Common case weather

The variety and dynamism of weather make it a subject of endless concern for the offshore sailor. Its fast-changing complexity can be baffling to the voyager. Here are four common weather scenarios that every sailor should recognize, have a basic understanding of, and know how to handle.


Hurricanes and mid-latitude storms are best handled by being given a wide berth, as there are few realistic or practical techniques in dealing with these monstrous weather phenomena if they are directly encountered. Though many weather and seamanship textbooks discuss such aspects of hurricanes as “safe” and “dangerous” semicircles, in reality all portions of these systems are dangerousfor both small and large vessels.

Avoidance is the essential tactic, but it is not always easy. Hurricanes develop and move in ways that frequently seem to defy analysis and prediction. There are, however, three aspects of hurricane behavior that are consistent and need to be fully understood. These are: 1) track forecasting errors; 2) acceleration and the influence of the jet stream; and 3) transformation into mid-latitude low pressure systems.

Hurricane track forecasting has recognized limits and errors, with error magnitude varying depending upon factors such as the computer model being used to simulate a particular hurricane, the amount and quality of data available, and the forecast interval. A detailed discussion of forecast models is found on the National Hurricane Center home page:

In general, track error forecasting for a hurricane is 100 miles either side of a predicted track for each 24-hour forecast period. Thus, for a 72-hour forecast, an error of 300 (3 x 100) miles to the left or right of an official predicted track is applicable, and for a 96-hour forecast, 400 (4 x 100) miles left and right of track is applied.

This rule of thumb is seen when calculating an average error for the seven computer models used in analyzing Atlantic hurricanes during the 1996-97 hurricane season. Errors are listed below.

Therefore, when a hurricane’s track is plotted, a 100-mile error for each 24-hour period must be applied, and a vessel within this adjusted area must take action as if a hurricane were bearing directly toward them, which may become the case.

Hurricanes move at varying speeds, often plodding along for days before suddenly accelerating. Hurricane Mitch (1998) is a perfect example, first meandering along and stalling in the Caribbean for more than a week before being influenced by upper-level winds and accelerating to the NE at 35 knots. Mitch moved from a position near Honduras to the Azores, a distance of 2,400 miles, in less than three days. So a lesson here is to closely monitor a hurricane’s movement at intervals no greater than every six hours, the official forecast interval for the National Weather Service’s Tropical Prediction Center.

Since official analyses and forecasts are by definition several hours old when received, a more accurate method of tracking and analyzing hurricane development and movement is using real-time satellite imagery. Imagery is continually available from NOAA polar-orbiting and geostationary satellites (presently there are three polar satellites, NOAA 12, 14, 15 and two geostationary satellites, GOES 8, 10) via real-time capture, Internet access, and wefax broadcast.

When hurricanes do move outside tropical regions (0° to 30° north and south) and into mid-latitudes (30° to 60° north and south) they lose tropical characteristics, developing cold and warm fronts as cold air is entrained. As this transformation occurs, a hurricane also loses its focused eye and begins to spread out. This spreading disperses energy and brings increasingly stronger winds farther from a dissipating hurricane center. Thus, contrary to what you might think, as a hurricane dissipates in mid-latitudes (becoming a mid-latitude low-pressure system) wind and sea conditions often worsen for a vessel lying on a hurricane’s outskirts.

Hurriance Mitch was a good example: as it moved NE from the Caribbean and became a mid-latitude storm (34 to 63 knots) its size increased, from 200 miles to nearly 600 miles in diameter, as energy contained within its center spread outward.

Hurricanes are very powerful weather features that dominate weather over thousands of miles horizontally and to the top of the atmosphere vertically. They should be treated with great respect and monitored continuously.

Rapidly intensifying low

Often called meteorological “bombs,” rapidly intensifying lows (RIL) are mid-latitude low pressure systems that develop so quickly and with such great force they often catch mariners by surprise. One indication of RIL development is a drop in surface pressure of one millibar per hour for 12 to 18 consecutive hours. However, a drop of one mb per hour for six consecutive hours is sufficient evidence of deteriorating weather to warrant action.

For a RIL to form there needs to be several supporting elements within the atmosphere:· Strong upper-level (500 mb) winds with speeds of 100 knots and greater· A well-formed and distinct upper-level trough that shows continuing signs of development· Warm surface conditions with moisture present, such as from the Gulf Stream and Kuroshio current· Surface air masses showing distinct temperature and dewpoint differences and located beneath an upper-level trough.

