


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 

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, inturn, 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 24 inches. 







