Laserfiche WebLink
<br />Vertical soundings were constructed usin~1 the airborne vertical measurements <br />collected by the U.S. Postal Service prior to conducting mail service ovel' the <br />Rockies. These measurements were used to assist in determinin~J the thrust <br />which could be produced by piston engines to carry a payload aloft. The <br />soundings were collected by an aircraft which spiraled up to about 20,000 feet. <br />Accuracy tolerances of the collected vertical temperature and dew point were <br />reasonable given the fact that they were used to compute a life and death <br />situation for the pilots. HMS made adjustments to allow for the downwind <br />location of the soundings compared to the location of Masonville. HMS used <br />average September heavy rain event temperatures for altitudes above 20,000 <br />feet gathered during the past 18 years of flash flood prediction work along the <br />Front Range. <br /> <br />The rainfall observation used to verify this storm was routinely taken at 400 PM <br />in the afternoon. The observation reported the rainfall over the previous 24 hour <br />period, Le., from 400 PM yesterday to 400 PM today. Since the day prior to the <br />Masonville storm was verified as a flood producing day in the neighborin!~ l3ig <br />Thompson and S1. Vrain basins, it is possible that rainfall also occurred at the <br />Masonville rain gage in the 24 hours before the Masonville storm. <br /> <br />Additionally, the Masonville storm began shortly before 400 PM and continued <br />during the normal 400 PM observation time. The gage with the storm rainfall <br />was read near 600 PM. It is quite reasonable to consider that the observer did <br />not venture out during the height of the Masonville storm to empty the rain gage <br />of the prior day's rain. It is quite possible lmd even likely that the estimated <br />7 inches in 30 minutes and 8.1 inch stoml total rainfall was an exag!~eration <br />based on the accumulation of more than one storm's rainfall contribution <br />over a 26 hour period ratl1er than a 1 hour period. <br /> <br />Another reality check on the Masonville storm is possible if the surface and <br />upper air observations obtained from the Elureau of Heclamation files are used to <br />reconstruct the storm rainfall by using the HMS CSM technique. The HMS CSM <br />methodology calculated rainfall for the Masonville storm and estimated a peak <br />30-minute rainfall of only 3.5 inches and a storm total of only 5.15 inches in 90 <br />minutes. The HMS values represent a 36 percent reduction in the HMR I'eported <br />rainfall for this storm. However, if the obsented surface and upper air <br />observations are used to calculate an estimated thunderstorm rainfall for the <br />prior day, an estimate of 2.45 inches in an hour is obtained. The sum of these <br />two storm days' rainfall is an estimated 7.6 inches or about 94 percent of the <br />observed Masonville 1 hour value. Note that this summed value is near the ratio <br />of the CSM computed rainfall to the observed rainfaU for the other storms. <br /> <br />The HMS CSM storm values were calculated using the worst case scenario for <br />the day's observations. It is not physically possible to recreate or explain the <br />reported rainfall associated with this storm from the observed atmospheric <br />structure. The Colorado State University weather observations indicate that <br /> <br />29 <br />