High Tropospheric Divergence in Developing Lows

If FUSING heights are indicated aloft over the expected low position, the AMOUNT MUST BE ESTIMATED to determine the ACTUAL height difference to which the rule will apply. In some cases the height rises aloft over the expected position of the low may be quite large, indicating the development of a high-latitude ridge aloft, which tends to block the eastward progress of the low. This may result in rapid deceleration of the low, with falling and/or recurvature to the north. In such a case, the forecast position of the low is revised in light of the changing circulation aloft. The above technique works only when lows are expected to move northeastward out of a heat source, such as the Southern Plains. When a low moves into the Southern Plains from the west or northwest, there is frequently no accompanying cooling in the low troposphere, since the low is moving toward the heat source. Some rules for the falling and deepening of surface lows in relation to upper contours areas follows: l Filling is indicated when a surface low moves l Surface lows tend to fill when the associated upper-level trough weakens. l Surface lows tend to fill when they move toward values of higher thickness lines. l When the associated upper trough deepens, the surface low also deepens. l Surface lows deepen when they move toward lower thickness values. l Waves develop along fronts when the 700-hPa windflow is parallel to the front, or nearly so. l During periods of southerly flow at 700 hPa along the east coast of the United States, secondary storms frequently develop in the vicinity of Cape Hatteras. High Tropospheric Divergence in Developing Lows In the case of developing (dynamic) cyclones, horizontal divergence is at a maximum in the 400- to into or ahead of the major ridge position of the 500-hPa 200-hPa stratum, and the air above must sink and warm level. adiabatically to maintain equilibrium. See figure 3-8. Figure 3-8.-Vertical circulation over developing low. 3-11