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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 l.jpg

Shear Lines, FrontsandAreas of Confluence/Diffluence

Mike Davison

HPC International Desks

Shearlines vs fronts l.jpg
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.

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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 l.jpg
    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 l.jpg
    Example Directional Diffluence/Confluence

    Directional Confluence

    Directional Diffluence

    Speed diffluence confluence l.jpg
    Speed Diffluence/Confluence

    Speed Confluence



    Speed Diffluence

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    Convergent or Divergent?

    Directional Confluence


    Speed Diffluence

    Convergent or divergent12 l.jpg
    Convergent or Divergent?

    Speed Confluence


    Directional Diffluence

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    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.