Cruising Weather Sources

There is so much weather data available today that capturing and absorbing it can be an overwhelming project. We look for a few good sources that apply to the area in which we are cruising-and then concentrate on those.

We start this process with a look at the 500mb data. In areas covered by NOAA fax charts, (North Atlantic and Pacific), 500mb data is broadcast. In most of the rest of the world we use the MaxSea weather “Chopper” to access this data. As the 500mb is a straight output from the computer models, the source is not as important as with surface forecasts.

Next, we download surface pressure and wind data. The best source for this is usually the local government forecast. NOAA’s Marine Prediction Center forecasts are excellent, as are those from the UK Met Service, and the Met Services of New Zealand and Australia.

We also look to the raw model data from the MaxSea Chopper. Outside the US we’ve found the Oceanic Experimental model to usually have good correlation in the first 48 to 72 hours. Beyond this period, the results can give you a feel for risk factors, but for actual routing, while better than nothing, these have to be viewed with a healthy dose of skepticism.

In many areas of the world, the local marine forecasters will issue a discussion of the current forecast, how the different models they use are correlating, and information on the possibility of upgrading or downgrading warnings. Where possible, we always review this data, as it gives us a much better feel for the underlying thinking.

It is often possible to call the local marine forecast office and speak to the duty forecaster. Once they’ve finished with the latest forecast, there is often time for a chat. We like to get a feel for what they are looking at which is not in the formal forecast. There will often be risk factors or potential changes that could occur, but for which the forecast process has not yet yielded enough data to enter this into the normal weather message. Speaking with the local weather guys can also yield information on clues to watch for in the government forecasts and our own on-board observations.

We put the most emphasis on the data which we can observe on board. Trends in barometric pressure, wind speed and direction, and clouds give a really good feel for what’s coming at us. And if there’s a conflict between what our observations are telling us and the forecast, we normally go with our own observations. In the olden days we noted this data hourly in our log book. Now we use the MaxSea Data Trend Center, which graphically tracks wind direction, speed, and barometric pressure.

We also use our radar as a close-in weather tool. We can pick out frontal boundaries, squall lines, and individual squalls at the maximum distance of the radar. On Beowulf this was as much as 50+ miles with the radar display offset. On the new boat, with a more powerful radar, we expect this range to extend out to 70 or 80 miles.

The Chopper data is accessed using an e-mail source. We’ve done this with SailMail and our SSB receiver, with Iridium and Globalstar sat phones, and with land lines, when they are available. We particularly like the “subscribe” function, where we can designate what data we want, and then it is automatically sent to our e-mail address each day.

The easiest way for us to get government faxes is with a dedicated fax receiver-although you can also pick these up with the SSB radio and a demodulator. When we’ve been in Globalstar Sat Phone range, we’ve sometimes found it helpful to download the NOAA fax charts directly from the web (Iridium is usually too slow for this). The advantage here is timing-we can get the latest chart a couple of hours earlier on the Internet than when it is broadcast via SSB. If we’re crossing the Gulf Stream, that two or three hour head start can make a big difference!

Another good source of weather are cruisers reporting in to ham or SSB nets. However, we’ve often found that this information is not in a consistent format. Is the wind direction given in true or magnetic? Are wind speeds true or apparent? And when was the last time the barometer was calibrated? When we hear data which is way off what we would normally expect, we discount it, or ask for a clarification.

With Internet or e-mail access it is now possible to get buoy and land-based observations as well. We have not used these in the past, but when we take off cruising on the new boat we will do so-if there is data in the areas that we’re cruising.

Over the years we’ve used a number of different routing services. These have usually been for out-of-season voyages through the tropics, where there were hurricane threats to evaluate. We like having someone with whom we can discuss the data in detail, and ask questions which may not have been answered by the forecasts or discussions we’ve received onboard. However, experience has taught us (the hard way) never to relinquish the final decision-making authority to land-based routers.

The problem with all of this data is interpreting it. The basics of weather analysis and making the appropriate tactical decisions is pretty simple. But you do have to have an understanding of the fundamentals. When we’ve been ashore or at anchor for a long period of time, we always go back and skim through our notes on weather interpretation and tactics. It all comes back pretty fast, but we don’t want to be scratching our heads with a gale bearing down on us offshore.

A final word on this subject. Understanding forecasting and tactics can seem intimidating. From the outside looking in the subject looks complex and difficult to learn. But there are so many resources available today that learning the basics and practicing are easier than they have ever been. Our advice is to put this skill at the top of the to-do list. It will be fun on land, and pay huge dividends once you start to cruise.

Posted by Steve Dashew  (October 11, 2004)

2 Responses to “Cruising Weather Sources”

  1. Wayne martin Says:

    I have used maxsea for years but never have understood what weather model maxsea chopper uses. Do you know?

  2. Steve Dashew Says:

    Howdy Waye:
    Maxsea used to have a choice in weather models, with the primary being GFS. Not sure now as we have not used it for years.