**Shear Lines,
Fronts
and
Areas of Confluence/Diffluence**

Mike Davison | |

HPC International Desks |

Shear Lines: are associated with wind shifts (direction and speed). | |||

A line or narrow zone across which there is an abrupt change in the horizontal wind component parallel to this line | |||

A line of maximum horizontal wind shear. | |||

An area of directional wind confluence along the tail end of a surface front. | |||

Lacks the baroclinicity/density discontinuity of surface fronts | |||

Fronts: The interface or transition zone between two air masses of different density. | |||

Density depends on temperature and moisture content | |||

Fronts either lie along shear lines or can lag behind them. |

**Wind Confluence and
Diffluence
in the Caribbean**

**Wind Confluence and
Diffluence
in South America**

We can express the divergence equation in a simplified form, with two terms: | |||

Direction | |||

Speed | |||

The direction and speed terms, in-turn, can be expressed as directional/speed diffluence and confluence | |||

Confluence is not equal to Convergence | |||

Diffluence is not equal to Divergence |

Divergence/convergence calculations need to take into account the direction and speed terms. | ||

This is done through objective analysis | ||

Streamline analysis is a subjective technique, and it only shows directional diffluence and confluence. | ||

It does not show convergence/divergence |

**Example Directional
Diffluence/Confluence**

During the Winter Months | |||

Convective activity over the Gulf of Mexico and gulf coast of Mexico tends to be capped below 500 hPa (20,000 ft) | |||

Evaluate upper dynamics for CONVERGENCE or DIVERGENCE before forecasting thunderstorms. | |||

If models forecast post frontal northerlies of 25Kt or greater, and dew point temperature of 20C (68F) or greater over southern Mexico. | |||

Forecast 12 hrs rainfall accumulation of at least 2-4 inches. |

**Example of “Ideal”
Conditions for Heavy Rainfall**

**Example Front/Shear Line
Caribbean**