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 iscoupled with divergence aloft, the contribution of either maybe canceled by the

other. An estimate of the effects of each must be made before a decision is reached.

When surface pressure falls occur near the center and in the forward quadrants of the high, the high will weaken.

When a cold high that is moving southward is being heated from below, it will weaken, unless the heating is compensated by intensification of the ridge aloft. The amount of intensification can be determined by correlating the contributions of height change at the 500-hPa level as progged and the change in thickness as advected.

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