Efficient weather

Voyagers often wonder how to get the best weather information offshore aboard a small vessel. The key to more efficient and effective weather gathering is knowing what weather information you really need and which products fill those needs.

Many mariners focus too much on what’s available and not enough on what they need. With the variety of available weather products growing faster than our ability to access them offshore, the shotgun approach of getting every bit of weather information possible is not practical. The result is wasted time, money and communications resources, obtaining information that does not tell you what you need to know.

Asking the right weather questions is your first step if you want to gather the most useful weather information in the most economical way.

For example, if you want to know what the weather is like in the Eastern Pacific, a conventional weatherfax product offers a familiar, useful overview. If you want to know the wind speed and direction in Bermuda for the next seven days, a Buoyweather.com text forecast gives you precisely what you’re looking for in a file smaller than 2 kb. If you want to know what the wind speed and direction will be at various locations in the northeast Caribbean for the next five days, grib (gridded binary) data is the best tool.

Unless otherwise noted, assume most wind forecasts are driven only by pressure gradients, and that is certainly the case in all of the forecasts mentioned above. But there are factors that might cause wind to deviate from that caused by pressure gradients. Would you like to know if there is a risk of higher winds, or winds significantly different from the forecast? Adding parameters such as precipitation and lifted index to your grib data may help answer those questions. If you’re extremely bandwidth limited and in the South Pacific, fleet code data answers the question, “What is the pressure gradient?” very efficiently.

Voyagers’ questions vary

The weather questions you ask may be different from those of others. If you are sailing on an open-water passage, you will want to know wind speeds and directions, chances for squalls and their likely intensity, sea states and sea surface currents. All of this information is available at sea in minute detail for virtually any location on Earth to almost anyone with a minimal ability to send and receive data.

If you’re among the growing ranks of coastal or offshore trawler folk, sea state is probably your most vital query, though the other factors mentioned for sailors hold some interest. Ask yourself, “What do I want to know about sea state?” In addition to sea height, you probably need a forecast for direction and interval of the waves, as well as some guidance on whether seas will be coming from more than one direction. If you know you need all of this information, you can save lots of time and possibly expense by eliminating any product that fails to answer your questions.

Those on a fast multihull in most conditions can focus less on sea surface currents and more on the other questions.

Think beyond the previous questions to focus on which environmental factors hold the key to your plans. If you are navigating a tricky reef, you may want to know how much cloud cover is forecast. If your radar is out or you’re near shipping lanes or tricky coastal areas, fog may be an important factor. In many situations you may want to validate the forecast with satellite, buoy or vessel reports from your region.

Low-bandwidth weather products

Almost any useful weather product on the Internet is available in a format optimized for low-bandwidth communications methods such as HF email, satellite phones and cell phones. If the product you want is not available in a practical fashion, there is probably a more efficient product that answers the same basic weather questions. Winlink’s catalog now has about 900 weather products covering all common cruising grounds (Winlink 2000 is a global email system for amateur radio users). Ocens’ WeatherNet boasts more than 25,000 products covering almost every location on Earth and works with almost any data connection, including email. These providers and others continue to add content. You can’t get it all, so select only the products that provide the answers you really need. Your needs may change from day to day, and though there will be some products you get every day, some you will obtain only to answer less frequent queries.

Let’s study one weather scenario and see how each of several common weather products answers our weather questions. Most of us have access to weatherfax products via some combination of HF radio/computer or dedicated fax/Navtex receiver. If your question is general – “What’s the weather like in the Western Atlantic?�VbCrLf – then weatherfax products may be a good choice. If you really want to know the forecast wind speed and direction 80 miles off the South Carolina coast tomorrow afternoon, you’ll have to obtain the correct weatherfax and spend time interpreting it. If you’ve done it correctly, you will have some idea whether winds will be light and from the southeast or strong and from the west.

How would you characterize the general weather situation off the South Carolina coast depicted by the 48-hour weatherfax forecast for 12z on June 29, 2005, in Figure 1? With a broad, weak area of high pressure essentially stationary near Bermuda and no indication of anything to the contrary, you might guess weather would be fair.

