Brown, Nate Mantua and David Battisti

The Pacific North America (PNA) pattern has long been recognized as a leading pattern of the observed seasonal, interannual, and interdecadal climate variability over the Pacific and North America, primarily in the cold half of the year. This work focusses on day-to-day variations in the PNA pattern and their association with cool season (October-March) extreme temperature, wind, and precipitation events at selected locations across the US. In the Pacific Northwest there are very strong and statistically significant shifts in the frequency of extreme temperature, snowfall, and precipitation events associated with changes in the PNA pattern. During positive PNA days, extreme cold temperature, heavy precipitation, freeze events and low elevation snow days are rare. During negative PNA days the opposite relations hold: there is a regional pattern of increased frequencies for extreme cold temperatures, freeze events, heavy precipitation, and low elevation snowfall. While previous research into the PNA pattern has focussed on seasonal and longer time scales, Renwick and Wallace (1995) identified the PNA pattern as the most predictable circulation pattern for deterministic weather forecasts at lead times of 6 to 10 days. The National Weather Service is now making operational model-based weather forecasts for the PNA index out to lead times of 14 days, and these forecasts have shown promising skill. We believe that studies like ours can now be combined with existing 14-day PNA forecasts to provide value-added "extreme weather" risk forecasts for affected regions (notably, the Pacific Northwest, parts of the Great Lakes region, and the Southeast US).

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