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).

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

Confluence is not equal to Convergence

Diffluence is not equal to Divergence

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

Speed
Diffluence/Confluence

Convergent or Divergent?

Evaluation of Shear Lines

Front/Shear Line Evaluation

Slide 16

Slide 17

Slide 18

Slide 19

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.

Example of “Ideal” Conditions for Heavy Rainfall

Example Front/Shear Line Caribbean

Example Front/Shear Line South America

Identify Features

Satellite Interpretation

Analysis


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.


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


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.