Shear Lines, Fronts and Areas of Confluence/Diffluence

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Shear Lines, Fronts and Areas of Confluence/Diffluence

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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

- 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

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

- We can express the divergence equation in a simplified form, with two terms:
- Direction
- Speed

- Confluenceis not equal toConvergence
- Diffluenceis not equal toDivergence

- 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

Directional Confluence

Directional Diffluence

Speed Confluence

B

A

Speed Diffluence

Directional Confluence

A

Speed Diffluence

Speed Confluence

A

Directional Diffluence

Evaluation of Shear Lines

Front and Shear Line

Front and Shear Line

Front and Shear Line

Front and Shear Line

- 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.

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