The freeze of December 1964 that occurred before the flood
An atypical cold spell began in Oregon on December 13, 1964, that froze the soil, and it was followed by unusually heavy snow.
Subsequently, an atmospheric river brought persistent, heavy, warm rain. The temperature increased by 30 to 40 °F (17 to 22 °C). This melted the snow, but left the soil frozen and impermeable. Some places received the equivalent of a year's rain in just a few days. Albany received 13 inches (330 mm) of rain in December, almost double its average December rainfall of 7 inches (200 mm). Detroit recorded an extra 18 inches (460 mm) of rain, and at Crater Lake, where the average normal December rainfall is 12 inches (300 mm), there was over 38 inches (970 mm) of rain. As rivers and streams overflowed and the soil became saturated, mudslides occurred, roads closed, and reservoirs overflowed. Many towns were isolated. By the end of the flood, every river in Oregon was above flood stage, and more than 30 major bridges were impassable.
Heavy warm rain and melting snow caused more flooding in late January 1965, after the waters had begun to recede from the December flood. More mudslides occurred in places that had withstood the December flooding, and there were more deaths. Many streams in the northern San Joaquin Valley reached higher flows than they had in December.