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301g?" Usually at least 50" of snow falls, and the average November-March precipitation total is about eight inches. It was dryll 0, , The mild and snow-free winter was pleasant in many ways (if you could ignore the downslope winds that buffeted the Front Range), but water offici8Is became increasingly nervous. The mountain snowpack statisties didn't sound too bad - 89"10 of average on April I. However, there was almost no snow on the ground at elevations below 9,000 feet, and south facing SlOPes were bare even higher. .The Front Range foothills had. re- mained free of snow for much of the winter. Where was all the water for spring and summer going to come from? Forecasts for a dry spring and summer by increasingly confident long-range fo~ers at the NOAA Climate Prediction Center, created even more uneasiness. We in the climate business have seen many dramatic "turn arounds" in the past with total flip-flops in the weather pattern. - the onset of extremely dry weather just when it looks like we're all going to drown in mud or sink in snow, and sudden wet spells when we're at the very brink of drought. Springtime' is particularly dynamic in this regard. Indeed, spring 1999 was far from over and Mother Nature had not yet spoken. The change in weather pattern occurred already by March 31. We just hadn't realized it. Instead of dry, westerly winds whipping across the state, a deep trough of.low pressure was developing over the Southwest and Colorado Plateau region. For the next 6 weeks, one storm system after another dropped down into the Southwest, and then moved slowly eastward across or just south of Colorado. The first of these storms was a 5-day snowstorm that . brought greatIy-apprecia\e4 moisture to SW Colorado. Wolf Creek Pass measnred88 iuches of new snow with nearly 5" of water content April 1-5. Then came the first majorthlinderstotIl1s of the season April13th with locally heavy I-inch rains at Denver, Burlington, and a few other areas of eastern Colorado and heavy, wet snows in and near the foothills followed by 3 days of on-and-off snow- showers with freezing, blUstery northerly winds. These early storms were helpful, but much more was still to come. On the 21st of April, rain showers spread eastward across western Colorado and turned to snow in the mountains. Rains developed east of the mountains during the cveuing and became widespread and steady. Nearly all of Colorado received some piecipitation, but "upslope" winds from the east concentrated the heaviest precipitation in a north-south hand along the Front Range. Rains changed to snow in many areas and continued all day on the 22nd and much of the 2300. Rain and snow began again late on the 24th and ended gradually on the 25th with a few lingering showers on the 26th. Foothills locations ended up with two tofour feet of sloppy wet snow. Much of the snow melted as it landed along the Front Range urban corrider, bnt 5-day precipitation totals ~.,;\ were substantial. Examples inclnded Pueblo, 1.49"; Colorado Springs, 1.76"; Denver, 2.27"; Greeley, 2.40"; Fort Collins, 3.42" and Boulder, 3.45". Dry air usually returns to Colorado on the heels Qf passing storms, but as this slow-moving system drifled sl~wly eastward towards the Midwest, very moist air rentained ready for the next storm. Already by the afternoon of the 27th, showers and thunderstorms began to develqp, and southeasterly winds developed in advance of a rapidly developing low pressure.area over Nevada, Arizona and southern California. With the ground a1ready soaked along the Front Range, and with deep snows in place over the foothills, concerns over inadequate water supplies began to shifl to "what are we going to do if we get any more rain?" Well, we got more rain. Rains became widespread on the evening of the 28th, especially along th,e Front Range. Southeasterly winds increased. Thunder rumblediand localized heavy rains in the Colorado Springs area be- haved much more like summer convective storms than. springtime storm systems. A burst of heavy rain fu the Colorado Springs area around niidnightsent tribularies roaring towards Fountain Creek and on toWard the Arkan- sas River. The center of the storm system remained nearly stJItiouary over northern Arizona and southern Utah on the 2'9th and 30th. This allowed strong, steady southeasterly winds to ..' . continue to pump'saturated air into eastern Colorado. . Watersheds that drain towards the southeast wereespe- cially favored for heavy rains and flooding in this weather pattern. Two river basins separated by mOre than 100 miles proved to be most vulnerable. The Pondre Rjver in . northern Colorado and Fountain Creek in the Pike$Pellk area both drain towards the southeast and both experi- enced particularly heavy rains and flooding although other tributaries such as the St. Charles River that flows from the Wet Mountains southwest of Pueblo to the Arkansas River also received excessive rainfall. In northern Colorado, rainfall rates were moderate, peaking near 0.25" per hour for severa1 consecutive hours on April 30th. Rainfall rates were somewhat highclr at times in the Arkansas Valley closer to the storm's original moisture source, the Gulf of Mexico. Compared to,sum- mer storms with rainfall rates that can exceed three inehes per hour, these rains seemed trivial. However, with rains simultaneously falling over hundreds of square miles . "volume flooding" began to occur. "Volume flooding" is a term used by hydrologists that distinguishes the. local "flash flood" that occurs when very high intensity tains fall over a relatively smll11 area from the flooding that occurs on large river systems when moderate rains fall over a large area and over a long period of time. There was additioual contribution to runoff from melting $nows in the Front Range foothills that added to th,e high vol- umes. "Rain on snow," a common contributing factor for 76 . '=-- , _"-*,;_i..;:'iliii,,,~>,..' ~"",, "."