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Shear Lines, Fronts and Areas of Confluence/Diffluence. Mike Davison HPC International Desks. Shearlines vs. Fronts. Shear Lines: are associated with wind shifts (direction and speed).

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

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**Shear Lines, FrontsandAreas of Confluence/Diffluence**Mike Davison HPC International Desks**Shearlines vs. Fronts**• 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.**Divergence of the Wind**• 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 • Confluenceis not equal toConvergence • Diffluenceis not equal toDivergence**Divergence (Cont)**• 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**Directional Confluence Directional Diffluence**Speed Diffluence/Confluence**Speed Confluence B A Speed Diffluence**Convergent or Divergent?**Directional Confluence A Speed Diffluence**Convergent or Divergent?**Speed Confluence A Directional Diffluence**Forecast Hints**• 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.

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