Major Numerical Forecast Failures over the Northeast Pacific

Abstract

Strong North Pacific storms that impact the North American west coast are sometimes poorly predicted in the short term (up to 48 h) by operational models, with cyclone position errors of hundreds of kilometers and central pressure errors of tens of millibars. These major numerical forecast failures still occur despite continuing improvements in modeling and data assimilation. In this paper, the frequency and intensity of sea level pressure errors at buoy and coastal locations are documented by comparing the National Centers for Environmental Prediction (NCEP) Eta Model forecasts to observations and through case studies of two poorly forecast cyclones from the 2001/02 winter season. Using data from October 1999 through March 2003 at coastal and offshore sites along the west coast of North America, it was found that large forecast errors (48-h sea level pressure errors greater than 10 mb) by the Eta Model occur 10?15 times each winter, and extremely large errors (48-h errors greater than 15 mb) occur 3?4 times per winter. Such substantial forecast errors are often associated with large position errors of surface low pressure centers. For example, storms associated with large 48-h forecast errors greater than 10 mb at nearshore and coastal sites had average forecast position errors of 453 km and mean absolute central pressure errors of 7.5 mb. To illustrate the nature of such large forecast errors, two major cyclones that were poorly predicted by several operational models are examined. The 7?8 February 2002 storm was a compact, but powerful, cyclone that struck western Oregon with strong winds, injured four people, and produced extensive damage and power outages. The 24-h numerical forecasts for this event were poor and had a variety of solutions. Two operational models forecast lows of sufficient depth, but displaced them more than 150 km to the east or southeast of the verifying position. Three other operational models did not produce a low at all but only predicted weak troughs. The comparison of the initial conditions of the various models revealed large differences, with the more accurate models starting with sharper, more intense features. The 13?14 December 2001 storm developed rapidly offshore of British Columbia, Canada, and brought extensive rain, winds, and snow to the mountains along the west coast. The 48-h forecasts of sea level pressure by five different operational numerical models had very large errors, with cyclone position errors greater than 400 km and central pressure errors on the order of 10 mb. Differences among the initial conditions of these operational models were smaller than in the February case. Comparison of the initial conditions to surface observations revealed potentially significant errors in the vicinity of the incipient cyclone.