It is useful to separate the facts from the hyperbole to understand what is really going on, both meteorologically and hydrologically.

The December 2005 Weather
The current weather pattern is not the same as the one that caused the most damaging flood on record for the Truckee River in early January 1997. The recent, wet storms here are the direct result of the polar jet stream dropping way south to almost 20 degrees latitude north of the equator in the Pacific Ocean. Then the jet stream curved back and became caught up in (intersected) a strong zonal flow that was driving storms into California. The storms themselves have been stacked up clear across the Pacific to eastern Asia and the Philippines waiting to get to the Sierra.

In contrast, a classic Pineapple Express occurs when both the polar and subtropical jet streams actually merge to funnel a long, unbroken fetch of subtropical moisture from the Hawaiian Islands region directly toward the West Coast.

Despite the impressive precipitation totals contributed so far by the series of relatively juicy storm systems, the subtropical moisture being tapped by each surge is relatively limited. In addition, each frontal passage has dropped snow at lower and lower altitudes, indicative of cooler air behind it. Although local ski resorts and holiday skiers have been disappointed that snow levels have risen above 8,000 feet at times, the heavy wet snow accumulating on the upper slopes is building the base that will provide skiing and riding on for months to come. And there is more good news: There is likely to be a substantial increase in the elevation of the water in Lake Tahoe. It will improve boating and river sports next summer.

New Year's Flood of 1997 Compared to Now
This flood caused nearly $650 million damage in western Nevada, a disaster of extreme magnitude. Why won't it happen again this year? Here is a short analysis of the combination of factors at that time, including Lake Tahoe's elevation and the rapid melt of snow pack just before the '97 flood. When this week's storms and the current snow pack are compared to the situation around New Years '97, there are significant differences.
--First, the two previous years (1995 & 1996) had been very wet in the Sierra and in western Nevada and Lake Tahoe's water level was within 12 inches of its legal maximum of 6,229 feet above sea level. There was little room for additional water storage in the lake.

--Second, a major winter storm on December 21 and 22, 1996, dumped heavy snow in the Sierra (about 6 to 7 feet at lake level) and up to three feet in the valleys of western Nevada. This set up an above-average snow pack.

--Third, a vigorous Pineapple Express weather pattern (merged polar flows) brought extremely heavy rain into the Sierra at the end of Dec. 1996, with rain up to levels as high as 11,700 feet on January 1. This means there was an incredible amount of rainfall and snowmelt runoff pouring out of the Sierra Nevada from December 30 to January 6, 1997. At Blue Canyon, nearly 19 inches of rain fell in just three days, with storm total precipitation in the Feather River drainage exceeding 35 inches. Indicative of the exceptional proportions of the runoff, an estimated 25 inches of rain and snowmelt cascaded out of Squaw Creek Basin, which caused substantial structural damage in Palisades Tahoe.

--Fourth, Lake Tahoe was filled over its legal limit (to its highest level since 1917). This forced Federal Watermaster, Garry Stone, to fully open all 17 gates in the Tahoe dam at the start of the Truckee River. The resulting flood crest inundated the Truckee River floodplain from Squaw Creek to beyond Reno, Nevada. Nearly every bridge across Highway 89 between Tahoe City and Truckee was destroyed or seriously damaged. When homes and businesses are included, there were $11 million in damages in the upper Truckee River watershed alone. In several eastern Sierra watersheds, the 1997 runoff exceeded 100-year flood flows.

So it appears that the critical ingredients needed to generate a significant flood event this season are not currently present...and the snow pack is building!

Mark McLaughlin is a weather historian and writer of four award-winning books on the region. He lives on the North Shore.