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Envelopment: A process for selecting the largest value from any set of data. In <br />estimating PMP, the maximum and transposed rainfall data are plotted on graph paper, <br />and a smooth curve is drawn through the largest values. <br />Front: The interface or transition zone between two air masses of different parameters. <br />The parameters describing the air masses are temperature and dew point. <br />General storm: A storm event, which produces precipitation over areas in excess of 500 <br />square miles, has a duration longer than 6 hours, and is associated with a major synoptic <br />weather feature. <br />Local storm: A storm event that occurs over a small area in a short time period. <br />Precipitation rarely exceeds 6 hours in duration and the area covered by precipitation is <br />less than 500 square miles. Frequently, local storms will last only 1 or 2 hours and <br />precipitation will occur over areas of up to 200 square miles. Precipitation from local <br />storms will be isolated from general -storm rainfall. Often these storms are <br />thunderstorms. <br />Low Level Jet stream: A band of strong winds at an atmospheric level well below the <br />high troposphere as contrasted with the jet streams of the upper troposphere. <br />Mass curve: Curve of cumulative values of precipitation through time. <br />Mid - latitude frontal system: An assemblage of fronts as they appear on a synoptic chart <br />north of the tropics and south of the polar latitudes. This term is used for a continuous <br />front and its characteristics along its entire extent, its variations of intensity, and any <br />frontal cyclones along it. <br />Moisture maximization: The process of adjusting observed precipitation amounts <br />upward based upon the hypothesis of increased moisture inflow to the storm. <br />Observational day: The 24 -hour time period between daily observation times for two <br />consecutive days at cooperative stations, e.g., 6:00PM to 6:00PM. <br />One - hundred year rainfall event: The point rainfall amount that has a one - percent <br />probability of occurrence in any year. Also referred to as the rainfall amount that on the <br />average occurs once in a hundred years. <br />Precipitable water: The total atmospheric water vapor contained in a vertical column of <br />unit cross - sectional area extending between any two specified levels in the atmosphere; <br />commonly expressed in terms of the height to which the liquid water would stand if the <br />vapor were completely condensed and collected in a vessel of the same unit cross - section. <br />The total precipitable water in the atmosphere at a location is that contained in a column <br />or unit cross - section extending from the earth's surface all the way to the "top" of the <br />atmosphere. The 300 -mb level is considered the top of the atmosphere in this study. <br />9 <br />