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Old 06-09-2000
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Michael Carr is on a distinguished road
Visualizing Weather, Part One

This is Part One in an ongoing series of articles by weather guru Michael Carr.

 
As nearly every mariner learns, local conditions can be different from those forecast.
 
All marine weather information is provided to sailors in a linear, deterministic format, that is, specific weather events and features are predicted to occur at a specific time at a specific location. Actual weather, however, is not deterministic, it is chaotic. In a chaos world events are non-linear, and as energy is injected or fed into a weather process it both modifies the weather and is modified itself. Chaos dynamics are often described by using the allegorical "butterfly effect", where an imperceptible event in one location may be amplified over time causing large events at remote locations. The name refers to a butterfly in the Amazon flapping its wings, initiating air movement which amplifies over time until it develops into a large weather event thousands of miles away.

Why is it important to distinguish and understand the difference between deterministic and chaos theories? Because sailors need to realize that weather forecasting is more a presentation of possibilities than probabilities—more a presentation of relationships that lead to possibilities. Translated to the real world these concepts suggest sailors should react to possibilities, not focus on exact deterministic predictions.

Deterministic forecasting predicts weather events to occur at specific locations at specific times and is the best tool marine meteorologists have to represent a chaos world. Deterministic weather forecasting is undertaken using a technique called ensemble forecasting where environmental data and assumptions are used by multiple weather analysis and forecasting models. Meteorologists evaluate each individual model output and then make an appropriate "ensemble" of these individual forecasts, injecting human intelligence into the process to derive a quality product. These weather forecasts, in the form of voice, text, and charts, are then disseminated to sailors for their use.

Reliable deterministic forecasting can presently be done out to five days, or 120 hours, and though outlooks exist for ten days, these 240 hour long range forecasts are not very useful for sailors who need more detailed and specific data. Every deterministic forecast, whether it be for surface winds, upper airflow patterns, or sea-state is constructed of several components. For example, a surface forecast chart will contain information on wind speed and direction, pressure, as well as the location and movement of high and low pressure systems and fronts. In an ideal world, each of these components will be correct and "spot on", but if one component is off for some reason the entire forecast should not be discounted, for as a whole the forecast may, and probably is, correct and useful.

For example, a forecast may predict 20 knot winds, and yet on site winds are 30 knots. The forecast for pressure, wind direction, front movement, and high and low pressure centers, however, may still be correct. In such a case, you should make an adjustment for the wind speed, possibly due to a local microscale event such as sea breeze or Gulf Stream North wall effect, which is bringing a localized increase in wind speed.

 
Combining situational awareness with forecasts and satellite imagery is the best way to balance a chaotic world.
 
A good way to balance a chaos world with deterministic forecasting is to use a "weather triangle" analysis procedure. Begin by developing skills in situational awareness, fancy words for knowing what is going on around you. What is the present wind speed and direction, swell height and direction, clouds, barometric pressure, and temperature? What is the trend of each, increasing, decreasing, or staying constant?

Now compare these on-scene "real world" conditions with official Marine Prediction Center (MPC) deterministic forecasts. How do they compare—are they in "sync" with your world, or are there differences which need to be examined and resolved? Finally, examine satellite imagery, either captured real time or downloaded from the Internet, assuming you are still dockside or have underway imaging capacities.

Satellite imagery is so useful because it is data intensive, high resolution, and shows weather features in real time. Both visible and infrared images are available, so comparison of temperature, texture, cloud structure, and sea surface temperature are readily and easily compared. Using GPS input images can be geo-referenced allowing proper time-space referencing.

 
Understanding weather underway is an ongoing task.
 
An ongoing comparison of on-scene conditions with official warnings, analysis, forecasts, and satellite imagery provides a reliable and consistent means for understanding formation, movement, and dissipation of weather features. Understanding weather is done through comparisons, detecting trends, and demonstrating causality. Weather events don't just occur—there is connection between upper airflow, surface temperatures and topography. When information on these inputs is viewed properly, weather is easily visualized.

In order to make valuable use of this new visualization of the weather, each sailor needs to add another ingredient. For a sailor to exercise prudent seamanship and avoid dangerous heavy weather conditions he must next understand his vessel's safe operating limits and response characteristics. Knowing these limits and characteristics allows maximum effective use of the weather forecasts, specifically wind and sea conditions.

"What if" routes and tactics can be simulated prior to actual implementation, which allows the sailor to know how his vessel will respond under known conditions. Accurate routing is of particular importance during night sailing when observation of swell and sea conditions is more difficult.

Therefore, there are two equally important factors affecting a sailor's development of a weather visualization and ultimate use of this forecast to determine an optimal sailing route:

 Understanding weather analysis and forecasts and their accuracy.
 Predicting vessel speed and motion under the forecast conditions.

Next month, Michael Carr's series on understanding weather continues with Part Two.

 

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