On 1-2 January 1997 and 31 December 2005 two of the most damaging floods in modern times affected the Reno-Sparks, Nevada metropolitan area when the Truckee River was forced over its banks. The Truckee River runs directly through this urban region within a complex sloping valley to the lee of the Sierra Nevada Mountain Range. Excessive property damage occurred during these floods in a rapidly developing urban region. The flooding was associated with Truckee River stream flow rates in excess of 15,000 CFS at the USGS station gauge in downtown Reno, Nevada. This flooding was less the result of snow melt than excessive rains exceeding 200 mm in 18 hours that spilled over the Carson and Sierra Crest Ranges dividing Nevada from California. Such extreme lee side precipitation in a semi-arid location, which is very unusual, drained into the narrow and normally shallow Truckee River that runs from the Sierra Nevada Mountains directly through the metropolitan Reno-Sparks region of northwestern Nevada. Flooding was particularly damaging in the downstream portion of the urban region in Sparks, Nevada where the flood plain is broad, uniformly low in elevation and unobstructed by topographic elements.

In depth observational and numerical modeling studies have provided insight into multi-scale processes that commenced 24-72 hours prior to the extreme spillover event and therefore can be employed to form a paradigm of what forecasters can expect prior to extreme flooding. Key diagnosed processes include the following: 1) an extraordinarily strong polar jet streak that was developing over the east central Pacific Ocean 48-72 hours prior to the heavy precipitation and was elongated from just north of the Hawaiian Islands to the Pacific coast of North America, 2) the establishment of a very strong and elongated polar jet streak exit region that propagated southeastward along the California coast ~24 hours prior to the heavy rainfall, 3) a mid-level and very high speed flow of warm and moist air within the 700 – 500 hPa layer above the windward and lee side of the Sierra Nevada during the heavy rainfall in which horizontal moisture flux values approached or exceeded in magnitude what is typical for planetary boundary layer moisture fluxes during heavy precipitation, 4) a deep moist neutral atmosphere extending from the lee side of the Sierra Nevada to nearly the tropopause during the heavy precipitation event and 5) a lee side meso-beta scale low pressure area embedded within a broader meso-alpha scale trough of low pressure during the heavy precipitation event.

Numerical simulations of these events were unique in that an adaptive (unstructured) grid numerical model was employed with very detailed resolution in the complex terrain just west of the Reno-Sparks metropolitan area. By placing the highest grid resolution in the most complex terrain an efficient simulation of terrain-induced effects on the rainfall could be generated. Such a grid enabled very fine horizontal resolution to be placed on the sloping terrain in the western suburbs of the metropolitan area thus enabling an estimate of the gradient of precipitation between the Reno Airport and National Weather Service as well as between the nearby Carson Mountain Range and the urban region that included the western suburbs of Reno. The simulated meso-beta and meso-gamma scale precipitation patterns and near surface atmospheric features delineate complex variations in surface and atmospheric variables within a metropolitan valley location during an orographically-modified heavy precipitation event. The possible utility of such a modeling system for generating forecasts of a high impact flooding event within an urban region will be discussed.