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References:

American Meteorological Society Glossary of Meteorology

Theater Climatology South America

Normally expect the shear line to lie parallel to the surface front as the air mass moves over the continents (particularly true over North America). As the front moves into subtropical areas (generally north of 35S over South America, Gulf of Mexico over North America) the shear line races ahead of the front.

Characteristics distinguishing a front include a pressure trough, a change in wind direction, moisture discontinuity, and certain characteristic cloud and precipitation forms.

This is a low level wind chart, with areas of wind diffluence along the blue asymptote, while the confluence is marked by the red asymptote. This is the typical streamline analysis for the evaluation of areas of directional wind confluence and diffluence.

The divergence of the total wind, in a simplified form, depends on the objective evaluation of the direction and speed terms. Subjective evaluation of the individual terms is not enough to accurately evaluate the proper areas of total wind divergence/convergence. You could have a directionally confluent flow that is speed diffluent. The net result could be convergent or divergent depending on which one of the two terms is the dominant one.

Forecasters mistakenly refer to the confluent asymptote as the “convergent” asymptote. Since convergence has to be determined objectively, not subjectively, it is more appropriate to use the term “confluent asymptote.”

Point A shows an area of speed diffluence. The winds are progressively accelerating upstream, which in-turn will result in the separation of the air parcels.

Point B shows an area of speed confluence, as the winds quickly decelerate as they approach the point. This in-turn will result in the compression of the air parcels.

This is an 850 hPa chart of winds and isotachs. The analysis shows directional confluence on point “A”. Simultaneously, the flow is speed diffluent on the same point as the winds accelerate downstream from this point. The question to ask is: “Is the flow convergent because of the directional confluence, or divergent due to the speed diffluence?” Once again, proper evaluation will require objective analysis.

This is an 850 hPa chart of winds, isotachs (cyan contours), wind divergence in dashed blue lines, and the wind convergence in solid red contours. Note that on the previously identified area, where directional wind confluence and speed diffluence were noted, the wind flow is divergent. The dominant term is the speed term.

This is an 850 hPa chart of winds and isotachs. The analysis shows directional diffluence on point “A”. Simultaneously, the flow is speed confluent on the same point. The question is, is the flow divergent because of the directional diffluence, or convergent due to the speed confluence?

This is an 850 hPa chart of winds, isotachs (cyan contours), wind divergence in dashed blue lines, and the wind convergence in solid red contours. Note that on the previously identified area, where directional wind diffluence and speed confluence were noted, the wind flow is convergent. The dominant term, once again, is the speed term.

This animation shows the typical evolution of a front and a shear line across the Caribbean Basin/Gulf of Mexico. The following figures will describe in detail.

In this example, a surface front is clearing the gulf coast of the Sern USA. Strong baroclinicity allows both the front and the shear Line to propagate in unison.

As the system moves across the Gulf of Mexico, the air mass starts to modify, with the shear line separating or racing ahead of the weakening baroclinic band near the southern end of the cold front. The waning dynamics allow the front to lag behind the shear line.

As the system moves across the Gulf of Mexico into the western Caribbean, the air mass continues to modify, with the shear line separating or racing ahead of the weakening baroclinic band.

Strong post frontal northerlies across southern Mexico (states of Chiapas, Tabasco and Campeche), will generally favor shallow convective activity due to orographic forcing. Dew point temperatures of 65F or greater will result in mixing ratios values of 12 g/kg. The high moisture content and the terrain forcing will result in rainfall maxima of 6-8 inches during a 24 hrs period!! This is an area where synoptic scale models such as the MRF, AVN and NOGAPS generally under estimate rainfall accumulation.

The shear line will support northerly flow across nrn Honduras and through Guatemala, with orographic forcing supporting shallow convection/cloud cover. Rainfall amounts are not as intense as they are normally seen over southern Mexico.

Farther into the Caribbean, the separation between the two boundaries becomes more pronounced.

This chart shows the 1000 hPa dew point temperatures in degrees C and the 850 hPa winds. Note the strong northerly flow across southern Mexico of 20-30Kt. The magenta colored oval shows the area where heavy rainfall amounts are likely to precipitate.

This chart shows the 850 hPa winds and isotherms at an interval of 02 degrees. The proper placement of a surface front is along the temperature gradient, while the shearline will manifest along the confluent asymptote.

This chart shows the 850 hPa winds and isotherms at an interval of 04 degrees. The proper placement of a surface front is along the temperature gradient, while the shear line will manifest along the confluent asymptote.

This is an 850 hPa wind plot, with the mean sea level pressure in solid contours (red) and the equivalent potential temperature in dashed blue lines. The temperature profile clearly shows a tight gradient north of the islands, with another boundary farther south and east across the srn windward islands.

The satellite imagery shows a narrow boundary north of Puerto Rico, and a secondary boundary to the east of the Lesser Antilles.

Evaluation of the satellite imagery and the low level wind/temp plot suggests that a front extends to the north of the islands while a shear line manifests farther south into the northern portions of South America.