RILs often form in fall and spring when temperature differences are accentuated; they often become as powerful as hurricanes. They can be avoided if on-scene conditionsbarometric pressure, cloud development, wind and sea conditionsare watched and weather charts, both surface and upper air, are examined several times a day, ideally every six hours as updated charts become available.Marine Prediction Center meteorologists annotate surface charts with the term RAPIDLY INTENSIFYING LOW when they detect such a system forming, and these words should be taken with the same seriousness as a hurricane warning. And, since these systems develop to full strength in less than 24 hours, there is little or no time for contemplation once a RIL is detected.

Frontal passage

Mid-latitude low-pressure systems develop from a meeting and mixing of cold and warm air masses. Leading edges of mixing air masses are designated as cold and warm fronts and are areas that indicate significant temperature differences. Temperature difference represents pressure, or density, difference, and this translates into wind production.

Detecting and then using or avoiding frontal passage will make a significant difference in weather conditions for several days. Though cold and warm fronts both represent edges of air masses, they behave quite differently. Cold fronts move faster than warm fronts and are more dynamic. This is because in the formation of low pressure systems, cold fronts gain support from strong upper-level flow (as depicted on 500-mb charts) longer than warm fronts.

In the Northern Hemisphere, fronts associated with low pressure systems follow a movement sequence as shown in the illustration on page 85. Notice that the strongest surface winds are found initially preceding a cold front; they then move to a low’s north sector, then to the west, southwest, and finally to near the low’s center. This movement coincides with areas where the greatest temperature differences are found as warm and cold air mix during the process of becoming a homogeneous air mass.

Changes in wind speed and direction preceding and following a frontal passage can be used to advantage if detected early, and detection is accomplished using weather charts, text forecasts, on-scene observations, and satellite imagery. Along the U.S. East Coast, for example, taking departure directly behind a cold front provides several days of consistent W and NW following winds, and is an ideal departure scenario for a voyage to Bermuda or points east.Warm fronts are not as focused as cold fronts as they are located under weaker jet stream winds and have their energy spread over a larger area. Winds across a warm front tend to shift from SE to S and SW, and this is most significant in indicating the subsequent approach of an associated cold front.High pressure zonesWhen a pebble is dropped in a pool of water, its impact on the water’s surface produces ripples that, clustered together, move outwards. Areas of high pressure have the same dynamics; air descends from the upper atmosphere, impacts the Earth’s surface and moves outward. Friction slows the outward movement of air after it impacts the Earth’s surface, so it piles up and forms “ripples” that define a high-pressure region’s outer edge.Wind around high pressure is found within these ripples as they indicate an area containing energy. Locating a “ripple” area is accomplished using an assortment of tools and techniques, including surface and upper-air analysis and forecast charts, satellite imagery, barometric pressure, and clues from clouds.

Analysis and forecast charts produced by the NWS Marine Prediction Center are highly accurate in their analysis and prediction of Atlantic and Pacific high-pressure areas. These charts are broadcast via high-frequency SSB and are simultaneously available on the Internet at the Marine Prediction Centers site;

Using surface and upper-air charts, areas of high pressure are easily located and tracked. Especially useful are 48-hour and 96-hour charts, as these products include 24-hour positions of both low and high pressure areas, and in a sense place weather in motion. Real-time as well as re-broadcast satellite imagery provides a highly accurate method of locating and tracking high pressure because satellite imagery is data intensive. Image resolution (pixel size) is available as both 4 km and 1.1 km, and using image zooming and color enhancement extraordinary detail can be obtained from each image.

Using a combination of visible and infrared satellite images, both day and night analysis of clouds found within a high’s outer “ripples” can be located and tracked, and this is where wind is found. Generally a combination of low, middle, and high clouds are found within a high’s outer “ripple” area, with clouds developing shapes and orientation dependent on wind strength. Areas showing only low-level cumulus clouds having random orientation represent a high’s center where winds are light or calm.

Cloud shape is an excellent indicator of surface winds. Symmetrical or donut-type clouds indicate winds speeds of less than 10 knots. Elongated donut shapes are seen when wind speed increases to between 11 and 20 knots. When clouds take on a banana shape, winds are up to 21 and 30 knots, and when clouds become elongated and resemble peapods winds have reached 30-plus knots.

By Ocean Navigator