During major flood events, forecast preparation increases to twice daily, plus additional releases are issued corresponding to significant changes in meteorological conditions or large biases between forecasts and observed values. During the fall months, forecasts extend to 3 days, and in the spring snowmelt events forecasts are issued for 6 days. Flood events historically occur approximately once in every two years in the northwest. In Western Oregon, floods occur from large frontal-system storms coming off the Pacific Ocean. Many times, the larger floods are associated with melting snow and frozen ground.
Flash floods can occur in late spring and throughout the summer.
Because of their unpredictability, the National Weather Service offers flash flood warnings in areas where high levels of thunderstorm activity are forecast.