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I I I I I I I I I I I I I I I I I I. I A POTENTIAL CONTRlBUTION TO THE UNDERST ANPING OF REA VY PRECIPITATION PRODUCING THUNDERSTORMS John F. Henz Henz Meteorological Services 2480 W. 26th Avenue, Suite 31 OB Denver. Colorado 80211 1. INTRODUCTION A lrigJ'ir.."nt problem facing operational meteor. ologists on the localleve1 is the refinement of tegional quantitative precipitation forecasts (QPF ) into a local product. This problem is especially trouble!lome when thUlldetstolIllS are the source of the precipitation over small urban or stream basins pf less than 2,5QO'squaIl'mi1es. ",,':' .. ';: .~-' - '--, The problem is frequentlycompounde4 in urban areas by the rapid nmotr response of highly impervious areas to high intensity rainfall associated with thunderstolIllS. In these areas a premium is placed on the ability of the operational meteorologist ,to identifY a potential t1ash flood situation as soon as it is possible. In these situations even the ,."h"n~ heavy p=ipitation detection ~d display q>pabilities ",fthe WSR-88D may be \00 $jow toallowJeSPOnse rather than reaction by local officials,to at1ash flood situation. Considerable research effort was d4-ected at , researching the meso-synoptic structure of the atmosphere associated with t1ash flooding events after the tragic Big Thompson, Colorado t1ash flood ofJuly31 ~dAugust 1,IQ76.Maddox,et 19xx ~d Chappell, et al, '19xx provided numerous pattern recognition techniques, composite soundings and ' kinematic forecast techniques to assist the operational meteorologist in identifying the presense of the t1ash flooding threat. These techniques have provided invaluable cperational insights into the successful predictionoft1ashflooding potential across most of the country. ." .., . Another operaticnal step which must be addressed once the t1ash floodpotenwiI has been detected is the quantification, of the heavy precipitation forecast. . Simply stated: How much rain can be produced by a thundentorm or tbundentorm complex over a given period of time? While the pattern recognition techniques reported have provided insight into predicting the potential heavy rain situation ~dother key insight relates to the amount cf rainfall that can be expected. A step in answering this petplexing question in the quantitlication prediction can be 8CC01IIp1ished through the analysis of implied therma1 cbanlcteristics of the predicted thunderstorm updraft on Skew.T, Log P diagrams. 11ll: reaminder of this paper will share the autho!'s experience with ibe, ,,' technique and discuss some of its applications and possible physical interpretations. .. 2.0 Updraft Analyses - Skew T, Log P The Flash Flood Prediction Program (F2P2) , . sponsored by the, Urban Drainage, &. Flood Control District of Denver, Colorado ( uDFCD ) has afforded the author an opportunity to prod""" daily t1ash ,flood potential predictions since I ~.79 for the period from IS April to 15 September for a 1600 square mile area lI\II!OWlding the Denver, Colorado metropolitan area. An integral partofthe daily analyses program is directed at the kinematic analysis of the vertical " structure of the almosph";' as 'it is :depict,ed on a. ' Skew T, Log P diagram through the use of both .. forecast and observed atmospheric soundings,.. The daily sounding analysis lll1Iline is rather detai1ed and is conducted on a standard Skew T, Log P diagram which has been modified by, Benz Meterological Services (HMS) for use in the QPF process. The modified diagram is presented in Figure I, Note the following differences in the modified fOllll: . A schematic presentation of the vertical profile of the Rocky Mountains ~ Denver, Colorado on the lower left hand comer of the diagram: -','~' . A precipitable water, index calculation table which uses mixing ratio values and a formula to calculate the surface to 500mb p=iptiable water index. . A stability data table which is used to record the temperature (T) and dew point (Td) of the surface lifted parcel, the average temperature deviation of a surface lifted parcel (Del T), the updraft depth (De1Z) and the depth of the updraft's warm layer.