Rules for Forecasting the Intensity of Highs

mass, and generally accompanied by cooling in the stratosphere. This is analogous to the deepening of lows where the decrease in mass, generally accompanied by warming at high levels, overcompensates the cooling in the troposphere. The evidence, therefore, indicates that high- level changes, undoubtedly due to dynamic mechanisms in the upper troposphere, are largely responsible for deepening and filling of surface pressure systems. This fact is of considerable prognostic value if the dynamic processes that induce these mass and density changes can be detected on the working charts. Rules for Forecasting the Intensity of Highs The following rules are for forecasting the intensity of surface highs: � Intensification of surface highs is indicated, and should be forecasted, when cold air advection is occurring in the stratum between 1,000 hPa and 500 hPa when either no height change is occurring or forecasted at 500 hPa, when convergence is indicated at and above 500 hPa or both, and when the cold advection is increasing rapidly. A high also intensifies when the 3-hour pressure tendency rises are occurring near the center, and in the rear quadrants of the high. When a moving surface high that is not subjected to heating from below is associated with a well- defined upper ridge, the change in intensity is largely governed by changes in intensity of the upper-level ridge. � Weakening of surface highs is indicated and should be forecasted when the cold air advection is decreasing, or is replaced by warm air advection in the lower tropospheric stratum, with either no height change at 500 hPa or when divergence is occurring or forecasted at and above 500 hPa, or when both are occurring at the same time. � When warm air and low tropospheric advection is coupled with convergence aloft, or when cold air and low tropospheric advection is coupled with divergence aloft, the contribution of either maybe canceled by the Figure 3-10.—Visual, local noon, first day. 3-14