“What is the forecast wind speed and direction off the South Carolina coast, south of Cape Hatteras?�VbCrLf The 1,016-mb and 1,020-mb isobars are relatively close together in this area, so you might guess winds SSW at 15 to 20 knots.

NOAA’s wind/wave forecast chart (Figure 2) suggests that’s pretty close – answering the same wind question with: SSE at 15 to 20 knots.

NOAA’s text forecast for June 29 gives a similar answer:

CAPE HATTERAS TO MURRELLS INLET �� OUT TO 250 NM. THIS AFTERNOON �� S TO SE WINDS 15 TO 20 KT. SEAS 3 TO 5 FT. SCATTERED SHOWERS AND TSTMS �� MAINLY W PORTION.

Even on June 29, the synopsis in NOAA’s offshore forecast only mentions the high pressure east of the area. There was more going on in the area, and only in the Marine Weather Discussion did meteorologists suggest uncertainties and other scenarios.

MARINE WEATHER DISCUSSION, 142 AM EDT TUE 28 JUN 2005

THE CANADIAN MODEL CONTINUES TO SHOW LOW PRES DEVELOPING ALONG THE CAROLINA COAST BY THU NIGHT �� THEN TRACKING NE ACROSS THE NT2 WATERS. WILL CONTINUE TO DOWNPLAY THIS SOLUTION AND FOLLOW A SOLUTION CLOSER TO THE GFS.

This is a good example of a situation where you should ask, “Is the forecast verified by current observational data?�VbCrLf By the evening of June 28, there were squalls with more than 30 knots of wind throughout much of this area. NOAA’s technical text discussion product mentioned one weather model – the Canadian model – had been consistently predicting a closed low would form in this area on June 30, and several other models hinted at various degrees of trough activity.

A satellite-observed wind product would have demonstrated the forecast was not correct for this area even on June 28. On the morning of June 29, satellite wind observations continued to show nasty weather (Figure 3), with SSE winds about 20 knots east of 76� W but much stronger winds of 30 to 50 knots in squalls west of 76� W. Squalls and stronger winds occurred all along a trough that developed just south of New England, to the Carolina coast, through Florida and into the Gulf of Mexico.

Aside from reading the discussion product, you might have known the weather was taking a turn for the worse with the Buoyweather.com forecast. It predicted south winds near 20 knots, along with 100 percent cloud cover, a falling barometer, possible thunderstorms and a slightly unstable atmosphere.

But that doesn’t illustrate all the events taking place in the region. Look at the Global Forecast System grib (Figure 4). Notice the south winds near 20 knots, east of 76� W, southwest winds near 20 knots west of 76� W, and the focused area of heavy rain off the South Carolina coast? The wind direction convergence, generally unstable atmosphere and area of heavy rainfall suggest winds in squalls are likely to be much stronger than the 20 knots of gradient wind forecast.

In the above scenario, the role of filters is interesting. NOAA filtered all but the most general information. Such filtering is appropriate, since weather phenomenon of interest to you might not be important to a tanker ship captain or beachgoers. NOAA’s broad constituency necessitates a high level of filtering. Buoyweather.com text filtered everything except what was forecast by one model for one precise location, but it offered more details of potential interest to us. The GFS grib filtered other forecast solutions but gave us a real feel for the bigger picture as well as very specific information. The NOAA discussion product suggested the forecast carried some degree of uncertainty, and we might need to dig a little deeper in our analysis. Doing so, we could have guessed things might turn out nasty.

Getting specific answers

The best weather products for the job will vary, depending on the vessel and situation, but should always have these things in common: They offer relatively specific answers to your questions; you understand what the products are telling you; you understand what the products are not telling you.

When you have finished your weather analysis, ask yourself, “Do I want to be in that place at that time?�VbCrLf In the sport of flying, pilots often say the worst thing is to be in the air wishing you were on the ground. If you use the right weather products and use them to make good decisions, you should find yourself in the right place at the right time. If you turn out to be wrong, go back and find out what happened, what you should have known, so you’ll know next time.

Practice weather forecast scenarios when you have good Internet access, become familiar with the products, and plan for how you will access the products when you are at sea. If you find a useful weather forecast or observational product not currently available in Ocens’ WeatherNet, Winlink or your bandwidth-efficient download resource, ask your provider to add the product.