FLOOD04822 - Laserfiche WebLink

Home Browse Search

In effect, the EPA is used to identify the "wc)rsf case" individual components of the atmospheric structure of the historical extreme precipitation events. The Extreme General Storm Atmosphere is obtained by "piecing together" the components from general storms of record including the complex convective general storms and general rain patterns whictl contributed to peak 72-hour general storm rainfall. This process allows contributions from convective rains and general rains. Note that the HMR PMP methodology selects or identifies the storm which provides the maximum stress to water storage facilities (e.g., dams and spillways). In the HMS methodology, layers of atmospheric structure taken from different events are "pieced together" to ~Ienerate a hybrid or composite sounding which produces the "most stressing atmosphere". The Extreme General Storm Atmosphere Gomponents were selected from the data in Table 3 of the historical extreme preGipitation events. The earlier HMR site-specific general storm PMP calculations supported the fact that the heaviest PMP 1-hour and 6-hour rainfalls were produced by complex convective general storms of less than 12 hours duration. The heaviest PMP 24-hour through 72-hour rainfalls were associated with continuing general rainfall with potential convective contributions dlJring the second 24-tlour period. While these different contributions were apparent, no mechanism in the methodology exists to "connect" the rainfall from the shorter duration complex convective genel'ai storms to the longer, steady accumulating general rains. The HMS methodology allows the selection of components of both general storm systems to effect a maximization of precipitation opportunity. This is equivalent to deriving PMP from contributions "among storms" (see HMR 55A, Section 10.1.1). The Extreme General Storm Atmosphere is based on a composite of the following Extreme General Storm precipitation Event components: 1. Surface conditions: Temperature 80 degrees F and surface (6,000 'feet msl) dew point 66 degrees F (both vcllues are climatic maxima for Colorado Springs), 2. Vertical temperature, moisture and wind structure taken frc>m thEl Big Thompson Canyon event for elevaticlns above 700 mb and from Ithe Plum Creek event for elevations below 700 mb, 3. Low level jet forcing (I.e., convergence into terrain barriers) of 40 knots for thunderstorm rainfall (as occurred with the Big Thomps;on Canyon event) and 30 knots forcing for genelral rain (as occurred in the Big Elk Meadow event), 4. Precipitable Water Index (PWI) of 1.65 inches (assumes saturated conditions from the surface, taken as 6,0100 feet msl to 18,000 feElt msl, with a surface dew point of 66 degre'es F), 39