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Mike Davison |
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HPC International Desks |
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Shear Lines: are associated with wind shifts
(direction and speed). |
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A line or narrow zone across which there is an
abrupt change in the horizontal wind component parallel to this line |
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A line of maximum horizontal wind shear. |
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An area of directional wind confluence along the
tail end of a surface front. |
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Lacks the baroclinicity/density discontinuity of
surface fronts |
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Fronts:
The interface or transition zone between two air masses of different
density. |
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Density depends on temperature and moisture
content |
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Fronts either lie along shear lines or can lag
behind them. |
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We can express the divergence equation in a
simplified form, with two terms: |
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Direction |
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Speed |
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The direction and speed terms, in-turn, can be
expressed as directional/speed diffluence and confluence |
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Confluence is not equal to Convergence |
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Diffluence is not equal to Divergence |
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Divergence/convergence calculations need to take
into account the direction and speed terms. |
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This is done through objective analysis |
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Streamline analysis is a subjective technique,
and it only shows directional diffluence and confluence. |
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It does not show convergence/divergence |
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During the Winter Months |
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Convective activity over the Gulf of Mexico and
gulf coast of Mexico tends to be capped below 500 hPa (20,000 ft) |
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Evaluate upper dynamics for CONVERGENCE or
DIVERGENCE before forecasting thunderstorms. |
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If models forecast post frontal northerlies of
25Kt or greater, and dew point temperature of 20C (68F) or greater over
southern Mexico. |
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Forecast 12 hrs rainfall accumulation of at
least 2-4 inches. |
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Notes
