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<br />/ <br /> <br />1S then estimated from an adiabatic diagram by using the saturation mixing ratio <br />at cloud base. <br />The next step is to evaluate upstream rawinsonde data, e.g., Ely, Salt Lake <br />City or Lander, plus current 700 mb and 500 mb analyses and prognostic charts, <br />to develop a water available forecast for the next 24-hour period. This detailed <br />operational forecast is prepared each morning and is valid from 11:00 MST today <br />until 11:00 H5T tomorrow. An actual cross-sectional sketch of the expected <br />cloud conditions over the target area is prepared each morning to document the <br />forecast. After preparing the detailed forecast, the hourly weather observations <br />and satell ite pictures from the NWS are monitored to insure that actual c"loud <br />conditions over the target area are close to the forecast cloud conditions. <br />Following the receipt of new NWS synoptic data each evening, and again the fol- <br />lowing morning, the forecast is re-evaluated and adjustments are made as <br />neces sa ry. <br /> <br />2. NUCLEI AVAILABLE <br /> <br />The availability of natural ice nuclei in the cloud is another key para- <br />meter in determining seedability. Favorable situations are those which have an <br />insufficient concentration of natural nuclei to efficiently utilize the avail- <br />able cloud liquid water. Cloud top temperature provides an indication of the <br />number of natural ice crystals available for the natural preCipitation process; <br />the colder the temperature, the more natural ice crystals are available in the <br />system (the concentration of ice nuclei is assumed to increase exponentially <br />with decreasing temperature). Operationally, it has been useful to evaluate <br />the upper temperature of the precipitation production region of a cloud system <br />by first determining whether it is either a pretrough or posttrough weathl~r <br />situation. <br />Since the cloud depths are greater and temperatures warmer in pretrough <br />conditions, for convenience, the temperature of the 500 mb level is used to <br />represent the temperature of the ice crystal producing region. There can be <br />clouds above the 500 mb level; however, it is believed that this higher cloudi- <br />ness does not contribute substantially to the main precipitation process of the <br />orographic portion of the cloud system. The range in the 500 mb temperature that <br />has been determined to be favorable is from -12oC to -27oC. <br /> <br />-2- <br /> <br />All <br />