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<br />~ <br /> <br />Af\~ r~-tlGQ: ~.~GiJ~~h~e.~g(~~lA>~( <br />ee:t ("'--('lIt 'if F'/ 5~ ~{,lIl"':D._ <br /> <br />0: <br /> <br />e <br /> <br />AN OBJECTIVE FORECASTING TECHNIQUE FOR <br />HIGH PLAINS 'l'HUNDERSTORM DEVELOPMENT <br /> <br />Stephen H. Cohen <br /> <br />Environmental Research & Technology, Inc. <br />Denver, Colorado <br /> <br />David A. Matthews <br /> <br />1. <br /> <br />INTRODUCTION <br /> <br />U.S. Bureau of Reclamation <br />Denver, Colorado <br /> <br />-"" <br /><- \ <br />"'~ <br /> <br />e, <br /> <br />Since 1975 the Bureau of Reclamation's <br />office of. Atmospheric Resources Management has <br />been conducting a weather modification research <br />program in the vicinity of GoodLand, Kansas. This <br />project is part of the Hign Plains Cooperative <br />Program (HIPLEX). Daily on-site meteorological <br />forecasts are required for two general purposes. <br />The first is to aid the site project director in <br />daily operational planning and decision making. <br />The morning forecast emphasizes the type, tiwe <br />and location of cloud development. <br /> <br />A variety of products have been exam- <br />ined for application to the High Plains forecast- <br />ing including a numerical clOUd model. Th~MESOCU <br />model (Kre~tzberg and Perkey, 197b) was used to <br />obJect~vely analyze the ava~lable potential insta- <br />bll~ty ana cnermodynam~c potent~al for cloud growth. <br />Ava1lable potenEral instability (API) is the amount <br />of releasable instability (positive area on a <br />thermodynamic diagram) which would exist if the <br />atmosphere were lifted by mesoscale or synoptic <br />./" scale forcing. API may be considered as available <br />convective potential energy. This paper presents <br />( examples of an objective forecasting technique <br />{ ~at combines ~sentrop~c ana screamline analysis <br />wi th MESOCU, <br /> <br />2. <br /> <br />BACKGROUND <br /> <br />The terrain of the central High Plains <br />slopes from west to east, with average elevations <br />of over 5000 feet in eastern Colorado and Wyoming, <br />to near 4000 feet in western Kansas (Figure 1). <br />Higher elevations due to extensions of the Rocky <br />Mountains onto the High Plains a~e found along the <br />northern Colorado border, through the east-central <br />portion of the state and along the Colorado-New <br />Mexico boundary. <br /> <br />. <br /> <br />Because of the large variation of <br />height throughout the area of interest, the weather <br />patterns below 50 kPa are not readily apparent from <br />standard analyses that use nearly horizontal pres- <br />sure or elevation surfaces, Cramer (1972) has <br />shown that surface isentropic maps and large-scale <br />isentropic'cross sections give a fairly detailed <br />p~cture of temperature, stability and possible <br /> <br />~ <br /> <br />wind conditions in terrain of widely varying ele-: <br />vation. <br /> <br />Isentropic cross.-section analyses from <br />Denve:, Colorado to Dodge City, Kansas for 2 years <br />(1976-1977) indicate that due to inc:reased heating <br />.md mixing along the sloping terrain of eastern <br />Colorado, a mixed, dry-adiabatic la~'ec from the <br />3urface to between 70 kpa and 50 kPct usually devel- <br />'}pes by late afternoon. The portion of the region <br />lenerally east of the Kansas border i3 subject to <br />Less of an'.elevation change influen'~e, and mixing <br />.~il.l mostly be confined to a layer below 70 kPa. <br /> <br /> <br />rro' <br /> <br /> <br />. <br />Goodland <br /> <br />. La Junta <br /> <br />I <br />I <br />\ <br />I <br />I, <br />i <br />I <br /> <br />Figure 1. <br />eastern Colorado <br /> <br />Miles <br />r-- , <br />1 50 <br />Major terrain features of <br /> <br />10'0 <br /> <br />179 <br />