Friday, July 16, 2010

Some Weather Sources

Like most sailors, having some advance knowledge of the weather is, at least, nice and, at most, vital to one's safety. The latter was best shown by our experience with a soon to be huge low-pressure area that developed off the Maryland coast when we were going from the Chesapeake to St. Thomas. Before we left I monitored the weather carefully for almost a week looking for the right conditions for our departure. We needed winds other than from the north to northeast for at least 36 hours so that we could get across the Gulf Stream (even fairly moderate winds blowing into the current can be dangerous) and we, of course, wanted to avoid gale conditions. Ainia is a heavy, powerful vessel so I did not mind a good blow though (up to 25 knots+ sustained is fine).

As it turned out there was a very narrow window that suited our departure. The previous days had had NE winds so were a no go; boats that left the day after we did ended up with gale conditions for 48 hours and ended up going into Bermuda to lick their wounds. One of these boats was still in Bermuda more than a week later waiting for suitable conditions to keep going. Once we were gone the weather forecast further informed what we did. The low was forecast to move northward and only a little eastward and deepen greatly. In fact, within four days it was centered at the southern tip of Greenland with a central pressure of 982 mb (similar to that of a minor hurricane). A cold front built from Greenland to Cuba. Since I knew that nastiness was building to the north, I made the tactical decision that we should push southeasterly as hard as we good - for example, by not taking in a precautionary reef before nightfall. It all worked well and we got south in a hurry and quite safely.

Weather resources we used
There are many resources available depending on whether you are offshore or just moving around in the Caribbean. For most you need as SSB radio or at least a good shortwave receiver (ie a receiver with a good antenna setup) Those we used were:
  • "Herb" - Herb Hilgenberg is a legend in cruising circles. A former cruiser himself, his hobby is to provide detailed, personal forecasts to cruisers across much of the North Atlantic. He works out of his house in Burlington, Ontario and uses shortwave (SSB) to communicate with boats at 2000 hrs Zulu (GMT) each day. The first day you contact him he wants to know your location and destination. Sometimes he wants to know the met conditions where you are (eg wind speed and direction, sea state, etc). After that he tracks you. After he knows which boats are calling in he will go through the 'fleet' from north to south and give you your weather and often suggestions on routing to avoid bad stuff and take best advantage of good stuff. I found it helpful to listen to reports and forecasts for other boats to get a better sense of the big picture.
  • Weather fax - A wide range of North Atlantic weather products are broadcast by the US government from stations in Boston (areas further north) and New Orleans (areas further south). These include current conditions, wind and sea state predictions, and 5oo mB charts for a variety of times eg 24 hours, 48 hours, up to 96 hours in some cases. There are not very detailed and you have to listen at the right time to get the right chart (one chart typically takes about five minutes or so to be received. To get these you need to have a SSB or other shortwave radio, a computer and appropriate software (like JVComm that I have mentioned), along with the schedule and frequencies. A couple of years ago the government was going to end this service, thinking it was obsolete, but relented in the face of huge opposition. Similar weather fax services can be found throughout much of the world.
  • Chris Parker - Parker offers the main weather service used throughout the Caribbean and Gulf of Mexico. He broadcasts from his home in Florida on various frequencies at various times in the morning (you have to experiment a bit to see which one is best where you are). You can subscribe to his service (something like $150/yr I think) and this allows you call him and ask for advice on the best day to go from A to B. Many people just listen in though. He starts with an overview and then listens in for subscribers. We did not subscribe and usually found that someone wanted to make a similar trip to where we were going. His reports are very detailed and tend to be quite conservative - which is good for a marine weather guy I think! He also reports on wave conditions. This is important because large northerly swells are common in the Caribbean in the winter. These are created by storms that could be 1000 miles away. You could be sitting in a protected anchorage wanting to sail 50 miles to the next island. There might have been no strong winds for a week where you are - so you might think the conditions ideal for your passage until you find 10' swells from some distant storm. I think if I was spending another winter in the Caribbean I might subscribe to Parker's service because it is a good one.
  • - If you have internet access this is a very handy site. It gives wind and wave forecasts several days ahead in much of the world. The information is in GRIB form and you have to do your own interpretation of where the frontal systems are though. Forecast periods in the near-term are quite short and you almost animate the maps to see what will happen to wind strengths and wave heights. Note that all of the weather fax charts are also available online.
  • Miscellaneous weather sources - Depending on where you are there might also be weather available from local cruiser webs on VHF, SSB webs (for example in the Bahamas where Parker's reception was a bit iffy), or even from island broadcast stations on AM or FM. The quality of these forecasts varied widely
We did not use a custom weather routing service. These tend to be very good and very costly too. The Caribbean 1500 boats that left Norfolk about six hours before we did got forecasts from one of these services and they were told to ignore the customary wisdom that you head pretty much east to 65*W before turning south. Many of these boats went straight down the rhumb line to their destination and had exceptionally rapid trips. This routing worked because the massive depression we had destroyed the normal trade wind pattern below about 25*N.

1 comment:

Rhys said...

Hi, Bruce. I didn't realize that low went as far as 982 wonder we kept your boat moving.

I was crewing on the recent Lake Ontario 300 on a Catalina 470 and we saw 60 knots in one of several squalls on the 17th of July. That's my new "record", but of course, the "seas" were nothing like we saw for amplitude.

Drop me a line. We'd like to have you two over for dinner.