OCR Text |
Show During a storm period, it was observed that precipitation will be nearly continuous at Alta so long as certain meteorological conditions are present. The first of these is dynamic lifting of the air mass. This usually occurs when the curvature of the flow or wind shear is cyclonic, or there is positive vorticity advection, etc. When anticy-clonic curvature is observed, precipitation seldom falls. One exception to this rule is when over- running moisture moves over the area from the Pacific Northwest well in advance of a disturbance. This may cause continuous light orographic snow to fall, even though other conditions do not favor precipitation. It can be shown that during a storm, precipitation generally will fall until the curvature of the flow becomes anticyclonic ( or vorticity advection becomes negative), at which time precipitation will nearly always cease. The other factors of importance are wind velocity and direction, which determine the mechanical lifting. As expected, the more nearly perpendicular the wind direction is to the mountain range, and the stronger the winds are, the more pronounced is the orographic effect. At Alta, upper wind directions of 240° to 300° usually are associated with the heaviest precipitation. Light winds, winds nearly parallel to the Wasatch Mountains, or winds within an easterly component are generally associated with relative light or no precipitation. Examples of the latter conditions are found in storms in which a cold low moves directly over Northern Utah. The upper winds become light and variable, leaving only dynamic lifting to produce precipitation. In this situation, similar amounts of precipitation fall at both valley and mountain locations. Another example is storms originating over California which result in a strong moist southerly flow of |