The relationship between forecast accuracy over the northeast Pacific and flow regime

Despite significant improvements in numerical weather prediction, such as improved data assets, increased model resolution and improved model physics, large short-term forecast failures of significant weather events still occur. Past studies have indicated that 48-hr forecast errors of sea level pressure larger than 10 hPa occur more than 10 times each winter season at west coast and offshore observation sites. The 2006 - 2007 winter season has exhibited similar forecast error statistics. The periods of large forecast errors can occur either episodically (i.e. one or two forecast periods of large error surrounded by longer periods of very small forecast errors) or clustered in periods of relatively large forecast errors. In order to improve numerical models and to understand their limitations, it is important to look further into these periods of large errors and examine under which circumstances they typically occur. For example, do certain significant weather phenomenon, such as west coast windstorms associated with deep surface low pressure centers, exhibit larger forecast errors than other, less severe, low pressure centers? Do forecast errors increase in frequency or magnitude under particular flow regimes, such as large-scale southwesterly flow impinging on the west coast? In this study, we examine the character of forecast errors further from a phenomenological perspective by compositing periods of large forecast error, and by examining forecast accuracy within varying flow regimes. Initial results indicate that large errors are associated with an upper level flow regime that exhibits more troughiness along the west coast, and lower heights and a stronger jet across the Pacific than climatology. However, large forecast errors are associated with surface low pressure centers and their attendant frontal systems at all stages of development, and are not confined to developing or rapidly deepening storms. Additional results will be presented that relate forecast accuracy to a variety of upper-level flow regimes, such as southwesterly or northwesterly flow, that occur on 1 - 2 week timescales.