A series of intense surges of subtropical moisture inundated western Oregon in February, 1996. The combination of record-breaking rain, warm temperatures, and a deep snowpack led to severe flooding throughout northern sections of the state. This flood was of comparable magnitude to the December, 1964 flood, the largest in Oregon since floodcontrol reservoirs were built in the 1940's and 1950's.
The event began with unusually high amounts of snow in the middle and high elevations of the Cascades and Coast Range. In mid-January, the snowpack was poor; the average for high-elevation sites (NRCS SNOTEL stations) in the Willamette drainage was only 29% of average on January 17th. The next two weeks, however, saw prodigious snowfall totals (in many locations, several feet per day for many days). By January 31, the average snowpack for the Willamette drainage had risen to 112% of average. It appeared to be a good snow year after all.
An intense cold spell during the week of the 29th resulted in very low temperatures in the northern half of the state. Many Willamette Valley stations had lows in the teens for 4 or 5 consecutive days. A number of eastern Oregon locations had lows well below zero. A moderate storm on February 3rd dropped rain on top of frozen soil and roads, causing a major freezing rain episode throughout the Willamette Valley. Traffic was slowed or completely halted in many locations. The hardest hit was the Portland area, where icy conditions lasted for three days, and which also experienced wind chill factors of -20F or lower.
Then on February 6th, a strong subtropical jet stream reached Oregon. This warm, very humid air mass, which originated near the Equator in the western Pacific (near the Date Line), brought record rainfall amounts to northern sections of the state. Although such subtropical storms are by no means rare, it is unuual for them to persist with such intensity for such a long period of time (3-4 days). Among the more notable precipitation amounts were:
Laurel Mountain Coast Range: 8.20 inches in 24 hours; more than 23 inches in 3 days
Corvallis: 3.26 inches in 24 hours (new record for any day in February; 108 years of record)
Eugene: 5.17 inches in 24 hours
Newport: 7.71 inches in 3 days
In addition to the wet conditions, temperatures were unusally mild. In the Willamette Valley, daily minimum temperatures were higher than normal maximum values for early February. Nighttime lows in the mid-50's were quite common. The freezing level quickly moved upward, to 7,000 - 8,000 feet. Rain fell even at mountain pass level.
The warm rain and air temperatures quickly began to erode the snowpack. Streams rose quickly on the 6th and 7th, reaching flood stage in many locations. At Vida on the McKenzie River, the flow jumped from 4,000 cfs on the 5th to over 20,000 cfs on the 6th. Major and minor tributaries throughout western Oregon jumped their banks. Gradually the levels in the major tributaries and the main stem rivers increased as well. At this point, water levels in the Willamette were well above flood stage. When peaks were reached (mostly Feb. 9 and 10), the levels rivaled those observed in 1964.
Comparisons with 1964 are inevitable, since that flood event is considered the largest in this area since flood control dams were completed in the 1950's. The major difference is the extent: whereas the 1996 flood affected mostly northwestern Oregon, the 1964 event extended from Northern California through Washington, and from the coast well into Idaho.
The 1996 flood capped a most unusual weather year in Oregon. A very wet November, big wind storm in December, and the snow and ice of January merely set the stage for the flood event. Doubtless this will be perceived as one of the most interesting (and damaging) winters in the history of Oregon.
George H. Taylor, State Climatologist