Shear Lines, Fronts and Areas of Confluence/Diffluence

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# Shear Lines, Fronts and Areas of Confluence - PowerPoint PPT Presentation

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, 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.

### Wind Divergence

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

### Evaluation of Shear Lines

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.