Cyclogenesis and deepening are closely related to cyclonic flow or cyclonic vorticity aloft If you recall from the discussion of vorticity in chapter 1, vorticity is the measure of the path of motion of a parcel plus the wind shear along the path of motion. Thus, we have the following rules for the relationship of vorticity aloft to the deepening or falling of surface lows:

. Increasing cyclonic (positive) relative vorticity induces downstream surface pressure falls. 

. Increasing antic cyclonic (negative) relative vorticity induces downstream surface pressure rises.

. A wave will be unstable and deepen if the 700-hPa wind field over it possesses cyclonic relative vorticity.

. A wave will be stable if the 700-hPa wind over it possesses anticyclonic vorticity.

. If there are several waves along a front, the one with the most intense cyclonic vorticity aloft will develop at the expense of the others. This is usually the one nearest the axis of the trough.

The amount of deepening of eastern United States lows moving northeastward into the Maritime Provinces of Canada frequently can be predicted by estimating the number of contours at the 200- or 300-hPa level that would be traversed by the surface low during the forecast period. For a close approximation, multiply the 200-hPa current height difference in tens of meters by 3/4 to obtain the surface pressure change in hectopascals. For example: a 240-meter height difference at 200 hPa results in a pressure change of 18 hPa at the surface.

If FALLING heights are indicated aloft, the AMOUNT of fall need not be estimated. The deepening of the surface low and greater advective cooling, associated with the occlusion process, appear to compensate for the upper height falls.

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:

. Filling is indicated when a surface low moves into or ahead of the major ridge position of the 500-hPa level.

. Surface lows tend to fill when the associated upper-level trough weakens.

. Surface lows tend to fill when they move toward values of higher thickness lines.

. When the associated upper trough deepens, the surface low also deepens.

. Surface lows deepen when they move toward lower thickness values.

. Waves develop along fronts when the 700-hPa windflow is parallel to the front, or nearly so.

. 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.

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