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Weather Considerations

by Vic-Maui Archives - Author Unknown, October 20th, 2009

Sailboat racing is like going to battle. A sound plan of attack is critical to success. To  win the battle, the yacht racer needs a sound strategic and tactical plan to overcome the competition. Mission planning begins with understanding the environment in which  we operate. Every successful skipper and navigator should be armed with knowledge of the weather and how to interpret weather charts at sea. This will promote correct decisions to safely make the boat go fast in the right direction. The key is sailing the yacht in the maximum wind over the least distance. To that end, the following is a brief explanation of how to use the most reliable weather information and where to obtain it.

Developing a strategic and tactical plan for a long distance race such as Vic Maui Cup involves many considerations. The effects of the weather should be one of them. Understanding surface and upper level charts, defining the ridge axis, adjusting the route with changing weather patterns, diurnal pressure changes, and squalls are some of the most important. They are discussed below.

The first aspect of mission planning includes developing an intended route based upon the indicated position and intensity of the Pacific High pressure system and the orientation of the ridge axis. It begins with understanding performance of atmospheric models.  Understanding models are important because this will be your fundamental source of weather patterns received in the form of fax charts over HF-SSB radio. From that we can derive trends in surface features depicted on charts. It is the trends we are after, not necessarily absolute values of wind speed and direction. If we are assured relatively more wind speed at a favorable direction than our competitor, we are in good position for success regardless of the true wind speed and direction. Basic model verification is relatively easy.

A good method is to compare the analysis with a forecast chart for the same valid time. For example, receive a 24 hr surface pressure forecast map that is valid for let's say May 15, 2000 1200Z. Then wait for the May 15 1200Z analysis. Compare the two. If they are close, between 1-2 mb, then it is a good chance the model is handling the physical processes in the atmosphere with reasonable accuracy. The same procedure can be done with other chart series including upper level (U/L) charts such as 500 mb heights. Upper level maps are particularly important because they describe atmospheric processes reflected at the surface. In other words, surface features depicted on weather maps are a direct result of processes occurring in the upper atmosphere. After a little experience your knowledge of pattern recognition will increase and help you determine if the models are providing an accurate picture. After verifying model performance, a route can be planned. As mentioned earlier, the goal is to sail the boat in the most wind over the least distance. The most wind is found where isobars are closest together and the least distance is determined, for the most part, by where the ridge axis is crossed. Let's examine each.

Choosing where to cross the ridge axis determines where you will be sailing the remainder of the race. The ridge axis can be defined as an area of maximum clockwise turning of  the wind flow around a high pressure system. A perpendicular line drawn from points tangent along the maximum isobar curvature visually identifies the axis. It is the base of the ?U' in the southeast corner of the High. (See figure) Using the forecast maps,  determine the intensity in mb of the High center and if it is moving. For illustration purposes, we'll start with the High center of 1027 mb located near 38 00 N 155 00W, its climatological intensity and position for July. If it is forecasted to be south of this position with the same or greater intensity, the pressure gradient is generally increased in the region southeast to south of the High center as indicated by tighter packing of the isobars on the surface weather map. This is because surface pressure changes are little surrounding the equator to near 20 degrees North.   Therefore, more wind is to the south, since there is a direct relationship between pressure gradient and wind speed.

Additionally, the High center, being closer to rhumbline, should be avoided at all costs. On the other hand, if the High center is north of its normal position with the same or even stronger intensity, then pressure gradient is generally evenly distributed to the south. The isobaric pattern is more equally spaced. In that case, it usually is not beneficial to sail more distance on a southern route when there is equal wind across the racecourse. Deciding if the wind will be stronger south is the determining factor in where you plan to cross the ridge.   It is a direct relationship. If the High is south, then generally it pays to cross the ridge farther south. How far south is determined by how the boat performs in various wind  conditions. After determining where to cross the ridge, then a route is easily  developed. Choose an estimated position on the ridge axis and after leaving the  coast head for it in the fastest way possible. It is not likely you will get to that point, however. It is critical to monitor the HF-FAX weather charts and adjust the proposed route in response to fluctuations in the High center and its associated isobars using the philosophy described above.

A handy instrument to have aboard is a calibrated barometer. Readings should be taken once an hour and compared with data obtained from surface pressure charts for the  another check on model performance.   Diurnal variation in surface pressure in lower latitudes is caused by atmospheric tides due to the effects of the sun and moon. This  effect can produce about a 1-2 mb fluctuation about the norm in the lower latitudes  and the tropics. As much as 3 mb have been observed. Maximum values occur at 1000 and 2200 local time (not PST) with minimums occurring at 0400 and 1600 local time. Be aware of a fall in the barometer below the known value of the diurnal change. It can mean significant changes in weather, such as an approaching tropical cyclone. Squalls, on the other hand, have relatively constant pressure except near their center. Tropical cyclones will be discussed later.

Now, let's move on to squalls.

Squalls usually occur in the region south to south west of the High center. Expect to deal with squalls for about 3 days towards the end of the race. Cool air traveling from the north is warmed and gains moisture as it moves southerly over a thousand miles of ocean. Local areas of  instability due to temperature differences cause pockets of rising air. As the air rises, it cools, condensing moisture and clouds form.If the instability is strong enough with accelerating vertical motion then squalls may develop. Squalls are small-scale feature of disturbed weather embedded in the trade winds Cloud tops tower over surrounding clouds and appear dark as they are laden with moisture. They are steered by upper level winds usually 15-20 degrees to the right of the trade wind direction. A boat sailing on starboard jibe with an apparent  wind direction of 160-165 degrees will have squalls affecting them from near dead astern. Correct use of them can be a significant factor in gaining considerable distance towards the finish. ULDB's have the ability to stay in front of a squall for long periods of time or until the squall dissipates. Heavier boats should position  themselves in the south west side of the direction of movement ahead of the squall on starboard jibe. Before the squall there is a sudden drop in temperature and increase wind velocity. This is the beginning of the downdraft winds ahead of the squall. As you get closer to it's track be prepared for further increase in wind strength. Good indications that you are crossing the squall's track are a backing shift (headed on starboard jibe) and decrease in winds.

As you cross in front, jibe to port and towards the southwest corner. This reduces the chances of getting caught in the  lull behind the squall. Sailing as deep as you can on port keeps the boat in the strongest wind for the longest time. If you are on the northwest side of the direction of travel on port jibe, then the opposite is true. The wind will veer approaching the direction of travel on port pole. This is OK too, as long as you stay on port pole as the squall moves away. Wind speeds can exceed double the value of normal trade winds. So if you are observing 20 kt trade winds, then you can expect an average of 40 kts in or near the squall. Squalls are stronger as night advances due to more cooling at the cloud tops raising instability.


Below are listed the recommended charts to receive over HF Radiofax while racing Vic Maui 2002

All Surface Analysis & Forecasts/Progs  to 96 HR
All 500 MB Heights Analysis & Forecasts/Progs  to 96 HR
All Satellite Imagery
Wind /Sea Analysis
Wind/Wave Prog

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